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:

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

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Can someone help ?

Code:

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#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;
}

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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;
}

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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:

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

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Thank you.