C++ :: Static Constant Member Variable

What is the problem with the following code is? It compiles with Visual C++ 2012 but does not with g++:

//a.h

#ifndef Loaded
#define Loaded
using namespace std;
class MyClass{
public:
static const int MyStaticValue = 200;

[Code] ....

If I try to compile this using the command

g++ a.cpp b.cpp

I get an "undefined reference to 'MyClass::MyStaticValue'" error for the line "A = MyClass::MyStaticValue;" in main(). The strange thing is that if I change the line to "A = (int) MyClass::MyStaticValue;" it works fine and the output is

200
200

as expected.

The code also compiles under g++ if I move the defintion of MyStaticValue from a.h to a.cpp by const int MyClass::MyStaticValue = 200;