C ++在头文件中声明函数原型的麻烦

Question

I have been given the following code to work with within a MatrixTest.cpp function:

Matrix matrix = Matrix::Zeros(2,4)

The aim is to "create a 2x4 matrix of zeros with the static Zeros" and I need to be able to add something to the header file "Matrix.h" which allows "MatrixTest.cpp" to compile for the line of code above. This is the code in my header file so far:

#ifndef MATRIX_H_
#define MATRIX_H_

class Matrix {
protected:
    // These are the only member variables allowed!
    int noOfRows;
    int noOfColumns;
    double *data;

    int GetIndex (const int rowIdx, const int columnIdx) const;

public:
    Matrix (const int noOfRows, const int noOfCols);
    Matrix (const Matrix& input);
    Matrix& operator= (const Matrix& rhs);
    ~Matrix ();

    Matrix Zeros(const int noOfRows, const int noOfCols);
};

#endif /* MATRIX_H_ */

This gives the error in my .cpp file that I cannot call member function Matrix Matrix::Zeros(int, int) without object. But surely Zeros is my object and my Matrix class is my type?

If I change my code in the header file to the following:

static Zeros(const int noOfRows, const int noOfCols);

Then I get an error within my .h file saying "forbids declaration of 'Zeros' with no type and an error within my .cpp file saying "conversion from 'int' to non-scalar type 'Matrix' requested"

I'm confused since I would've thought that my type is Matrix as it appears underneath the class Matrix and that since Matrix::Zeros(2,4) follows the constructor Matrix(const int noOfRows, const int noOfCols) then there wouldn't be a conversion issue from 'int' to non-scalar type.

Can anybody help with this as I seem to be going back and forth between these errors?

Answer 1

The signature of the function should be

static Matrix Zeros(const int noOfRows, const int noOfCols);

The static keyword is not the return type, Matrix is. Rather the static keyword states that you do not need an instance of a Matrix to call the method, instead you can call it as

Matrix matrix = Matrix::Zeros(2,4)

To be clear, if you did not use the word static then you would have to do something like

Matrix a{};
Matrix matrix = a.Zeros(2,4);

but you can see that the Zeros method doesn't depend on the state of a so it would make sense for the method to be static instead.

Answer 2

Since static is not a return type here, and your function is returning a Matrix , that would be your return type.

Changing your function signature to static Matrix Zeros(const int noOfRows, const int noOfCols); should do the trick.