to_string is not a member of std, says g++ (mingw)

#include <string>
#include <sstream>

namespace patch
{
    template < typename T > std::string to_string( const T& n )
    {
        std::ostringstream stm ;
        stm << n ;
        return stm.str() ;
    }
}

#include <iostream>

int main()
{
    std::cout << patch::to_string(1234) << '\n' << patch::to_string(1234.56) << '\n' ;
}

do not forget to include #include <sstream>

This is a known bug under MinGW. Relevant Bugzilla. In the comments section you can get a patch to make it work with MinGW.

This issue has been fixed in MinGW-w64 distros higher than GCC 4.8.0 provided by the MinGW-w64 project. Despite the name, the project provides toolchains for 32-bit along with 64-bit. The Nuwen MinGW distro also solves this issue.

As suggested this may be an issue with your compiler version.

Try using the following code to convert a long to std::string:

#include <sstream>
#include <string>
#include <iostream>

int main() {
    std::ostringstream ss;
    long num = 123456;
    ss << num;
    std::cout << ss.str() << std::endl;
}