"to_string" isn't a member of "std"?

you may want to specify the C++ version with

g++ -std=c++11 tmp.cpp -o tmp

I don't have gcc 4.8.1 at hand , but in older versions of GCC, you can use

g++ -std=c++0x tmp.cpp -o tmp

At least gcc 4.9.2 I believe also support part of C++14 by specifying

g++ -std=c++1y tmp.cpp -o tmp

Update: gcc 5.3.0 (I am using the cygwin version) supports both -std=c++14 and -std=c++17 now.

to_string works with the latest C++ versions like version 11. For older versions you can try using this function

#include <string>
#include <sstream>

template <typename T>
std::string ToString(T val)
{
    std::stringstream stream;
    stream << val;
    return stream.str();
}

By adding a template you can use any data type too. You have to include #include<sstream> here.