C convert hex string to signed integer

Question

I want to convert a hex string to a 32 bit signed integer in C       So  for example  I have the hex string  fffefffe    The binary representation of this is 11111111111111101111111111111110   The signed integer representation of this is  -65538     How do I do this conversion in C     This also needs to work for non-negative numbers   For example  the hex string  0000000A   which is 00000000000000000000000000001010 in binary  and 10 in decimal

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This worked for me     string string test    80123456   unsigned long x  signed long val   std  stringstream ss  ss  lt  lt  std  hex  lt  lt  string test  ss  gt  gt  x     ss  gt  gt  val      if I try this val   0 val    signed long x      However  if I cast the unsigned result I get val   0x80123456

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Andy Buchanan  as far as sticking to C   goes  I liked yours  but I have a few mods   template  lt typename ElemT gt  struct HexTo       ElemT value      operator ElemT   const  return value       friend std  istream amp  operator gt  gt  std  istream amp  in  HexTo amp  out            in  gt  gt  std  hex  gt  gt  out value          return in             Used like  uint32 t value   boost  lexical cast lt HexTo lt uint32 t gt   gt   0x2a      That way you don t need one impl per int type

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use std  stringstream  unsigned int x     std  stringstream ss  ss  lt  lt  std  hex  lt  lt   fffefffe   ss  gt  gt  x    the following example produces -65538 as its result    include  lt sstream gt   include  lt iostream gt   int main         unsigned int x         std  stringstream ss      ss  lt  lt  std  hex  lt  lt   fffefffe       ss  gt  gt  x         output it as a signed type     std  cout  lt  lt  static cast lt int gt  x   lt  lt  std  endl      In the new C  11 standard  there are a few new utility functions which you can make use of  specifically  there is a family of  string to number  functions  http   en cppreference com w cpp string basic string stol and http   en cppreference com w cpp string basic string stoul   These are essentially thin wrappers around C s string to number conversion functions  but know how to deal with a std  string  So  the simplest answer for newer code would probably look like this   std  string s    0xfffefffe   unsigned int x   std  stoul s  nullptr  16       NOTE  Below is my original answer  which as the edit says is not a complete answer  For a functional solution  stick the code above the line  -    It appears that since lexical cast lt  gt  is defined to have stream conversion semantics  Sadly  streams don t understand the  0x  notation  So both the boost  lexical cast and my hand rolled one don t deal well with hex strings  The above solution which manually sets the input stream to hex will handle it just fine   Boost has some stuff to do this as well  which has some nice error checking capabilities as well  You can use it like this   try       unsigned int x   lexical cast lt int gt   0x0badc0de      catch bad lexical cast  amp            whatever you want to do        If you don t feel like using boost  here s a light version of lexical cast which does no error checking   template lt typename T2  typename T1 gt  inline T2 lexical cast const T1  amp in        T2 out      std  stringstream ss      ss  lt  lt  in      ss  gt  gt  out      return out      which you can use like this      though this needs the 0x prefix so it knows it is hex unsigned int x   lexical cast lt unsigned int gt   0xdeadbeef

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Here s a simple and working method I found elsewhere   string hexString    7FF   int hexNumber  sscanf hexString c str      x    amp hexNumber     Please note that you might prefer using unsigned long integer long integer  to receive the value  Another note  the c str   function just converts the std  string to const char     So if you have a const char  ready  just go ahead with using that variable name directly  as shown below  I am also showing the usage of the unsigned long variable for a larger hex number  Do not confuse it with the case of having const char  instead of string    const char  hexString    7FFEA5     Just to show the conversion of a bigger hex number unsigned long hexNumber    In case your hex number is going to be sufficiently big  sscanf hexString    x    amp hexNumber     This works just perfectly fine  provided you use appropriate data types per your need

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For a method that works with both C and C    you might want to consider using the standard library function strtol      include  lt cstdlib gt   include  lt iostream gt  using namespace std   int main         string s    abcd       char   p      long n   strtol  s c str     amp  p  16        if     p    0       my bad edit was here         cout  lt  lt   not a number   lt  lt  endl            else           cout  lt  lt  n  lt  lt  endl

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just use stoi stol stoll for example  std  cout  lt  lt  std  stol  quot fffefffe quot   nullptr  16   lt  lt  std  endl   output  4294901758

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I had the same problem today  here s how I solved it so I could keep lexical cast lt     typedef unsigned int    uint32  typedef signed int      int32   class uint32 from hex      For use with boost  lexical cast       uint32 value  public      operator uint32   const   return value        friend std  istream amp  operator gt  gt   std  istream amp  in  uint32 from hex amp  outValue                 in  gt  gt  std  hex  gt  gt  outValue value            class int32 from hex      For use with boost  lexical cast       uint32 value  public      operator int32   const   return static cast lt int32 gt   value          friend std  istream amp  operator gt  gt   std  istream amp  in  int32 from hex amp  outValue                 in  gt  gt  std  hex  gt  gt  outvalue value            uint32 material0   lexical cast lt uint32 from hex gt    0x4ad     uint32 material1   lexical cast lt uint32 from hex gt    4ad     uint32 material2   lexical cast lt uint32 gt    1197      int32 materialX   lexical cast lt int32 from hex gt    0xfffefffe     int32 materialY   lexical cast lt int32 from hex gt    fffefffe        etc       Found this page when I was looking for a less sucky way  -   Cheers  A

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Try this  This solution is a bit risky  There are no checks  The string must only have hex values and the string length must match the return type size  But no need for extra headers   char hextob char ch        if  ch  gt    0   amp  amp  ch  lt    9   return ch -  0       if  ch  gt    A   amp  amp  ch  lt    F   return ch -  A    10      if  ch  gt    a   amp  amp  ch  lt    f   return ch -  a    10      return 0    template lt typename T gt  T hextot char  hex        T value   0      for  size t i   0  i  lt  sizeof T  2    i          value    hextob hex i    lt  lt   8 sizeof T -4  i 1        return value       Usage   int main         char str 4      f   f   f   f        std  cout  lt  lt  hextot lt int16 t gt  str    lt  lt    n       Note  the length of the string must be divisible by 2

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Working example with strtoul will be    include  lt cstdlib   include  lt iostream  using namespace std   int main          string s    fffefffe       char   p      long n   strtoul  s c str      p  16         if     p    0               cout  lt  lt   not a number   lt  lt  endl           else             cout  lt  lt  n  lt  lt  endl            strtol converts string to long  On my computer numeric limits lt long gt   max   gives 0x7fffffff  Obviously that 0xfffefffe is greater than 0x7fffffff  So strtol returns MAX LONG instead of wanted value  strtoul converts string to unsigned long that s why no overflow in this case   Ok  strtol is considering input string not as 32-bit signed integer before convertation  Funny sample with strtol    include  lt cstdlib   include  lt iostream  using namespace std   int main          string s    -0x10002       char   p      long n   strtol  s c str      p  16         if     p    0               cout  lt  lt   not a number   lt  lt  endl           else             cout  lt  lt  n  lt  lt  endl            The code above prints -65538 in console

[c++] C++ convert hex string to signed integer

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