Copy a string: Difference between revisions

Standard C++ strings are not guaranteed to be stored contiguous, as needed for C functions, and it's only possible to get pointers to constant C strings. Therefore if the C function modifies the string, it has to be copied into a C string. There are several ways, depending on the needs:

Using a <tt>std::vector</tt> allows C++ to manage the memory of the copy. Since C++2003 (TR1), std::vector is guaranteed to be contiguous; but even before, there was no implementation which wasn't.

#include <string>

#include <vector>

std::string s = "Hello";

{

// copy into a vector

std::vector<char> string_data(s.begin(), s.end());

// append '\0' because C functions expect it

string_data.push_back('\0');

// call C function

modify(&string_data[0]);

// copy resulting C string back to s

s = (&string_data[0]);

} // string_data going out of scope -> the copy is automatically released

Another way is to manage your memory yourself and use C functions for the copy:

#include <string>

#include <cstring> // For C string functions (strcpy)

std::string s = "Hello";

// allocate memory for C string

char* cs = new char[s.length() + 1];

// copy string into modifyable C string (s.c_str() gives constant C string)

std::strcpy(cs, s.c_str());

// call C function

modify(cs);

// copy result back into s

s = cs;

// get rid of temporary buffer

delete[] cs;

If the C function calls free or realloc on its argument, new[] cannot be used, but instead malloc must be called:

#include <string>

#include <cstring>

#include <cstdlib> // for malloc

std::string s = "Hello";

// allocate memory for C string

char* cs = (char*)malloc(s.length()+1);

// copy the string

std::strcpy(cs, s.c_str());

// call C function which may call realloc

modify(&cs);

// copy result back to s

s = cs;

// get rid of temporary buffer

free(cs); // delete or delete[] may not be used here