模板参数变量/动态实例化

Question

I am an absolutely beginner in C++, so I really would appreciate your help!

I am currently implementing a Matrix Class with Template Parameters for datatype, dimension width and dimension height.

template <class T, int rows, int columns> class Matrix

In the multiplication function I have to create the result matrix (Height Matrix A x Width Matrix B), but I receive the error "template value cannot appear in a constant expression".

// overload * for matrix multiplication 
   template <class T, int rows, int columns>
   Matrix<T, rows, columns> operator*(Matrix<T, rows, columns> a, Matrix<T, rows, columns> b) {
     Matrix <T, rows, columns> result = new Matrix<T, a->height, b->width>;
// make multiplication here

Any ideas how to make a new Matrix with the given type and the height from A resp. width from B?

Thank you so much!

Answer 1

How about make rows and columns as the parameters of the constructors?

template <class T> class Matrix
{        
public:           
   Matrix(int rows, int columns)
   {....}    
}

//then you can create a new instance with:

 Matrix <T>* result = new Matrix<T> (a->height, b->width)

Answer 2

Try to think of two different concepts when referring to your template class Matrix that stores the data of the matrix and the *operator which is a template method.

If you think of the *operator as a template method that multiplies template classes, then you realize that you need more template arguments and that those arguments are only partial given by one instance of the matrix class.

The error is given because a->width and a->height a not know at compile time and every template argument has to be known at compile time.

Answer 3

As the error says, you cannot set static (compile time) "fields" from dynamic (run time) values.

You could ( not saying you should ) use different template parameters for the sizes of your two matrices and create a new matrix with them :

template <class T, int rows_A, int columns_A, int rows_B, int columns_B>
Matrix<T, rows_A, columns_B> operator*(Matrix<T, rows_A, columns_A> a, Matrix<T, rows_B, columns_B> b) {
    Matrix <T, rows_A, columns_B> result;
    // ...
    return result;
}

And as Joe Z. says, you'll also have to check the validity of the operation before. Because columns_A must be equal to rows_B , you can factorize the two template parameters.

EDIT : mmmmmmm wrote this factorization in below comments :

template <class T, int rows_A, int columns_A_rows_B, int columns_B>
Matrix<T, rows_A, columns_B> operator*(
    Matrix<T, rows_A, columns_A_rows_B> a,
    Matrix<T, columns_A_rows_B, columns_B> b
) {
    Matrix <T, rows_A, columns_B> result;
    // ...
    return result;
}

Yet ,this code is not really good looking, neither easy to use. If you can, you could try the idea of making the sizes dynamic (not parameter templates), as suggested by Matt.

Answer 4

It is looks like in this line:

Matrix <T, rows, columns> result = new Matrix<T, a->height, b->width>;

Is error. You are trying to write new Address to variable which is declared on stack. Try this:

Matrix<T, int, int> *result = new Matrix<T, a->height, b->width>();