Question

I must have missed an obvious fact here — haven’t been programming C++ for a while. Why can’t I print the c-style string after assigning it to a const char* variable? But if I try to print it directly without assigning it works fine:

#include "boost/lexical_cast.hpp"

using namespace std;
using boost::lexical_cast;

int main (int argc, char** argv)
{
    int aa=500;
    cout << lexical_cast<string>(aa).c_str() << endl;   // prints the string "500" fine

    const char* bb = lexical_cast<string>(aa).c_str();
    cout << bb << endl;                                 // prints nothing

    return EXIT_SUCCESS;
}

Answer 1

The C String returned by c_str is only usable while the std::string from which it was obtained exists. Once that std::string is destroyed, the C String is gone too. (At that point, attempting to use the C String yields undefined behavior.)

Other operations may also invalidate the C String. In general, any operation that modifies the string will invalidate the pointer returned by c_str.