How to convert a std string to const char or char

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How can I convert an std  string to a char  or a const char

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Just see this   string str1  stackoverflow    const char   str2   str1 c str      However  note that this will return a const char     For a char    use strcpy to copy it into another char array

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If you just want to pass a std  string to a function that needs const char  you can use   std  string str  const char   c   str c str      If you want to get a writable copy  like char    you can do that with this   std  string str  char   writable   new char str size     1   std  copy str begin    str end    writable   writable str size        0      don t forget the terminating 0     don t forget to free the string after finished using it delete   writable    Edit  Notice that the above is not exception safe  If anything between the new call and the delete call throws  you will leak memory  as nothing will call delete for you automatically  There are two immediate ways to solve this   boost  scoped array  boost  scoped array will delete the memory for you upon going out of scope   std  string str  boost  scoped array lt char gt  writable new char str size     1    std  copy str begin    str end    writable get     writable str size        0      don t forget the terminating 0     get the char  using writable get       memory is automatically freed if the smart pointer goes     out of scope   std  vector  This is the standard way  does not require any external library   You use std  vector  which completely manages the memory for you   std  string str  std  vector lt char gt  writable str begin    str end     writable push back   0        get the char  using  amp writable 0  or  amp  writable begin

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C  17  C  17  upcoming standard  changes the synopsis of the template basic string adding a non const overload of data        charT  data   noexcept       Returns  A pointer p such that p   i     amp operator for each i in  0 size         CharT const   from std  basic string lt CharT gt   std  string const cstr              char const   p   cstr data       or  c str     CharT   from std  basic string lt CharT gt   std  string str              char   p   str data      C  11  CharT const   from std  basic string lt CharT gt   std  string str              str c str      CharT   from std  basic string lt CharT gt   From C  11 onwards  the standard says         The char-like objects in a basic string object shall be stored contiguously  That is  for any basic string object s  the identity  amp   s begin     n      amp  s begin     n shall hold for all values of n such that 0  lt   n  lt  s size                     const reference operator   size type pos  const   reference operator   size type pos         Returns    begin     pos  if pos  lt  size    otherwise a reference to an object of type CharT with value CharT    the referenced value shall not be modified                   const charT  c str   const noexcept const charT  data   const noexcept       Returns  A pointer p such that p   i     amp operator   i  for each i in  0 size          There are severable possible ways to get a non const character pointer   1  Use the contiguous storage of C  11  std  string foo  text    auto p    amp  foo begin      Pro   Simple and short Fast  only method with no copy involved    Cons   Final   0  is not to be altered   not necessarily part of the non-const memory    2  Use std  vector lt CharT gt   std  string foo  text    std  vector lt char gt  fcv foo data    foo data   foo size   1u   auto p   fcv data      Pro   Simple Automatic memory handling Dynamic   Cons   Requires string copy   3  Use std  array lt CharT  N gt  if N is compile time constant  and small enough   std  string foo  text    std  array lt char  5u gt  fca  std  copy foo data    foo data   foo size   1u  fca begin       Pro   Simple Stack memory handling   Cons   Static Requires string copy   4  Raw memory allocation with automatic storage deletion  std  string foo   text     auto p   std  make unique lt char   gt  foo size   1u   std  copy foo data    foo data     foo size     1u   amp p 0      Pro   Small memory footprint Automatic deletion Simple   Cons   Requires string copy Static  dynamic usage requires lots more code  Less features than vector or array   5  Raw memory allocation with manual handling  std  string foo   text     char   p   nullptr  try     p   new char foo size     1u     std  copy foo data    foo data     foo size     1u  p        handle stuff with p   delete   p    catch           if  p    delete   p      throw      Pro   Maximum  control    Con   Requires string copy Maximum liability   susceptibility for errors Complex

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char  result   strcpy  char  malloc str length   1   str c str

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Try this   std  string s reinterpret cast lt const char   gt  Data   Size

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If you just want to pass a std  string to a function that needs const char  you can use   std  string str  const char   c   str c str      If you want to get a writable copy  like char    you can do that with this   std  string str  char   writable   new char str size     1   std  copy str begin    str end    writable   writable str size        0      don t forget the terminating 0     don t forget to free the string after finished using it delete   writable    Edit  Notice that the above is not exception safe  If anything between the new call and the delete call throws  you will leak memory  as nothing will call delete for you automatically  There are two immediate ways to solve this   boost  scoped array  boost  scoped array will delete the memory for you upon going out of scope   std  string str  boost  scoped array lt char gt  writable new char str size     1    std  copy str begin    str end    writable get     writable str size        0      don t forget the terminating 0     get the char  using writable get       memory is automatically freed if the smart pointer goes     out of scope   std  vector  This is the standard way  does not require any external library   You use std  vector  which completely manages the memory for you   std  string str  std  vector lt char gt  writable str begin    str end     writable push back   0        get the char  using  amp writable 0  or  amp  writable begin

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If you just want to pass a std  string to a function that needs const char  you can use   std  string str  const char   c   str c str      If you want to get a writable copy  like char    you can do that with this   std  string str  char   writable   new char str size     1   std  copy str begin    str end    writable   writable str size        0      don t forget the terminating 0     don t forget to free the string after finished using it delete   writable    Edit  Notice that the above is not exception safe  If anything between the new call and the delete call throws  you will leak memory  as nothing will call delete for you automatically  There are two immediate ways to solve this   boost  scoped array  boost  scoped array will delete the memory for you upon going out of scope   std  string str  boost  scoped array lt char gt  writable new char str size     1    std  copy str begin    str end    writable get     writable str size        0      don t forget the terminating 0     get the char  using writable get       memory is automatically freed if the smart pointer goes     out of scope   std  vector  This is the standard way  does not require any external library   You use std  vector  which completely manages the memory for you   std  string str  std  vector lt char gt  writable str begin    str end     writable push back   0        get the char  using  amp writable 0  or  amp  writable begin

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If you just want to pass a std  string to a function that needs const char  you can use   std  string str  const char   c   str c str      If you want to get a writable copy  like char    you can do that with this   std  string str  char   writable   new char str size     1   std  copy str begin    str end    writable   writable str size        0      don t forget the terminating 0     don t forget to free the string after finished using it delete   writable    Edit  Notice that the above is not exception safe  If anything between the new call and the delete call throws  you will leak memory  as nothing will call delete for you automatically  There are two immediate ways to solve this   boost  scoped array  boost  scoped array will delete the memory for you upon going out of scope   std  string str  boost  scoped array lt char gt  writable new char str size     1    std  copy str begin    str end    writable get     writable str size        0      don t forget the terminating 0     get the char  using writable get       memory is automatically freed if the smart pointer goes     out of scope   std  vector  This is the standard way  does not require any external library   You use std  vector  which completely manages the memory for you   std  string str  std  vector lt char gt  writable str begin    str end     writable push back   0        get the char  using  amp writable 0  or  amp  writable begin

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C  17  C  17  upcoming standard  changes the synopsis of the template basic string adding a non const overload of data        charT  data   noexcept       Returns  A pointer p such that p   i     amp operator for each i in  0 size         CharT const   from std  basic string lt CharT gt   std  string const cstr              char const   p   cstr data       or  c str     CharT   from std  basic string lt CharT gt   std  string str              char   p   str data      C  11  CharT const   from std  basic string lt CharT gt   std  string str              str c str      CharT   from std  basic string lt CharT gt   From C  11 onwards  the standard says         The char-like objects in a basic string object shall be stored contiguously  That is  for any basic string object s  the identity  amp   s begin     n      amp  s begin     n shall hold for all values of n such that 0  lt   n  lt  s size                     const reference operator   size type pos  const   reference operator   size type pos         Returns    begin     pos  if pos  lt  size    otherwise a reference to an object of type CharT with value CharT    the referenced value shall not be modified                   const charT  c str   const noexcept const charT  data   const noexcept       Returns  A pointer p such that p   i     amp operator   i  for each i in  0 size          There are severable possible ways to get a non const character pointer   1  Use the contiguous storage of C  11  std  string foo  text    auto p    amp  foo begin      Pro   Simple and short Fast  only method with no copy involved    Cons   Final   0  is not to be altered   not necessarily part of the non-const memory    2  Use std  vector lt CharT gt   std  string foo  text    std  vector lt char gt  fcv foo data    foo data   foo size   1u   auto p   fcv data      Pro   Simple Automatic memory handling Dynamic   Cons   Requires string copy   3  Use std  array lt CharT  N gt  if N is compile time constant  and small enough   std  string foo  text    std  array lt char  5u gt  fca  std  copy foo data    foo data   foo size   1u  fca begin       Pro   Simple Stack memory handling   Cons   Static Requires string copy   4  Raw memory allocation with automatic storage deletion  std  string foo   text     auto p   std  make unique lt char   gt  foo size   1u   std  copy foo data    foo data     foo size     1u   amp p 0      Pro   Small memory footprint Automatic deletion Simple   Cons   Requires string copy Static  dynamic usage requires lots more code  Less features than vector or array   5  Raw memory allocation with manual handling  std  string foo   text     char   p   nullptr  try     p   new char foo size     1u     std  copy foo data    foo data     foo size     1u  p        handle stuff with p   delete   p    catch           if  p    delete   p      throw      Pro   Maximum  control    Con   Requires string copy Maximum liability   susceptibility for errors Complex

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Use the  c str   method for const char     You can use  amp mystring 0  to get a char   pointer  but there are a couple of gotcha s  you won t necessarily get a zero terminated string  and you won t be able to change the string s size  You especially have to be careful not to add characters past the end of the string or you ll get a buffer overrun  and probable crash    There was no guarantee that all of the characters would be part of the same contiguous buffer until C  11  but in practice all known implementations of std  string worked that way anyway  see Does     amp s 0     point to contiguous characters in a std  string    Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved  Best to use them immediately and then discard

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Use the  c str   method for const char     You can use  amp mystring 0  to get a char   pointer  but there are a couple of gotcha s  you won t necessarily get a zero terminated string  and you won t be able to change the string s size  You especially have to be careful not to add characters past the end of the string or you ll get a buffer overrun  and probable crash    There was no guarantee that all of the characters would be part of the same contiguous buffer until C  11  but in practice all known implementations of std  string worked that way anyway  see Does     amp s 0     point to contiguous characters in a std  string    Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved  Best to use them immediately and then discard

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char  result   strcpy  char  malloc str length   1   str c str

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I am working with an API with a lot of functions get as an input a char    I have created a small class to face this kind of problem  I have implemented the RAII idiom   class DeepString           DeepString const DeepString amp  other           DeepString amp  operator  const DeepString amp  other           char  internal         public          explicit DeepString  const string amp  toCopy                internal  new char toCopy size   1                          strcpy internal  toCopy c str                        DeepString     delete   internal             char  str   const   return internal             const char  c str    const   return internal          And you can use it as   void aFunctionAPI char  input        other stuff  aFunctionAPI  Foo      this call is not safe  if the function modified the                         literal string the program will crash std  string myFoo  Foo    aFunctionAPI myFoo c str       this is not compiling aFunctionAPI const cast lt char  gt  myFoo c str        this is not safe std  string                                                    implement reference counting and                                                    it may change the value of other                                                   strings as well  DeepString myDeepFoo myFoo   aFunctionAPI myFoo str       this is fine   I have called the class DeepString because it is creating a deep and unique copy  the DeepString is not copyable  of an existing string

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Use the  c str   method for const char     You can use  amp mystring 0  to get a char   pointer  but there are a couple of gotcha s  you won t necessarily get a zero terminated string  and you won t be able to change the string s size  You especially have to be careful not to add characters past the end of the string or you ll get a buffer overrun  and probable crash    There was no guarantee that all of the characters would be part of the same contiguous buffer until C  11  but in practice all known implementations of std  string worked that way anyway  see Does     amp s 0     point to contiguous characters in a std  string    Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved  Best to use them immediately and then discard

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Just see this   string str1  stackoverflow    const char   str2   str1 c str      However  note that this will return a const char     For a char    use strcpy to copy it into another char array

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Given say     std  string x    hello      Getting a  char    or  const char   from a  string   How to get a character pointer that s valid while x remains in scope and isn t modified further  C  11 simplifies things  the following all give access to the same internal string buffer   const char  p c str   x c str    const char  p data    x data    char  p writable data   x data       for non-const x from C  17  const char  p x0       amp x 0          char  p x0 rw    amp x 0       compiles iff x is not const      All the above pointers will hold the same value - the address of the first character in the buffer   Even an empty string has a  first character in the buffer   because C  11 guarantees to always keep an extra NUL 0 terminator character after the explicitly assigned string content  e g  std  string  this 0that   9  will have a buffer holding  this 0that 0     Given any of the above pointers   char c   p n        valid for n  lt   x size                       i e  you can safely read the NUL at p x size      Only for the non-const pointer p writable data and from  amp x 0    p writable data n    c  p x0 rw n    c      valid for n  lt   x size   - 1                     i e  don t overwrite the implementation maintained NUL   Writing a NUL elsewhere in the string does not change the string s size    string s are allowed to contain any number of NULs - they are given no special treatment by std  string  same in C  03    In C  03  things were considerably more complicated  key differences highlighted     x data     returns const char  to the string s internal buffer which wasn t required by the Standard to conclude with a NUL  i e  might be   h    e    l    l    o   followed by uninitialised or garbage values  with accidental accesses thereto having undefined behaviour     x size   characters are safe to read  i e  x 0  through x x size   - 1  for empty strings  you re guaranteed some non-NULL pointer to which 0 can be safely added  hurray    but you shouldn t dereference that pointer     amp x 0    for empty strings this has undefined behaviour  21 3 4    e g  given f const char  p  size t n    if  n    0  return     whatever      you mustn t call f  amp x 0   x size     when x empty   - just use f x data           otherwise  as per x data   but    for non-const x this yields a non-const char  pointer  you can overwrite string content   x c str     returns const char  to an ASCIIZ  NUL-terminated  representation of the value  i e    h    e    l    l    o     0     although few if any implementations chose to do so  the C  03 Standard was worded to allow the string implementation the freedom to create a distinct NUL-terminated buffer on the fly  from the potentially non-NUL terminated buffer  exposed  by x data   and  amp x 0  x size     1 characters are safe to read  guaranteed safe even for empty strings     0        Consequences of accessing outside legal indices  Whichever way you get a pointer  you must not access memory further along from the pointer than the characters guaranteed present in the descriptions above   Attempts to do so have undefined behaviour  with a very real chance of application crashes and garbage results even for reads  and additionally wholesale data  stack corruption and or security vulnerabilities for writes   When do those pointers get invalidated   If you call some string member function that modifies the string or reserves further capacity  any pointer values returned beforehand by any of the above methods are invalidated   You can use those methods again to get another pointer    The rules are the same as for iterators into strings    See also How to get a character pointer valid even after x leaves scope or is modified further below      So  which is better to use   From C  11  use  c str   for ASCIIZ data  and  data   for  binary  data  explained further below    In C  03  use  c str   unless certain that  data   is adequate  and prefer  data   over  amp x 0  as it s safe for empty strings         try to understand the program enough to use data   when appropriate  or you ll probably make other mistakes     The ASCII NUL   0  character guaranteed by  c str   is used by many functions as a sentinel value denoting the end of relevant and safe-to-access data   This applies to both C  -only functions like say fstream  fstream const char  filename       and shared-with-C functions like strchr    and printf     Given C  03 s  c str   s guarantees about the returned buffer are a super-set of  data   s  you can always safely use  c str    but people sometimes don t because    using  data   communicates to other programmers reading the source code that the data is not ASCIIZ  rather  you re using the string to store a block of data  which sometimes isn t even really textual    or that you re passing it to another function that treats it as a block of  binary  data   This can be a crucial insight in ensuring that other programmers  code changes continue to handle the data properly  C  03 only  there s a slight chance that your string implementation will need to do some extra memory allocation and or data copying in order to prepare the NUL terminated buffer   As a further hint  if a function s parameters require the  const  char  but don t insist on getting x size    the function probably needs an ASCIIZ input  so  c str   is a good choice  the function needs to know where the text terminates somehow  so if it s not a separate parameter it can only be a convention like a length-prefix or sentinel or some fixed expected length    How to get a character pointer valid even after x leaves scope or is modified further  You ll need to copy the contents of the string x to a new memory area outside x   This external buffer could be in many places such as another string or character array variable  it may or may not have a different lifetime than x due to being in a different scope  e g  namespace  global  static  heap  shared memory  memory mapped file    To copy the text from std  string x into an independent character array      USING ANOTHER STRING - AUTO MEMORY MANAGEMENT  EXCEPTION SAFE std  string old x   x     - old x will not be affected by subsequent modifications to x       - you can use   amp old x 0   to get a writable char  to old x s textual content    - you can use resize   to reduce expand the string      - resizing isn t possible from within a function passed only the char  address  std  string old x   x c str       old x will terminate early if x embeds NUL    Copies ASCIIZ data but could be less efficient as it needs to scan memory to    find the NUL terminator indicating string length before allocating that amount    of memory to copy into  or more efficient if it ends up allocating copying a    lot less content     Example  x     ab 0cd  - gt  old x     ab       USING A VECTOR OF CHAR - AUTO  EXCEPTION SAFE  HINTS AT BINARY CONTENT  GUARANTEED CONTIGUOUS EVEN IN C  03 std  vector lt char gt  old x x data    x data     x size              without the NUL std  vector lt char gt  old x x c str    x c str     x size     1       with the NUL     USING STACK WHERE MAXIMUM SIZE OF x IS KNOWN TO BE COMPILE-TIME CONSTANT  N      a bit dangerous  as  known  things are sometimes wrong and often become wrong  char y N   1   strcpy y  x c str         USING STACK WHERE UNEXPECTEDLY LONG x IS TRUNCATED  e g  Hello 0- gt Hel 0  char y N   1   strncpy y  x c str    N       copy at most N  zero-padding if shorter y N      0                    ensure NUL terminated     USING THE STACK TO HANDLE x OF UNKNOWN  BUT SANE  LENGTH char  y   alloca x size     1   strcpy y  x c str         USING THE STACK TO HANDLE x OF UNKNOWN LENGTH  NON-STANDARD GCC EXTENSION  char y x size     1   strcpy y  x c str         USING new delete HEAP MEMORY  MANUAL DEALLOC  NO INHERENT EXCEPTION SAFETY char  y   new char x size     1   strcpy y  x c str            or as a one-liner  char  y   strcpy new char x size     1   x c str        use y    delete   y     make sure no break  return  throw or branching bypasses this     USING new delete HEAP MEMORY  SMART POINTER DEALLOCATION  EXCEPTION SAFE    see boost shared array usage in Johannes Schaub s answer     USING malloc free HEAP MEMORY  MANUAL DEALLOC  NO INHERENT EXCEPTION SAFETY char  y   strdup x c str        use y    free y     Other reasons to want a char  or const char  generated from a string  So  above you ve seen how to get a  const  char   and how to make a copy of the text independent of the original string  but what can you do with it   A random smattering of examples      give  C  code access to the C   string s text  as in printf  x is   s    x c str     copy x s text to a buffer specified by your function s caller  e g  strncpy callers buffer  callers buffer size  x c str      or volatile memory used for device I O  e g  for  const char  p   x c str     p    p   p device    p   append x s text to an character array already containing some ASCIIZ text  e g  strcat other buffer  x c str     - be careful not to overrun the buffer  in many situations you may need to use strncat  return a const char  or char  from a function  perhaps for historical reasons - client s using your existing API - or for C compatibility you don t want to return a std  string  but do want to copy your string s data somewhere for the caller    be careful not to return a pointer that may be dereferenced by the caller after a local string variable to which that pointer pointed has left scope some projects with shared objects compiled linked for different std  string implementations  e g  STLport and compiler-native  may pass data as ASCIIZ to avoid conflicts

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Try this   std  string s reinterpret cast lt const char   gt  Data   Size

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Given say     std  string x    hello      Getting a  char    or  const char   from a  string   How to get a character pointer that s valid while x remains in scope and isn t modified further  C  11 simplifies things  the following all give access to the same internal string buffer   const char  p c str   x c str    const char  p data    x data    char  p writable data   x data       for non-const x from C  17  const char  p x0       amp x 0          char  p x0 rw    amp x 0       compiles iff x is not const      All the above pointers will hold the same value - the address of the first character in the buffer   Even an empty string has a  first character in the buffer   because C  11 guarantees to always keep an extra NUL 0 terminator character after the explicitly assigned string content  e g  std  string  this 0that   9  will have a buffer holding  this 0that 0     Given any of the above pointers   char c   p n        valid for n  lt   x size                       i e  you can safely read the NUL at p x size      Only for the non-const pointer p writable data and from  amp x 0    p writable data n    c  p x0 rw n    c      valid for n  lt   x size   - 1                     i e  don t overwrite the implementation maintained NUL   Writing a NUL elsewhere in the string does not change the string s size    string s are allowed to contain any number of NULs - they are given no special treatment by std  string  same in C  03    In C  03  things were considerably more complicated  key differences highlighted     x data     returns const char  to the string s internal buffer which wasn t required by the Standard to conclude with a NUL  i e  might be   h    e    l    l    o   followed by uninitialised or garbage values  with accidental accesses thereto having undefined behaviour     x size   characters are safe to read  i e  x 0  through x x size   - 1  for empty strings  you re guaranteed some non-NULL pointer to which 0 can be safely added  hurray    but you shouldn t dereference that pointer     amp x 0    for empty strings this has undefined behaviour  21 3 4    e g  given f const char  p  size t n    if  n    0  return     whatever      you mustn t call f  amp x 0   x size     when x empty   - just use f x data           otherwise  as per x data   but    for non-const x this yields a non-const char  pointer  you can overwrite string content   x c str     returns const char  to an ASCIIZ  NUL-terminated  representation of the value  i e    h    e    l    l    o     0     although few if any implementations chose to do so  the C  03 Standard was worded to allow the string implementation the freedom to create a distinct NUL-terminated buffer on the fly  from the potentially non-NUL terminated buffer  exposed  by x data   and  amp x 0  x size     1 characters are safe to read  guaranteed safe even for empty strings     0        Consequences of accessing outside legal indices  Whichever way you get a pointer  you must not access memory further along from the pointer than the characters guaranteed present in the descriptions above   Attempts to do so have undefined behaviour  with a very real chance of application crashes and garbage results even for reads  and additionally wholesale data  stack corruption and or security vulnerabilities for writes   When do those pointers get invalidated   If you call some string member function that modifies the string or reserves further capacity  any pointer values returned beforehand by any of the above methods are invalidated   You can use those methods again to get another pointer    The rules are the same as for iterators into strings    See also How to get a character pointer valid even after x leaves scope or is modified further below      So  which is better to use   From C  11  use  c str   for ASCIIZ data  and  data   for  binary  data  explained further below    In C  03  use  c str   unless certain that  data   is adequate  and prefer  data   over  amp x 0  as it s safe for empty strings         try to understand the program enough to use data   when appropriate  or you ll probably make other mistakes     The ASCII NUL   0  character guaranteed by  c str   is used by many functions as a sentinel value denoting the end of relevant and safe-to-access data   This applies to both C  -only functions like say fstream  fstream const char  filename       and shared-with-C functions like strchr    and printf     Given C  03 s  c str   s guarantees about the returned buffer are a super-set of  data   s  you can always safely use  c str    but people sometimes don t because    using  data   communicates to other programmers reading the source code that the data is not ASCIIZ  rather  you re using the string to store a block of data  which sometimes isn t even really textual    or that you re passing it to another function that treats it as a block of  binary  data   This can be a crucial insight in ensuring that other programmers  code changes continue to handle the data properly  C  03 only  there s a slight chance that your string implementation will need to do some extra memory allocation and or data copying in order to prepare the NUL terminated buffer   As a further hint  if a function s parameters require the  const  char  but don t insist on getting x size    the function probably needs an ASCIIZ input  so  c str   is a good choice  the function needs to know where the text terminates somehow  so if it s not a separate parameter it can only be a convention like a length-prefix or sentinel or some fixed expected length    How to get a character pointer valid even after x leaves scope or is modified further  You ll need to copy the contents of the string x to a new memory area outside x   This external buffer could be in many places such as another string or character array variable  it may or may not have a different lifetime than x due to being in a different scope  e g  namespace  global  static  heap  shared memory  memory mapped file    To copy the text from std  string x into an independent character array      USING ANOTHER STRING - AUTO MEMORY MANAGEMENT  EXCEPTION SAFE std  string old x   x     - old x will not be affected by subsequent modifications to x       - you can use   amp old x 0   to get a writable char  to old x s textual content    - you can use resize   to reduce expand the string      - resizing isn t possible from within a function passed only the char  address  std  string old x   x c str       old x will terminate early if x embeds NUL    Copies ASCIIZ data but could be less efficient as it needs to scan memory to    find the NUL terminator indicating string length before allocating that amount    of memory to copy into  or more efficient if it ends up allocating copying a    lot less content     Example  x     ab 0cd  - gt  old x     ab       USING A VECTOR OF CHAR - AUTO  EXCEPTION SAFE  HINTS AT BINARY CONTENT  GUARANTEED CONTIGUOUS EVEN IN C  03 std  vector lt char gt  old x x data    x data     x size              without the NUL std  vector lt char gt  old x x c str    x c str     x size     1       with the NUL     USING STACK WHERE MAXIMUM SIZE OF x IS KNOWN TO BE COMPILE-TIME CONSTANT  N      a bit dangerous  as  known  things are sometimes wrong and often become wrong  char y N   1   strcpy y  x c str         USING STACK WHERE UNEXPECTEDLY LONG x IS TRUNCATED  e g  Hello 0- gt Hel 0  char y N   1   strncpy y  x c str    N       copy at most N  zero-padding if shorter y N      0                    ensure NUL terminated     USING THE STACK TO HANDLE x OF UNKNOWN  BUT SANE  LENGTH char  y   alloca x size     1   strcpy y  x c str         USING THE STACK TO HANDLE x OF UNKNOWN LENGTH  NON-STANDARD GCC EXTENSION  char y x size     1   strcpy y  x c str         USING new delete HEAP MEMORY  MANUAL DEALLOC  NO INHERENT EXCEPTION SAFETY char  y   new char x size     1   strcpy y  x c str            or as a one-liner  char  y   strcpy new char x size     1   x c str        use y    delete   y     make sure no break  return  throw or branching bypasses this     USING new delete HEAP MEMORY  SMART POINTER DEALLOCATION  EXCEPTION SAFE    see boost shared array usage in Johannes Schaub s answer     USING malloc free HEAP MEMORY  MANUAL DEALLOC  NO INHERENT EXCEPTION SAFETY char  y   strdup x c str        use y    free y     Other reasons to want a char  or const char  generated from a string  So  above you ve seen how to get a  const  char   and how to make a copy of the text independent of the original string  but what can you do with it   A random smattering of examples      give  C  code access to the C   string s text  as in printf  x is   s    x c str     copy x s text to a buffer specified by your function s caller  e g  strncpy callers buffer  callers buffer size  x c str      or volatile memory used for device I O  e g  for  const char  p   x c str     p    p   p device    p   append x s text to an character array already containing some ASCIIZ text  e g  strcat other buffer  x c str     - be careful not to overrun the buffer  in many situations you may need to use strncat  return a const char  or char  from a function  perhaps for historical reasons - client s using your existing API - or for C compatibility you don t want to return a std  string  but do want to copy your string s data somewhere for the caller    be careful not to return a pointer that may be dereferenced by the caller after a local string variable to which that pointer pointed has left scope some projects with shared objects compiled linked for different std  string implementations  e g  STLport and compiler-native  may pass data as ASCIIZ to avoid conflicts

User · Answer

I am working with an API with a lot of functions get as an input a char    I have created a small class to face this kind of problem  I have implemented the RAII idiom   class DeepString           DeepString const DeepString amp  other           DeepString amp  operator  const DeepString amp  other           char  internal         public          explicit DeepString  const string amp  toCopy                internal  new char toCopy size   1                          strcpy internal  toCopy c str                        DeepString     delete   internal             char  str   const   return internal             const char  c str    const   return internal          And you can use it as   void aFunctionAPI char  input        other stuff  aFunctionAPI  Foo      this call is not safe  if the function modified the                         literal string the program will crash std  string myFoo  Foo    aFunctionAPI myFoo c str       this is not compiling aFunctionAPI const cast lt char  gt  myFoo c str        this is not safe std  string                                                    implement reference counting and                                                    it may change the value of other                                                   strings as well  DeepString myDeepFoo myFoo   aFunctionAPI myFoo str       this is fine   I have called the class DeepString because it is creating a deep and unique copy  the DeepString is not copyable  of an existing string

User · Answer

Use the  c str   method for const char     You can use  amp mystring 0  to get a char   pointer  but there are a couple of gotcha s  you won t necessarily get a zero terminated string  and you won t be able to change the string s size  You especially have to be careful not to add characters past the end of the string or you ll get a buffer overrun  and probable crash    There was no guarantee that all of the characters would be part of the same contiguous buffer until C  11  but in practice all known implementations of std  string worked that way anyway  see Does     amp s 0     point to contiguous characters in a std  string    Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved  Best to use them immediately and then discard

[c++] How to convert a std::string to const char* or char*?

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