Parenthesis operator overload

I was trying to implement a simple matrix class below is its code. Can any one tell me why the the print statement "CALLED CONST" isn't getting executed ? It was supposed to get printed while executing these two statements .

Code:

	int x=M(3,4)+5;
	cout<<M(3,4)<<endl;

Can someone help ?

Code:

#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

template<class T>
class Matrix
{
	private:
	unsigned r,c; 
	vector<T>data;

	public:
	Matrix(){};
	Matrix(unsigned _r,unsigned _c):r(_r),c(_c)
	{
		data.resize(r*c);
	}
	T& operator ()(unsigned _r,unsigned _c)
	{
		puts("CALLED");
		return data[c*_r+_c];
	}
	T operator ()(unsigned _r,unsigned _c) const
	{
		puts("CALLED CONST");
		return data[c*_r+_c];
	}
};

int main()
{
	Matrix<int>M(5,5);
	M(3,4)=19191;
	int x=M(3,4)+5;
	cout<<M(3,4)<<endl;
	return 0;
}

The compiler doesn't choose between the const and non-const overload based on your usage.

It chooses based on whether the instance is const or not.

Code:

int main()
{
	Matrix<int>M(5,5);
	M(3,4)=19191;

        const Matrix<int>& N = M;
	int x=N(3,4)+5;        
	cout<<N(3,4)<<endl;
	return 0;
}

BTW, your const overload should better return a const T (or a const T&). Otherwise the following will compile for user-defined types, but will only result in modifying a temporary.

Code:

	const Matrix<std::string>N(5,5);
	N(3, 4) = "Hello";

Thank you.