How can I Invoke a const member function in a class?

[peng-jr]

When I learn the STL,I find that we can define such a class:
class A {
public:
int a;
A(int x=0):a(x) {};
int GetA() { return a++;};
int GetA() const { return a;};
};

But I find when I call the GetA method,the non-const method will be invoked
always,for example:

void main()
{
A t(10);
cout<<t.GetA()<<endl; //call non-const method
cout<<t.GetA()<<endl; //call non-const method
}

Can anybody tell me how I invoke the const method without deleting the
non-const one?