How to parse a string to an int in C

Question

What s the C   way of parsing a string  given as char    into an int  Robust and clear error handling is a plus  instead of returning zero

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The C   String Toolkit Library  StrTk  has the following solution   static const std  size t digit table symbol count   256  static const unsigned char digit table digit table symbol count         0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xFF - 0x07    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x08 - 0x0F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x10 - 0x17    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x18 - 0x1F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x20 - 0x27    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x28 - 0x2F    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07     0x30 - 0x37    0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x38 - 0x3F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x40 - 0x47    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x48 - 0x4F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x50 - 0x57    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x58 - 0x5F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x60 - 0x67    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x68 - 0x6F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x70 - 0x77    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x78 - 0x7F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x80 - 0x87    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x88 - 0x8F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x90 - 0x97    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x98 - 0x9F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xA0 - 0xA7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xA8 - 0xAF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xB0 - 0xB7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xB8 - 0xBF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xC0 - 0xC7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xC8 - 0xCF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xD0 - 0xD7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xD8 - 0xDF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xE0 - 0xE7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xE8 - 0xEF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xF0 - 0xF7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xF8 - 0xFF      template lt typename InputIterator  typename T gt  inline bool string to signed type converter impl itr InputIterator begin  InputIterator end  T amp  v       if  0    std  distance begin end         return false     v   0     InputIterator it   begin     bool negative   false     if          it          it     else if   -      it               it        negative   true          if  end    it        return false     while end    it             const T digit   static cast lt T gt  digit table static cast lt unsigned int gt   it             if  0xFF    digit           return false        v    10   v    digit          if  negative        v    -1     return true      The InputIterator can be of either unsigned char   char  or std  string iterators  and T is expected to be a signed int  such as signed int  int  or long

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I think these three links sum it up    http   tinodidriksen com 2010 02 07 cpp-convert-int-to-string-speed  http   tinodidriksen com 2010 02 16 cpp-convert-string-to-int-speed  http   www fastformat org performance html   stringstream and lexical cast solutions are about the same as lexical cast is using stringstream   Some specializations of lexical cast use different approach see http   www boost org doc libs release boost lexical cast hpp for details   Integers and floats are now specialized for integer to string conversion     One can specialize lexical cast for his her own needs and make it fast  This would be the ultimate solution satisfying all parties  clean and simple   Articles already mentioned show comparison between different methods of converting  integers  lt -  strings  Following approaches make sense   old c-way  spirit karma  fastformat  simple naive loop     Lexical cast is ok in some cases e g  for int to string conversion   Converting string to int using lexical cast is not a good idea as it is 10-40 times slower than atoi depending on the platform compiler used   Boost Spirit Karma seems to be the fastest library for converting integer to string   ex   generate ptr char  int   integer number     and basic simple loop from the article mentioned above is a fastest way to convert string to int  obviously not the safest one  strtol   seems like a safer solution  int naive char 2 int const char  p        int x   0      bool neg   false      if   p     -             neg   true            p            while   p  gt    0   amp  amp   p  lt    9             x    x 10      p -  0              p            if  neg            x   -x            return x

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This is a safer C way than atoi    const char  str    123   int i   if sscanf str    d    amp i      EOF           error        C   with standard library stringstream   thanks CMS    int str2int  const string  amp str      stringstream ss str     int num    if  ss  gt  gt  num  fail                 ERROR        return num      With boost library   thanks jk    include  lt boost lexical cast hpp gt   include  lt string gt   try       std  string str    123       int number   boost  lexical cast lt  int  gt   str      catch  const boost  bad lexical cast  amp             Error     Edit  Fixed the stringstream version so that it handles errors   thanks to CMS s and jk s comment on original post

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You can use the a stringstream from the C   standard libraray   stringstream ss str   int x  ss  gt  gt  x   if ss        lt -- error handling      use x   else        not a number        The stream state will be set to fail   if a non-digit is encountered when   trying to read an integer    See Stream pitfalls for pitfalls of errorhandling and streams in C

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You can use Boost s lexical cast  which wraps this in a more generic interface  lexical cast lt Target gt  Source  throws bad lexical cast on failure

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The good  old C way still works  I recommend strtol or strtoul  Between the return status and the  endPtr   you can give good diagnostic output  It also handles multiple bases nicely

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I know this is an older question  but I ve come across it so many times and  to date  have still not found a nicely templated solution having the following characteristics    Can convert any base  and detect base type  Will detect erroneous data  i e  ensure the entire string  less leading trailing whitespace  is consumed by the conversion  Will ensure that  regardless of the type converted to  the range of the string s value is acceptable    So  here is mine  with a test strap   Because it uses the C functions strtoull strtoll under the hood  it always converts first to the largest type available   Then  if you are not using the largest type  it will perform additional range checks to verify your type was not over under flowed  For this  it is a little less performant than if one properly chose strtol strtoul   However  it also works for shorts chars and  to the best of my knowledge  there exists no standard library function that does that  too   Enjoy  hopefully someone finds it useful    include  lt cstdlib gt   include  lt cerrno gt   include  lt limits gt   include  lt stdexcept gt   include  lt sstream gt   static const int DefaultBase   10   template lt typename T gt  static inline T CstrtoxllWrapper const char  str  int base   DefaultBase        while  isspace  str   str       remove leading spaces  verify there s data     if   str      0     throw std  invalid argument  str  no data         nothing to convert         NOTE   for some reason strtoull allows a negative sign  we don t  if                 converting to an unsigned then it must always be positive      if   std  numeric limits lt T gt   is signed  amp  amp   str     -         throw std  invalid argument  str  negative              reset errno and call fn  either strtoll or strtoull      errno   0      char  ePtr      T tmp   std  numeric limits lt T gt   is signed   strtoll str   amp ePtr  base                                                  strtoull str   amp ePtr  base           check for any C errors -- note these are range errors on T  which may          still be out of the range of the actual type we re using  the caller          may need to perform additional range checks      if  errno    0                     if  errno    ERANGE    throw std  range error  str  out of range                  else if  errno    EINVAL    throw std  invalid argument  str  EINVAL                  else   throw std  invalid argument  str  unknown errno                    verify everything converted -- extraneous spaces are allowed     if  ePtr    NULL                    while  isspace  ePtr   ePtr                if   ePtr      0     throw std  invalid argument  str  bad data                 return tmp     template lt typename T gt  T StringToSigned const char  str  int base   DefaultBase        static const long long max   std  numeric limits lt T gt   max        static const long long min   std  numeric limits lt T gt   min         long long tmp   CstrtoxllWrapper lt typeof tmp  gt  str  base      use largest type         final range check -- only needed if not long long type  a smart compiler          should optimize this whole thing out     if  sizeof T     sizeof tmp     return tmp         if  tmp  lt  min    tmp  gt  max                    std  ostringstream err              err  lt  lt   str  value    lt  lt  tmp  lt  lt    out of    lt  lt  sizeof T    8                  lt  lt   -bit signed range                 if  sizeof T     1  err  lt  lt  min  lt  lt        lt  lt  max              else err  lt  lt   int  min  lt  lt        lt  lt   int  max      don t print garbage chars             err  lt  lt                   throw std  range error err str                return tmp     template lt typename T gt  T StringToUnsigned const char  str  int base   DefaultBase        static const unsigned long long max   std  numeric limits lt T gt   max         unsigned long long tmp   CstrtoxllWrapper lt typeof tmp  gt  str  base      use largest type         final range check -- only needed if not long long type  a smart compiler          should optimize this whole thing out     if  sizeof T     sizeof tmp     return tmp         if  tmp  gt  max                    std  ostringstream err              err  lt  lt   str  value    lt  lt  tmp  lt  lt    out of    lt  lt  sizeof T    8                  lt  lt   -bit unsigned range  0                 if  sizeof T     1  err  lt  lt  max              else err  lt  lt   int  max      don t print garbage chars             err  lt  lt                   throw std  range error err str                return tmp     template lt typename T gt  inline T StringToDecimal const char  str  int base   DefaultBase        return std  numeric limits lt T gt   is signed   StringToSigned lt T gt  str  base                                                 StringToUnsigned lt T gt  str  base      template lt typename T gt  inline T StringToDecimal T  amp out convertedVal  const char  str  int base   DefaultBase        return out convertedVal   StringToDecimal lt T gt  str  base                                         Test Strap                                       include  lt inttypes h gt   include  lt iostream gt   static bool  g anyFailed   false   template lt typename T gt  void TestIt const char  tName              const char  s  int base              bool successExpected   false  T expectedValue   0         define FAIL s     g anyFailed   true  std  cout  lt  lt  s         T x      std  cout  lt  lt   converting lt    lt  lt  tName  lt  lt    gt b    lt  lt  base  lt  lt        lt  lt  s  lt  lt           try                   StringToDecimal lt T gt  x  s  base                  get here on success only             if   successExpected                                    FAIL   -- TEST FAILED  SUCCESS NOT EXPECTED    lt  lt  std  endl                             else                                   std  cout  lt  lt    - gt                         if  sizeof T     1  std  cout  lt  lt  x                      else std  cout  lt  lt   int  x      don t print garbage chars                     if  x    expectedValue                                                    FAIL    FAILED  expected value    lt  lt  expectedValue  lt  lt                                                   std  cout  lt  lt  std  endl                          catch  std  exception  amp e                    if  successExpected                                    FAIL      -- TEST FAILED  EXPECTED SUCCESS                             lt  lt     got    lt  lt  e what    lt  lt       lt  lt  std  endl                             else                                   std  cout  lt  lt     expected exception encounterd      lt  lt  e what    lt  lt       lt  lt  std  endl                          define TEST t  s             TestIt lt t gt   t  s    VA ARGS      int main         std  cout  lt  lt                variable base tests                lt  lt  std  endl      TEST int   -0xF   0  true  -0xF       TEST int    0xF   0  true  0xF       TEST int   0xF   0  true  0xF       TEST int   -010   0  true  -010       TEST int    010   0  true  010       TEST int   010   0  true  010       TEST int   -10   0  true  -10       TEST int    10   0  true  10       TEST int   10   0  true  10        std  cout  lt  lt                base-10 tests                lt  lt  std  endl      TEST int   -010   10  true  -10       TEST int    010   10  true  10       TEST int   010   10  true  10       TEST int   -10   10  true  -10       TEST int    10   10  true  10       TEST int   10   10  true  10       TEST int   00010   10  true  10        std  cout  lt  lt                base-8 tests                lt  lt  std  endl      TEST int   777   8  true  0777       TEST int   -0111    8  true  -0111       TEST int    0010    8  true  010        std  cout  lt  lt                base-16 tests                lt  lt  std  endl      TEST int   DEAD   16  true  0xDEAD       TEST int   -BEEF   16  true  -0xBEEF       TEST int    C30   16  true  0xC30        std  cout  lt  lt                base-2 tests                lt  lt  std  endl      TEST int   -10011001   2  true  -153       TEST int   10011001   2  true  153        std  cout  lt  lt                irregular base tests                lt  lt  std  endl      TEST int   Z   36  true  35       TEST int   ZZTOP   36  true  60457993       TEST int   G   17  true  16       TEST int   H   17        std  cout  lt  lt                space deliminated tests                lt  lt  std  endl      TEST int   1337       10  true  1337       TEST int      FEAD   16  true  0xFEAD       TEST int      0711      0  true  0711        std  cout  lt  lt                bad data tests                lt  lt  std  endl      TEST int   FEAD   10       TEST int   1234 asdfklj   10       TEST int   -0xF   10       TEST int    0xF   10       TEST int   0xF   10       TEST int   -F   10       TEST int    F   10       TEST int   12 4   10       TEST int   ABG   16       TEST int   10011002   2        std  cout  lt  lt                int8 t range tests                lt  lt  std  endl      TEST int8 t   7F   16  true  std  numeric limits lt int8 t gt   max         TEST int8 t   80   16       TEST int8 t   -80   16  true  std  numeric limits lt int8 t gt   min         TEST int8 t   -81   16       TEST int8 t   FF   16       TEST int8 t   100   16        std  cout  lt  lt                uint8 t range tests                lt  lt  std  endl      TEST uint8 t   7F   16  true  std  numeric limits lt int8 t gt   max         TEST uint8 t   80   16  true  std  numeric limits lt int8 t gt   max   1       TEST uint8 t   -80   16       TEST uint8 t   -81   16       TEST uint8 t   FF   16  true  std  numeric limits lt uint8 t gt   max         TEST uint8 t   100   16        std  cout  lt  lt                int16 t range tests                lt  lt  std  endl      TEST int16 t   7FFF   16  true  std  numeric limits lt int16 t gt   max         TEST int16 t   8000   16       TEST int16 t   -8000   16  true  std  numeric limits lt int16 t gt   min         TEST int16 t   -8001   16       TEST int16 t   FFFF   16       TEST int16 t   10000   16        std  cout  lt  lt                uint16 t range tests                lt  lt  std  endl      TEST uint16 t   7FFF   16  true  std  numeric limits lt int16 t gt   max         TEST uint16 t   8000   16  true  std  numeric limits lt int16 t gt   max   1       TEST uint16 t   -8000   16       TEST uint16 t   -8001   16       TEST uint16 t   FFFF   16  true  std  numeric limits lt uint16 t gt   max         TEST uint16 t   10000   16        std  cout  lt  lt                int32 t range tests                lt  lt  std  endl      TEST int32 t   7FFFFFFF   16  true  std  numeric limits lt int32 t gt   max         TEST int32 t   80000000   16       TEST int32 t   -80000000   16  true  std  numeric limits lt int32 t gt   min         TEST int32 t   -80000001   16       TEST int32 t   FFFFFFFF   16       TEST int32 t   100000000   16        std  cout  lt  lt                uint32 t range tests                lt  lt  std  endl      TEST uint32 t   7FFFFFFF   16  true  std  numeric limits lt int32 t gt   max         TEST uint32 t   80000000   16  true  std  numeric limits lt int32 t gt   max   1       TEST uint32 t   -80000000   16       TEST uint32 t   -80000001   16       TEST uint32 t   FFFFFFFF   16  true  std  numeric limits lt uint32 t gt   max         TEST uint32 t   100000000   16        std  cout  lt  lt                int64 t range tests                lt  lt  std  endl      TEST int64 t   7FFFFFFFFFFFFFFF   16  true  std  numeric limits lt int64 t gt   max         TEST int64 t   8000000000000000   16       TEST int64 t   -8000000000000000   16  true  std  numeric limits lt int64 t gt   min         TEST int64 t   -8000000000000001   16       TEST int64 t   FFFFFFFFFFFFFFFF   16       TEST int64 t   10000000000000000   16        std  cout  lt  lt                uint64 t range tests                lt  lt  std  endl      TEST uint64 t   7FFFFFFFFFFFFFFF   16  true  std  numeric limits lt int64 t gt   max         TEST uint64 t   8000000000000000   16  true  std  numeric limits lt int64 t gt   max   1       TEST uint64 t   -8000000000000000   16       TEST uint64 t   -8000000000000001   16       TEST uint64 t   FFFFFFFFFFFFFFFF   16  true  std  numeric limits lt uint64 t gt   max         TEST uint64 t   10000000000000000   16        std  cout  lt  lt  std  endl  lt  lt  std  endl                lt  lt    g anyFailed       SOME TESTS FAILED        ALL TESTS PASSED                  lt  lt  std  endl       return  g anyFailed      StringToDecimal is the user-land method  it is overloaded so it can be called either like this   int a  a   StringToDecimal lt int gt   100      or this   int a  StringToDecimal a   100      I hate repeating the int type  so prefer the latter  This ensures that if the type of  a  changes one does not get bad results  I wish the compiler could figure it out like   int a  a   StringToDecimal  100         but  C   does not deduce template return types  so that s the best I can get   The implementation is pretty simple   CstrtoxllWrapper wraps both strtoull and strtoll  calling whichever is necessary based on the template type s signed-ness and providing some additional guarantees  e g  negative input is disallowed if unsigned and it ensures the entire string was converted    CstrtoxllWrapper is used by StringToSigned and StringToUnsigned with the largest type  long long unsigned long long  available to the compiler  this allows the maximal conversion to be performed   Then  if it is necessary  StringToSigned StringToUnsigned performs the final range checks on the underlying type  Finally  the end-point method  StringToDecimal  decides which of the StringTo  template methods to call based on the underlying type s signed-ness   I think most of the junk can be optimized out by the compiler  just about everything should be compile-time deterministic  Any commentary on this aspect would be interesting to me

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The good  old C way still works  I recommend strtol or strtoul  Between the return status and the  endPtr   you can give good diagnostic output  It also handles multiple bases nicely

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In the new C  11 there are functions for that  stoi  stol  stoll  stoul and so on   int myNr   std  stoi myString     It will throw an exception on conversion error   Even these new functions still have the same issue as noted by Dan  they will happily convert the string  11x  to integer  11    See more  http   en cppreference com w cpp string basic string stol

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You can use stringstream s  int str2int  const string  amp str      stringstream ss str     int num    ss  gt  gt  num    return num

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You can use Boost s lexical cast  which wraps this in a more generic interface  lexical cast lt Target gt  Source  throws bad lexical cast on failure

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You can use Boost s lexical cast  which wraps this in a more generic interface  lexical cast lt Target gt  Source  throws bad lexical cast on failure

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In C  you can use int atoi  const char   str    Parses the C-string str interpreting its content as an integral number  which is returned as a value of type int

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This is a safer C way than atoi    const char  str    123   int i   if sscanf str    d    amp i      EOF           error        C   with standard library stringstream   thanks CMS    int str2int  const string  amp str      stringstream ss str     int num    if  ss  gt  gt  num  fail                 ERROR        return num      With boost library   thanks jk    include  lt boost lexical cast hpp gt   include  lt string gt   try       std  string str    123       int number   boost  lexical cast lt  int  gt   str      catch  const boost  bad lexical cast  amp             Error     Edit  Fixed the stringstream version so that it handles errors   thanks to CMS s and jk s comment on original post

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You can use stringstream s  int str2int  const string  amp str      stringstream ss str     int num    ss  gt  gt  num    return num

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This is a safer C way than atoi    const char  str    123   int i   if sscanf str    d    amp i      EOF           error        C   with standard library stringstream   thanks CMS    int str2int  const string  amp str      stringstream ss str     int num    if  ss  gt  gt  num  fail                 ERROR        return num      With boost library   thanks jk    include  lt boost lexical cast hpp gt   include  lt string gt   try       std  string str    123       int number   boost  lexical cast lt  int  gt   str      catch  const boost  bad lexical cast  amp             Error     Edit  Fixed the stringstream version so that it handles errors   thanks to CMS s and jk s comment on original post

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You could use this defined method    define toInt x   atoi x c str        And if you were to convert from String to an Integer  you would just do the following   int main     string test    46   test2    56   int a   toInt test   int b   toInt test2   cout lt  lt a b lt  lt endl      The output would be 102

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I know three ways of converting String into int   Either use stoi String to int  function or just go with Stringstream  the third way to go individual conversion  Code is below   1st Method  std  string s1    4533   std  string s2    3 010101   std  string s3    31337 with some string    int myint1   std  stoi s1   int myint2   std  stoi s2   int myint3   std  stoi s3    std  cout  lt  lt   s1  lt  lt      lt  lt  myint1  lt  lt    n   std  cout  lt  lt   s2  lt  lt      lt  lt  myint2  lt  lt    n   std  cout  lt  lt   s3  lt  lt      lt  lt  myint3  lt  lt    n     2nd Method   include  lt string h gt   include  lt sstream gt   include  lt iostream gt   include  lt cstring gt  using namespace std    int StringToInteger string NumberAsString        int NumberAsInteger      stringstream ss      ss  lt  lt  NumberAsString      ss  gt  gt  NumberAsInteger      return NumberAsInteger    int main         string NumberAsString      cin  gt  gt  NumberAsString      cout  lt  lt  StringToInteger NumberAsString   lt  lt  endl      return 0       3rd Method - but not for an individual conversion   std  string str4    453   int i   0  in 0     453 as on for   i   0  i  lt  str4 length    i           in   str4 i       cout  lt  lt in-48

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You can use the a stringstream from the C   standard libraray   stringstream ss str   int x  ss  gt  gt  x   if ss        lt -- error handling      use x   else        not a number        The stream state will be set to fail   if a non-digit is encountered when   trying to read an integer    See Stream pitfalls for pitfalls of errorhandling and streams in C

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If you have C  11  the appropriate solutions nowadays are the C   integer conversion functions in  lt string gt   stoi  stol  stoul  stoll  stoull   They throw appropriate exceptions when given incorrect input and use the fast and small strto  functions under the hood   If you are stuck with an earlier revision of C    it would be forward-portable of you to mimic these functions in your implementation

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From C  17 onwards you can use std  from chars from the  lt charconv gt  header as documented here   For example    include  lt iostream gt   include  lt charconv gt   include  lt array gt   int main         char const   str    42       int value   0       std  from chars result result   std  from chars std  begin str   std  end str   value        if result error    std  errc  invalid argument              std  cout  lt  lt   Error  invalid format             else if result error    std  errc  result out of range              std  cout  lt  lt   Error  value too big for int range             else             std  cout  lt  lt   Success     lt  lt  result            As a bonus  it can also handle other bases  like hexadecimal

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I like Dan Moulding s answer  I ll just add a bit of C   style to it    include  lt cstdlib gt   include  lt cerrno gt   include  lt climits gt   include  lt stdexcept gt   int to int const std  string  amp s  int base   0        char  end      errno   0      long result   std  strtol s c str     amp end  base       if  errno    ERANGE    result  gt  INT MAX    result  lt  INT MIN          throw std  out of range  toint  string is out of range        if  s length      0     end      0           throw std  invalid argument  toint  invalid string        return result      It works for both std  string and const char  through the implicit conversion  It s also useful for base conversion  e g  all to int  0x7b   and to int  0173   and to int  01111011   2  and to int  0000007B   16  and to int  11120   3  and to int  3L   34   would return 123   Unlike std  stoi it works in pre-C  11  Also unlike std  stoi  boost  lexical cast and stringstream it throws exceptions for weird strings like  123hohoho    NB  This function tolerates leading spaces but not trailing spaces  i e  to int    123   returns 123 while to int  123      throws exception  Make sure this is acceptable for your use case or adjust the code   Such function could be part of STL

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What not to do  Here is my first piece of advice  do not use stringstream for this  While at first it may seem simple to use  you ll find that you have to do a lot of extra work if you want robustness and good error handling   Here is an approach that intuitively seems like it should work   bool str2int  int  amp i  char const  s        std  stringstream ss s       ss  gt  gt  i      if  ss fail                 not an integer         return false            return true      This has a major problem  str2int i   1337h4x0r   will happily return true and i will get the value 1337  We can work around this problem by ensuring there are no more characters in the stringstream after the conversion   bool str2int  int  amp i  char const  s        char              c      std  stringstream ss s       ss  gt  gt  i      if  ss fail      ss get c                not an integer         return false            return true      We fixed one problem  but there are still a couple of other problems   What if the number in the string is not base 10  We can try to accommodate other bases by setting the stream to the correct mode  e g  ss  lt  lt  std  hex  before trying the conversion  But this means the caller must know a priori what base the number is -- and how can the caller possibly know that  The caller doesn t know what the number is yet  They don t even know that it is a number  How can they be expected to know what base it is  We could just mandate that all numbers input to our programs must be base 10 and reject hexadecimal or octal input as invalid  But that is not very flexible or robust  There is no simple solution to this problem  You can t simply try the conversion once for each base  because the decimal conversion will always succeed for octal numbers  with a leading zero  and the octal conversion may succeed for some decimal numbers  So now you have to check for a leading zero  But wait  Hexadecimal numbers can start with a leading zero too  0x      Sigh   Even if you succeed in dealing with the above problems  there is still another bigger problem  what if the caller needs to distinguish between bad input  e g   123foo   and a number that is out of the range of int  e g   4000000000  for 32-bit int   With stringstream  there is no way to make this distinction  We only know whether the conversion succeeded or failed  If it fails  we have no way of knowing why it failed  As you can see  stringstream leaves much to be desired if you want robustness and clear error handling   This leads me to my second piece of advice  do no use Boost s lexical cast for this  Consider what the lexical cast documentation has to say      Where a higher degree of control is   required over conversions    std  stringstream and   std  wstringstream offer a more   appropriate path  Where   non-stream-based conversions are   required  lexical cast is the wrong   tool for the job and is not   special-cased for such scenarios    What   We ve already seen that stringstream has a poor level of control  and yet it says stringstream should be used instead of lexical cast if you need  a higher level of control   Also  because lexical cast is just a wrapper around stringstream  it suffers from the same problems that stringstream does  poor support for multiple number bases and poor error handling   The best solution  Fortunately  somebody has already solved all of the above problems  The C standard library contains strtol and family which have none of these problems   enum STR2INT ERROR   SUCCESS  OVERFLOW  UNDERFLOW  INCONVERTIBLE     STR2INT ERROR str2int  int  amp i  char const  s  int base   0        char  end      long  l      errno   0      l   strtol s   amp end  base       if   errno    ERANGE  amp  amp  l    LONG MAX     l  gt  INT MAX            return OVERFLOW            if   errno    ERANGE  amp  amp  l    LONG MIN     l  lt  INT MIN            return UNDERFLOW            if   s      0      end      0             return INCONVERTIBLE            i   l      return SUCCESS      Pretty simple for something that handles all the error cases and also supports any number base from 2 to 36  If base is zero  the default  it will try to convert from any base  Or the caller can supply the third argument and specify that the conversion should only be attempted for a particular base  It is robust and handles all errors with a minimal amount of effort   Other reasons to prefer strtol  and family     It exhibits much better runtime performance It introduces less compile-time overhead  the others pull in nearly 20 times more SLOC from headers  It results in the smallest code size   There is absolutely no good reason to use any other method

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In the new C  11 there are functions for that  stoi  stol  stoll  stoul and so on   int myNr   std  stoi myString     It will throw an exception on conversion error   Even these new functions still have the same issue as noted by Dan  they will happily convert the string  11x  to integer  11    See more  http   en cppreference com w cpp string basic string stol

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You can use the a stringstream from the C   standard libraray   stringstream ss str   int x  ss  gt  gt  x   if ss        lt -- error handling      use x   else        not a number        The stream state will be set to fail   if a non-digit is encountered when   trying to read an integer    See Stream pitfalls for pitfalls of errorhandling and streams in C

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You can use stringstream s  int str2int  const string  amp str      stringstream ss str     int num    ss  gt  gt  num    return num

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I like Dan s answer  esp because of the avoidance of exceptions  For embedded systems development and other low level system development  there may not be a proper Exception framework available    Added a check for white-space after a valid string   these three lines      while  isspace  end             end             Added a check for parsing errors too       if   errno    0      s    end             return INCONVERTIBLE           Here is the complete function     include  lt cstdlib gt   include  lt cerrno gt   include  lt climits gt   include  lt stdexcept gt   enum STR2INT ERROR   SUCCESS  OVERFLOW  UNDERFLOW  INCONVERTIBLE     STR2INT ERROR str2long  long  amp l  char const  s  int base   0        char  end    char   s      errno   0       l   strtol s   amp end  base        if   errno    ERANGE   amp  amp   l    LONG MAX             return OVERFLOW            if   errno    ERANGE   amp  amp   l    LONG MIN             return UNDERFLOW            if   errno    0      s    end             return INCONVERTIBLE                while  isspace  unsigned char  end             end               if   s      0      end      0             return INCONVERTIBLE             return SUCCESS

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The C   String Toolkit Library  StrTk  has the following solution   static const std  size t digit table symbol count   256  static const unsigned char digit table digit table symbol count         0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xFF - 0x07    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x08 - 0x0F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x10 - 0x17    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x18 - 0x1F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x20 - 0x27    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x28 - 0x2F    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07     0x30 - 0x37    0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x38 - 0x3F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x40 - 0x47    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x48 - 0x4F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x50 - 0x57    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x58 - 0x5F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x60 - 0x67    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x68 - 0x6F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x70 - 0x77    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x78 - 0x7F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x80 - 0x87    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x88 - 0x8F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x90 - 0x97    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0x98 - 0x9F    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xA0 - 0xA7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xA8 - 0xAF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xB0 - 0xB7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xB8 - 0xBF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xC0 - 0xC7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xC8 - 0xCF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xD0 - 0xD7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xD8 - 0xDF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xE0 - 0xE7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xE8 - 0xEF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xF0 - 0xF7    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     0xF8 - 0xFF      template lt typename InputIterator  typename T gt  inline bool string to signed type converter impl itr InputIterator begin  InputIterator end  T amp  v       if  0    std  distance begin end         return false     v   0     InputIterator it   begin     bool negative   false     if          it          it     else if   -      it               it        negative   true          if  end    it        return false     while end    it             const T digit   static cast lt T gt  digit table static cast lt unsigned int gt   it             if  0xFF    digit           return false        v    10   v    digit          if  negative        v    -1     return true      The InputIterator can be of either unsigned char   char  or std  string iterators  and T is expected to be a signed int  such as signed int  int  or long

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If you have C  11  the appropriate solutions nowadays are the C   integer conversion functions in  lt string gt   stoi  stol  stoul  stoll  stoull   They throw appropriate exceptions when given incorrect input and use the fast and small strto  functions under the hood   If you are stuck with an earlier revision of C    it would be forward-portable of you to mimic these functions in your implementation

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You could use this defined method    define toInt x   atoi x c str        And if you were to convert from String to an Integer  you would just do the following   int main     string test    46   test2    56   int a   toInt test   int b   toInt test2   cout lt  lt a b lt  lt endl      The output would be 102

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The good  old C way still works  I recommend strtol or strtoul  Between the return status and the  endPtr   you can give good diagnostic output  It also handles multiple bases nicely

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In C  you can use int atoi  const char   str    Parses the C-string str interpreting its content as an integral number  which is returned as a value of type int

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I like Dan Moulding s answer  I ll just add a bit of C   style to it    include  lt cstdlib gt   include  lt cerrno gt   include  lt climits gt   include  lt stdexcept gt   int to int const std  string  amp s  int base   0        char  end      errno   0      long result   std  strtol s c str     amp end  base       if  errno    ERANGE    result  gt  INT MAX    result  lt  INT MIN          throw std  out of range  toint  string is out of range        if  s length      0     end      0           throw std  invalid argument  toint  invalid string        return result      It works for both std  string and const char  through the implicit conversion  It s also useful for base conversion  e g  all to int  0x7b   and to int  0173   and to int  01111011   2  and to int  0000007B   16  and to int  11120   3  and to int  3L   34   would return 123   Unlike std  stoi it works in pre-C  11  Also unlike std  stoi  boost  lexical cast and stringstream it throws exceptions for weird strings like  123hohoho    NB  This function tolerates leading spaces but not trailing spaces  i e  to int    123   returns 123 while to int  123      throws exception  Make sure this is acceptable for your use case or adjust the code   Such function could be part of STL

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I know this is an older question  but I ve come across it so many times and  to date  have still not found a nicely templated solution having the following characteristics    Can convert any base  and detect base type  Will detect erroneous data  i e  ensure the entire string  less leading trailing whitespace  is consumed by the conversion  Will ensure that  regardless of the type converted to  the range of the string s value is acceptable    So  here is mine  with a test strap   Because it uses the C functions strtoull strtoll under the hood  it always converts first to the largest type available   Then  if you are not using the largest type  it will perform additional range checks to verify your type was not over under flowed  For this  it is a little less performant than if one properly chose strtol strtoul   However  it also works for shorts chars and  to the best of my knowledge  there exists no standard library function that does that  too   Enjoy  hopefully someone finds it useful    include  lt cstdlib gt   include  lt cerrno gt   include  lt limits gt   include  lt stdexcept gt   include  lt sstream gt   static const int DefaultBase   10   template lt typename T gt  static inline T CstrtoxllWrapper const char  str  int base   DefaultBase        while  isspace  str   str       remove leading spaces  verify there s data     if   str      0     throw std  invalid argument  str  no data         nothing to convert         NOTE   for some reason strtoull allows a negative sign  we don t  if                 converting to an unsigned then it must always be positive      if   std  numeric limits lt T gt   is signed  amp  amp   str     -         throw std  invalid argument  str  negative              reset errno and call fn  either strtoll or strtoull      errno   0      char  ePtr      T tmp   std  numeric limits lt T gt   is signed   strtoll str   amp ePtr  base                                                  strtoull str   amp ePtr  base           check for any C errors -- note these are range errors on T  which may          still be out of the range of the actual type we re using  the caller          may need to perform additional range checks      if  errno    0                     if  errno    ERANGE    throw std  range error  str  out of range                  else if  errno    EINVAL    throw std  invalid argument  str  EINVAL                  else   throw std  invalid argument  str  unknown errno                    verify everything converted -- extraneous spaces are allowed     if  ePtr    NULL                    while  isspace  ePtr   ePtr                if   ePtr      0     throw std  invalid argument  str  bad data                 return tmp     template lt typename T gt  T StringToSigned const char  str  int base   DefaultBase        static const long long max   std  numeric limits lt T gt   max        static const long long min   std  numeric limits lt T gt   min         long long tmp   CstrtoxllWrapper lt typeof tmp  gt  str  base      use largest type         final range check -- only needed if not long long type  a smart compiler          should optimize this whole thing out     if  sizeof T     sizeof tmp     return tmp         if  tmp  lt  min    tmp  gt  max                    std  ostringstream err              err  lt  lt   str  value    lt  lt  tmp  lt  lt    out of    lt  lt  sizeof T    8                  lt  lt   -bit signed range                 if  sizeof T     1  err  lt  lt  min  lt  lt        lt  lt  max              else err  lt  lt   int  min  lt  lt        lt  lt   int  max      don t print garbage chars             err  lt  lt                   throw std  range error err str                return tmp     template lt typename T gt  T StringToUnsigned const char  str  int base   DefaultBase        static const unsigned long long max   std  numeric limits lt T gt   max         unsigned long long tmp   CstrtoxllWrapper lt typeof tmp  gt  str  base      use largest type         final range check -- only needed if not long long type  a smart compiler          should optimize this whole thing out     if  sizeof T     sizeof tmp     return tmp         if  tmp  gt  max                    std  ostringstream err              err  lt  lt   str  value    lt  lt  tmp  lt  lt    out of    lt  lt  sizeof T    8                  lt  lt   -bit unsigned range  0                 if  sizeof T     1  err  lt  lt  max              else err  lt  lt   int  max      don t print garbage chars             err  lt  lt                   throw std  range error err str                return tmp     template lt typename T gt  inline T StringToDecimal const char  str  int base   DefaultBase        return std  numeric limits lt T gt   is signed   StringToSigned lt T gt  str  base                                                 StringToUnsigned lt T gt  str  base      template lt typename T gt  inline T StringToDecimal T  amp out convertedVal  const char  str  int base   DefaultBase        return out convertedVal   StringToDecimal lt T gt  str  base                                         Test Strap                                       include  lt inttypes h gt   include  lt iostream gt   static bool  g anyFailed   false   template lt typename T gt  void TestIt const char  tName              const char  s  int base              bool successExpected   false  T expectedValue   0         define FAIL s     g anyFailed   true  std  cout  lt  lt  s         T x      std  cout  lt  lt   converting lt    lt  lt  tName  lt  lt    gt b    lt  lt  base  lt  lt        lt  lt  s  lt  lt           try                   StringToDecimal lt T gt  x  s  base                  get here on success only             if   successExpected                                    FAIL   -- TEST FAILED  SUCCESS NOT EXPECTED    lt  lt  std  endl                             else                                   std  cout  lt  lt    - gt                         if  sizeof T     1  std  cout  lt  lt  x                      else std  cout  lt  lt   int  x      don t print garbage chars                     if  x    expectedValue                                                    FAIL    FAILED  expected value    lt  lt  expectedValue  lt  lt                                                   std  cout  lt  lt  std  endl                          catch  std  exception  amp e                    if  successExpected                                    FAIL      -- TEST FAILED  EXPECTED SUCCESS                             lt  lt     got    lt  lt  e what    lt  lt       lt  lt  std  endl                             else                                   std  cout  lt  lt     expected exception encounterd      lt  lt  e what    lt  lt       lt  lt  std  endl                          define TEST t  s             TestIt lt t gt   t  s    VA ARGS      int main         std  cout  lt  lt                variable base tests                lt  lt  std  endl      TEST int   -0xF   0  true  -0xF       TEST int    0xF   0  true  0xF       TEST int   0xF   0  true  0xF       TEST int   -010   0  true  -010       TEST int    010   0  true  010       TEST int   010   0  true  010       TEST int   -10   0  true  -10       TEST int    10   0  true  10       TEST int   10   0  true  10        std  cout  lt  lt                base-10 tests                lt  lt  std  endl      TEST int   -010   10  true  -10       TEST int    010   10  true  10       TEST int   010   10  true  10       TEST int   -10   10  true  -10       TEST int    10   10  true  10       TEST int   10   10  true  10       TEST int   00010   10  true  10        std  cout  lt  lt                base-8 tests                lt  lt  std  endl      TEST int   777   8  true  0777       TEST int   -0111    8  true  -0111       TEST int    0010    8  true  010        std  cout  lt  lt                base-16 tests                lt  lt  std  endl      TEST int   DEAD   16  true  0xDEAD       TEST int   -BEEF   16  true  -0xBEEF       TEST int    C30   16  true  0xC30        std  cout  lt  lt                base-2 tests                lt  lt  std  endl      TEST int   -10011001   2  true  -153       TEST int   10011001   2  true  153        std  cout  lt  lt                irregular base tests                lt  lt  std  endl      TEST int   Z   36  true  35       TEST int   ZZTOP   36  true  60457993       TEST int   G   17  true  16       TEST int   H   17        std  cout  lt  lt                space deliminated tests                lt  lt  std  endl      TEST int   1337       10  true  1337       TEST int      FEAD   16  true  0xFEAD       TEST int      0711      0  true  0711        std  cout  lt  lt                bad data tests                lt  lt  std  endl      TEST int   FEAD   10       TEST int   1234 asdfklj   10       TEST int   -0xF   10       TEST int    0xF   10       TEST int   0xF   10       TEST int   -F   10       TEST int    F   10       TEST int   12 4   10       TEST int   ABG   16       TEST int   10011002   2        std  cout  lt  lt                int8 t range tests                lt  lt  std  endl      TEST int8 t   7F   16  true  std  numeric limits lt int8 t gt   max         TEST int8 t   80   16       TEST int8 t   -80   16  true  std  numeric limits lt int8 t gt   min         TEST int8 t   -81   16       TEST int8 t   FF   16       TEST int8 t   100   16        std  cout  lt  lt                uint8 t range tests                lt  lt  std  endl      TEST uint8 t   7F   16  true  std  numeric limits lt int8 t gt   max         TEST uint8 t   80   16  true  std  numeric limits lt int8 t gt   max   1       TEST uint8 t   -80   16       TEST uint8 t   -81   16       TEST uint8 t   FF   16  true  std  numeric limits lt uint8 t gt   max         TEST uint8 t   100   16        std  cout  lt  lt                int16 t range tests                lt  lt  std  endl      TEST int16 t   7FFF   16  true  std  numeric limits lt int16 t gt   max         TEST int16 t   8000   16       TEST int16 t   -8000   16  true  std  numeric limits lt int16 t gt   min         TEST int16 t   -8001   16       TEST int16 t   FFFF   16       TEST int16 t   10000   16        std  cout  lt  lt                uint16 t range tests                lt  lt  std  endl      TEST uint16 t   7FFF   16  true  std  numeric limits lt int16 t gt   max         TEST uint16 t   8000   16  true  std  numeric limits lt int16 t gt   max   1       TEST uint16 t   -8000   16       TEST uint16 t   -8001   16       TEST uint16 t   FFFF   16  true  std  numeric limits lt uint16 t gt   max         TEST uint16 t   10000   16        std  cout  lt  lt                int32 t range tests                lt  lt  std  endl      TEST int32 t   7FFFFFFF   16  true  std  numeric limits lt int32 t gt   max         TEST int32 t   80000000   16       TEST int32 t   -80000000   16  true  std  numeric limits lt int32 t gt   min         TEST int32 t   -80000001   16       TEST int32 t   FFFFFFFF   16       TEST int32 t   100000000   16        std  cout  lt  lt                uint32 t range tests                lt  lt  std  endl      TEST uint32 t   7FFFFFFF   16  true  std  numeric limits lt int32 t gt   max         TEST uint32 t   80000000   16  true  std  numeric limits lt int32 t gt   max   1       TEST uint32 t   -80000000   16       TEST uint32 t   -80000001   16       TEST uint32 t   FFFFFFFF   16  true  std  numeric limits lt uint32 t gt   max         TEST uint32 t   100000000   16        std  cout  lt  lt                int64 t range tests                lt  lt  std  endl      TEST int64 t   7FFFFFFFFFFFFFFF   16  true  std  numeric limits lt int64 t gt   max         TEST int64 t   8000000000000000   16       TEST int64 t   -8000000000000000   16  true  std  numeric limits lt int64 t gt   min         TEST int64 t   -8000000000000001   16       TEST int64 t   FFFFFFFFFFFFFFFF   16       TEST int64 t   10000000000000000   16        std  cout  lt  lt                uint64 t range tests                lt  lt  std  endl      TEST uint64 t   7FFFFFFFFFFFFFFF   16  true  std  numeric limits lt int64 t gt   max         TEST uint64 t   8000000000000000   16  true  std  numeric limits lt int64 t gt   max   1       TEST uint64 t   -8000000000000000   16       TEST uint64 t   -8000000000000001   16       TEST uint64 t   FFFFFFFFFFFFFFFF   16  true  std  numeric limits lt uint64 t gt   max         TEST uint64 t   10000000000000000   16        std  cout  lt  lt  std  endl  lt  lt  std  endl                lt  lt    g anyFailed       SOME TESTS FAILED        ALL TESTS PASSED                  lt  lt  std  endl       return  g anyFailed      StringToDecimal is the user-land method  it is overloaded so it can be called either like this   int a  a   StringToDecimal lt int gt   100      or this   int a  StringToDecimal a   100      I hate repeating the int type  so prefer the latter  This ensures that if the type of  a  changes one does not get bad results  I wish the compiler could figure it out like   int a  a   StringToDecimal  100         but  C   does not deduce template return types  so that s the best I can get   The implementation is pretty simple   CstrtoxllWrapper wraps both strtoull and strtoll  calling whichever is necessary based on the template type s signed-ness and providing some additional guarantees  e g  negative input is disallowed if unsigned and it ensures the entire string was converted    CstrtoxllWrapper is used by StringToSigned and StringToUnsigned with the largest type  long long unsigned long long  available to the compiler  this allows the maximal conversion to be performed   Then  if it is necessary  StringToSigned StringToUnsigned performs the final range checks on the underlying type  Finally  the end-point method  StringToDecimal  decides which of the StringTo  template methods to call based on the underlying type s signed-ness   I think most of the junk can be optimized out by the compiler  just about everything should be compile-time deterministic  Any commentary on this aspect would be interesting to me

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You can use Boost s lexical cast  which wraps this in a more generic interface  lexical cast lt Target gt  Source  throws bad lexical cast on failure

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You can use stringstream s  int str2int  const string  amp str      stringstream ss str     int num    ss  gt  gt  num    return num

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I think these three links sum it up    http   tinodidriksen com 2010 02 07 cpp-convert-int-to-string-speed  http   tinodidriksen com 2010 02 16 cpp-convert-string-to-int-speed  http   www fastformat org performance html   stringstream and lexical cast solutions are about the same as lexical cast is using stringstream   Some specializations of lexical cast use different approach see http   www boost org doc libs release boost lexical cast hpp for details   Integers and floats are now specialized for integer to string conversion     One can specialize lexical cast for his her own needs and make it fast  This would be the ultimate solution satisfying all parties  clean and simple   Articles already mentioned show comparison between different methods of converting  integers  lt -  strings  Following approaches make sense   old c-way  spirit karma  fastformat  simple naive loop     Lexical cast is ok in some cases e g  for int to string conversion   Converting string to int using lexical cast is not a good idea as it is 10-40 times slower than atoi depending on the platform compiler used   Boost Spirit Karma seems to be the fastest library for converting integer to string   ex   generate ptr char  int   integer number     and basic simple loop from the article mentioned above is a fastest way to convert string to int  obviously not the safest one  strtol   seems like a safer solution  int naive char 2 int const char  p        int x   0      bool neg   false      if   p     -             neg   true            p            while   p  gt    0   amp  amp   p  lt    9             x    x 10      p -  0              p            if  neg            x   -x            return x

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I know three ways of converting String into int   Either use stoi String to int  function or just go with Stringstream  the third way to go individual conversion  Code is below   1st Method  std  string s1    4533   std  string s2    3 010101   std  string s3    31337 with some string    int myint1   std  stoi s1   int myint2   std  stoi s2   int myint3   std  stoi s3    std  cout  lt  lt   s1  lt  lt      lt  lt  myint1  lt  lt    n   std  cout  lt  lt   s2  lt  lt      lt  lt  myint2  lt  lt    n   std  cout  lt  lt   s3  lt  lt      lt  lt  myint3  lt  lt    n     2nd Method   include  lt string h gt   include  lt sstream gt   include  lt iostream gt   include  lt cstring gt  using namespace std    int StringToInteger string NumberAsString        int NumberAsInteger      stringstream ss      ss  lt  lt  NumberAsString      ss  gt  gt  NumberAsInteger      return NumberAsInteger    int main         string NumberAsString      cin  gt  gt  NumberAsString      cout  lt  lt  StringToInteger NumberAsString   lt  lt  endl      return 0       3rd Method - but not for an individual conversion   std  string str4    453   int i   0  in 0     453 as on for   i   0  i  lt  str4 length    i           in   str4 i       cout  lt  lt in-48

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The good  old C way still works  I recommend strtol or strtoul  Between the return status and the  endPtr   you can give good diagnostic output  It also handles multiple bases nicely

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This is a safer C way than atoi    const char  str    123   int i   if sscanf str    d    amp i      EOF           error        C   with standard library stringstream   thanks CMS    int str2int  const string  amp str      stringstream ss str     int num    if  ss  gt  gt  num  fail                 ERROR        return num      With boost library   thanks jk    include  lt boost lexical cast hpp gt   include  lt string gt   try       std  string str    123       int number   boost  lexical cast lt  int  gt   str      catch  const boost  bad lexical cast  amp             Error     Edit  Fixed the stringstream version so that it handles errors   thanks to CMS s and jk s comment on original post

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What not to do  Here is my first piece of advice  do not use stringstream for this  While at first it may seem simple to use  you ll find that you have to do a lot of extra work if you want robustness and good error handling   Here is an approach that intuitively seems like it should work   bool str2int  int  amp i  char const  s        std  stringstream ss s       ss  gt  gt  i      if  ss fail                 not an integer         return false            return true      This has a major problem  str2int i   1337h4x0r   will happily return true and i will get the value 1337  We can work around this problem by ensuring there are no more characters in the stringstream after the conversion   bool str2int  int  amp i  char const  s        char              c      std  stringstream ss s       ss  gt  gt  i      if  ss fail      ss get c                not an integer         return false            return true      We fixed one problem  but there are still a couple of other problems   What if the number in the string is not base 10  We can try to accommodate other bases by setting the stream to the correct mode  e g  ss  lt  lt  std  hex  before trying the conversion  But this means the caller must know a priori what base the number is -- and how can the caller possibly know that  The caller doesn t know what the number is yet  They don t even know that it is a number  How can they be expected to know what base it is  We could just mandate that all numbers input to our programs must be base 10 and reject hexadecimal or octal input as invalid  But that is not very flexible or robust  There is no simple solution to this problem  You can t simply try the conversion once for each base  because the decimal conversion will always succeed for octal numbers  with a leading zero  and the octal conversion may succeed for some decimal numbers  So now you have to check for a leading zero  But wait  Hexadecimal numbers can start with a leading zero too  0x      Sigh   Even if you succeed in dealing with the above problems  there is still another bigger problem  what if the caller needs to distinguish between bad input  e g   123foo   and a number that is out of the range of int  e g   4000000000  for 32-bit int   With stringstream  there is no way to make this distinction  We only know whether the conversion succeeded or failed  If it fails  we have no way of knowing why it failed  As you can see  stringstream leaves much to be desired if you want robustness and clear error handling   This leads me to my second piece of advice  do no use Boost s lexical cast for this  Consider what the lexical cast documentation has to say      Where a higher degree of control is   required over conversions    std  stringstream and   std  wstringstream offer a more   appropriate path  Where   non-stream-based conversions are   required  lexical cast is the wrong   tool for the job and is not   special-cased for such scenarios    What   We ve already seen that stringstream has a poor level of control  and yet it says stringstream should be used instead of lexical cast if you need  a higher level of control   Also  because lexical cast is just a wrapper around stringstream  it suffers from the same problems that stringstream does  poor support for multiple number bases and poor error handling   The best solution  Fortunately  somebody has already solved all of the above problems  The C standard library contains strtol and family which have none of these problems   enum STR2INT ERROR   SUCCESS  OVERFLOW  UNDERFLOW  INCONVERTIBLE     STR2INT ERROR str2int  int  amp i  char const  s  int base   0        char  end      long  l      errno   0      l   strtol s   amp end  base       if   errno    ERANGE  amp  amp  l    LONG MAX     l  gt  INT MAX            return OVERFLOW            if   errno    ERANGE  amp  amp  l    LONG MIN     l  lt  INT MIN            return UNDERFLOW            if   s      0      end      0             return INCONVERTIBLE            i   l      return SUCCESS      Pretty simple for something that handles all the error cases and also supports any number base from 2 to 36  If base is zero  the default  it will try to convert from any base  Or the caller can supply the third argument and specify that the conversion should only be attempted for a particular base  It is robust and handles all errors with a minimal amount of effort   Other reasons to prefer strtol  and family     It exhibits much better runtime performance It introduces less compile-time overhead  the others pull in nearly 20 times more SLOC from headers  It results in the smallest code size   There is absolutely no good reason to use any other method

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You can use the a stringstream from the C   standard libraray   stringstream ss str   int x  ss  gt  gt  x   if ss        lt -- error handling      use x   else        not a number        The stream state will be set to fail   if a non-digit is encountered when   trying to read an integer    See Stream pitfalls for pitfalls of errorhandling and streams in C

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I like Dan s answer  esp because of the avoidance of exceptions  For embedded systems development and other low level system development  there may not be a proper Exception framework available    Added a check for white-space after a valid string   these three lines      while  isspace  end             end             Added a check for parsing errors too       if   errno    0      s    end             return INCONVERTIBLE           Here is the complete function     include  lt cstdlib gt   include  lt cerrno gt   include  lt climits gt   include  lt stdexcept gt   enum STR2INT ERROR   SUCCESS  OVERFLOW  UNDERFLOW  INCONVERTIBLE     STR2INT ERROR str2long  long  amp l  char const  s  int base   0        char  end    char   s      errno   0       l   strtol s   amp end  base        if   errno    ERANGE   amp  amp   l    LONG MAX             return OVERFLOW            if   errno    ERANGE   amp  amp   l    LONG MIN             return UNDERFLOW            if   errno    0      s    end             return INCONVERTIBLE                while  isspace  unsigned char  end             end               if   s      0      end      0             return INCONVERTIBLE             return SUCCESS

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From C  17 onwards you can use std  from chars from the  lt charconv gt  header as documented here   For example    include  lt iostream gt   include  lt charconv gt   include  lt array gt   int main         char const   str    42       int value   0       std  from chars result result   std  from chars std  begin str   std  end str   value        if result error    std  errc  invalid argument              std  cout  lt  lt   Error  invalid format             else if result error    std  errc  result out of range              std  cout  lt  lt   Error  value too big for int range             else             std  cout  lt  lt   Success     lt  lt  result            As a bonus  it can also handle other bases  like hexadecimal

[c++] How to parse a string to an int in C++?

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