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

In C++11 you can use auto to do that:

auto Three=Multiply(One,Two);

In current C++ you cannot do this.

One way to avoid having to spell out the type’s name is to move the code dealing with Three into a function template:

template< int a, int b >
void do_something_with_it(const Matrix<a,b>& One, const Matrix<a,b>& Two)
{
  Matrix<a,b> Three = Multiply(One,Two);
  // ...
}

int main()
{
    Matrix<2,3> One;
    One.print();
    Matrix<3,5> Two;
    do_something_with_it(One,Two);
    return 0;
}

Edit: A few more notes to your code.

1. Be careful with using namespace std;, it can lead to very nasty surprises.
2. Unless you plan to have matrices with negative dimensions, using unsigned int or, even more appropriate, std::size_t would be better for the template arguments.
3. You shouldn’t pass matrices per copy. Pass per const reference instead.
4. Multiply() could be spelled operator*, which would allow Matrix<2,3> Three = One * Two;
5. print should probably take the stream to print to as std::ostream&. And I’d prefer it to be a free function instead of a member function. I would contemplate overloading operator<< instead of naming it print.