How to make compiler work out template class arguments at assigmnet?

Question

Here's the code. Is it possible to make last line work?

#include<iostream>
using namespace std;

template <int X, int Y>
class Matrix
{
    int matrix[X][Y];
    int x,y;
    public:
    Matrix() : x(X), y(Y) {}
    void print() { cout << "x: " << x << " y: " << y << endl; }
};

template < int a, int b, int c>
Matrix<a,c> Multiply (Matrix<a,b>, Matrix<b,c>)
{
    Matrix<a,c> tmp;
    return tmp;
}

int main()
{
    Matrix<2,3> One;
    One.print();
    Matrix<3,5> Two;
    (Multiply(One,Two)).print();    // this works perfect
    Matrix Three=Multiply(One,Two); // !! THIS DOESNT WORK
    return 0;
}

Answer 1

This wont be possible in C++03 but C++0x offers auto.

auto Three=Multiply(One,Two);

Answer 2

Templates are used at compilation time and are used to implement static polymorphism. This means you should know everything about your objects at the moment your code is being compiled.

Hence, here the compiler fails, because this would be too hard for it to know that Three should have (2,5) dimensions (at least at currently common standard).

If this is a question for "just-to-know", then OK, but in real code you should obviously use constructors to initialize matrix (and set it's dimensions).

Answer 3

No, when using a class template, you have to specify all template arguments explicitly.

If your compiler supports it, you can use auto from C++0x instead:

auto Three=Multiply(One,Two);

In g++, you can enable C++0x support using the -std=c++0x flag.