convert string to char* [duplicate]

There are many ways. Here are at least five:

/* * An example of converting std::string to (const)char* using five * different methods. Error checking is emitted for simplicity. * * Compile and run example (using gcc on Unix-like systems): * * $ g++ -Wall -pedantic -o test ./test.cpp * $ ./test * Original string (0x7fe3294039f8): hello * s1 (0x7fe3294039f8): hello * s2 (0x7fff5dce3a10): hello * s3 (0x7fe3294000e0): hello * s4 (0x7fe329403a00): hello * s5 (0x7fe329403a10): hello */ #include <alloca.h> #include <string> #include <cstring> int main() { std::string s0; const char *s1; char *s2; char *s3; char *s4; char *s5; // This is the initial C++ string. s0 = "hello"; // Method #1: Just use "c_str()" method to obtain a pointer to a // null-terminated C string stored in std::string object. // Be careful though because when `s0` goes out of scope, s1 points // to a non-valid memory. s1 = s0.c_str(); // Method #2: Allocate memory on stack and copy the contents of the // original string. Keep in mind that once a current function returns, // the memory is invalidated. s2 = (char *)alloca(s0.size() + 1); memcpy(s2, s0.c_str(), s0.size() + 1); // Method #3: Allocate memory dynamically and copy the content of the // original string. The memory will be valid until you explicitly // release it using "free". Forgetting to release it results in memory // leak. s3 = (char *)malloc(s0.size() + 1); memcpy(s3, s0.c_str(), s0.size() + 1); // Method #4: Same as method #3, but using C++ new/delete operators. s4 = new char[s0.size() + 1]; memcpy(s4, s0.c_str(), s0.size() + 1); // Method #5: Same as 3 but a bit less efficient.. s5 = strdup(s0.c_str()); // Print those strings. printf("Original string (%p): %s\n", s0.c_str(), s0.c_str()); printf("s1 (%p): %s\n", s1, s1); printf("s2 (%p): %s\n", s2, s2); printf("s3 (%p): %s\n", s3, s3); printf("s4 (%p): %s\n", s4, s4); printf("s5 (%p): %s\n", s5, s5); // Release memory... free(s3); delete [] s4; free(s5); } 

First of all, you would have to allocate memory:

char * S = new char[R.length() + 1]; 

then you can use strcpy with S and R.c_str():

std::strcpy(S,R.c_str()); 

You can also use R.c_str() if the string doesn't get changed or the c string is only used once. However, if S is going to be modified, you should copy the string, as writing to R.c_str() results in undefined behavior.

Note: Instead of strcpy you can also use str::copy.