How to call static library function in a C++ class?

I have class whose header file is defined as:

namespace mip {
    class CustomStatic {
        public:
            static const char* GetVersion();
    };
}

And class file is defined as:

#include "CustomStatic.h"

namespace mip {
    static const char* GetVersion() {
        return "hello";
    }
}

I am accessing this static function from my main class

#include "CustomStatic.h"

#include <iostream>

using std::cout;
using mip::CustomStatic;

int main() {
    const char *msg = mip::CustomStatic::GetVersion();
    cout << "Version " << msg << "\n";
}

When I try to compile it using-

g++ -std=c++11 -I CustomStatic.h  MainApp.cpp CustomStatic.cpp

I am getting error as:

Undefined symbols for architecture x86_64:
"mip::CustomStatic::GetVersion()", referenced from: _main in MainApp-feb286.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

your static function is not properly implemented in the cpp file...

you need to do something like

//.h
namespace mip
{
    class CustomStatic
    {
         public:
            static const char* GetVersion();
    };
}


//.cpp -> note that no static keyword is required...
namespace mip
{
    const char* CustomStatic::GetVersion()
    {
        return "hello";
    }
}

//use
int main(int argc, char *argv[])
{
    const char* msg{mip::CustomStatic::GetVersion()};
    cout << "Version " << msg << "\n";
}