no Operator "=" 匹配这些操作数，操作数类型 -- Matrix = Matrix

Question

我正在编写一个使用运算符重载的矩阵运算程序，但出现“=”运算符重载的错误这是完整的代码。我还通过在“=”运算符声明中使用 (return *this) 返回了我的参考对象，但不知道 为什么它不工作。 当我使用 C = A 时，它没有显示任何错误，但是当我使用 C = A + B 时，就会出现此错误

#include<iostream>
#include<iomanip>
#include<string.h>

using namespace std;

class Matrix{
    private:
        int rows;   //For rows
        int cols;   //For columns
        int **ptr; //pointer to pointer to point in the array 
    public:
    //Declaring Constructors and destructors
        Matrix(Matrix &);
        Matrix(int, int);
        ~Matrix();
    //Declaring other functions
        friend istream& operator>>(istream &, Matrix &);
        friend ostream& operator<<(ostream &,Matrix &);
        Matrix operator +(Matrix &);
        Matrix operator -(Matrix &);
        Matrix operator *(Matrix &);
        Matrix operator =(Matrix &);
        void transpose(Matrix &);
    //End of class definition
};

//Defining Copy constructor
Matrix::Matrix(Matrix &m){
    rows = m.rows;
    cols = m.cols;
    ptr = new int* [rows];
    for(int i= 0;i<rows;i++){
        ptr[i]=new int [cols];
    }
    for(int i =0 ;i<rows;i++){
        for(int j=0;j<cols;j++){
             ptr[i][j] = m.ptr[i][j];
         }
     }
}

//Defining parametrised constructor
Matrix::Matrix(int m, int n){
    rows = m;
    cols = n;
    ptr = new int* [m];
    for(int i =0;i<m;i++){
        ptr[i] = new int[n];
    }
}

//Defining oveloaded operator to take matrix as an input
istream & operator >>(istream &din , Matrix & m){
    for(int i =0;i<m.rows;i++){
        for(int j =0;j<m.cols;j++){
            din >> m.ptr[i][j];
        }
    }
    return din;
}

//Defining overoaded operator to display matrix 
ostream & operator << (ostream &dout,Matrix &m ){
    for(int i =0;i<m.rows;i++){
        for(int j = 0; j<m.cols;j++){
            dout<<setw(5)<< m.ptr[i][j];
        }
        dout<<endl;
    }
    return dout;
}

//Defining overloaded + opearator to add two matrices
Matrix Matrix::operator +(Matrix &m){
    Matrix temp( rows, cols);
    for(int i =0;i<rows;i++){
        for(int j =0;j<cols;j++){
            temp.ptr[i][j]= ptr[i][j] + m.ptr[i][j];
        }
    }
    return temp;
}

//Definig overloaded - operator to subtract two matrices
Matrix Matrix::operator -(Matrix &m){
    Matrix temp(rows,cols);
    for(int i=0;i<rows;i++){
        for(int j=0;j<cols;j++){
            temp.ptr[i][j] = ptr[i][j] - m.ptr[i][j];
        }
    }
    return temp;
}

//Defining overloaded *opeartor to perform matrix multiplication
Matrix Matrix::operator *(Matrix &m){
    Matrix temp(rows,cols);
    int product_sum;
    for(int i =0;i<rows;i++){
        for(int j=0;j<cols;j++){
            product_sum =0;
            for(int k=0;k<rows;k++){
                product_sum += ptr[i][k] * m.ptr[k][j];
            }
            temp.ptr[i][j] = product_sum;
        }
    }
    return temp;
}

//Defining overloaded = operator to assingn matrix value into another matrix  
Matrix Matrix::operator =(Matrix & m){
    for(int i =0;i<rows;i++){
        for(int j=0;j<cols;j++){
            ptr[i][j] = m.ptr[i][j];
        }
    }
    return *this;
}
//Definig a function to perform transpose of a matrix
void Matrix::transpose(Matrix &m){
    for(int i=0;i< rows;i++){
        for(int j=0;j<cols;j++){
            ptr[j][i] = m.ptr[i][j];
        }
    }
}
//Definig Destructor
Matrix::~Matrix(){
    for(int i=0;i<rows;i++){
        delete[] ptr[i];
    }
    delete[] ptr;
}
//Main Function
int main(){
    int row_1 ,row_2,col_1,col_2 ,chs;
    char choice , ch;
    cout<<"\n\n\t\t\t\t\tCreate Matrix A";
    cout<<"\nEnter the number of rows for matrix : ";
    cin>>row_1;
    cout<<"\nEnter the number of columns for matrix : ";
    cin>>col_1;
    Matrix A(row_1,col_1);
    cout<<"\nEnter the elements of Matrix row wise : ";
    cin >> A;
    cout<<"\nEntered Matrix \n";
    cout << A;
    cout<<"\n\n\t\t\tCreate Matrix B";
    cout<<"\nEnter the number of rows for matrix : ";
    cin>>row_2;
    cout<<"\nEnter the number of columns for matrix : ";
    cin>>col_2;
    Matrix B(row_2,col_2);
    cout<<"\nEnter the elements of Matrix row wise : ";
    cin >> B;
    cout<<"\nEntered Matrix\n";
    cout << B;
    Matrix C(col_1,row_1);
    C = A + B; //GETTING ERROR ON THIS LINE
    // **NO Operator "=" matches these operands , operand types -- Matrix = Matrix** 
    return 0;
}

Answer 1

Matrix Matrix::operator + 返回一个临时值。在 C++ 中，非常量引用不会绑定到临时对象。这是因为非常量引用通常用于修改对象，而临时修改可能会丢失。

但是重载的赋值运算符Matrix Matrix::operator =(Matrix & m)并没有修改m，所以真的可以通过const&取一个临时的：Matrix Matrix::operator =(Matrix const& m)。

此外，从技术上讲，返回类型可以是任何东西，甚至是 void，但实际上您应该遵循默认运算符的行为并返回 Matrix& - 对 *this 的引用，而不是副本。

另外，如果您使用std::vector 作为后备存储，则根本不需要这些实用方法！

Answer 2

问题是这个声明：

Matrix Matrix::operator =(Matrix & m);

你应该把它改成：

Matrix Matrix::operator =(const Matrix & m)

const 引用延长了对象的生命周期；查看更多关于延长对象生命周期的详细信息：Does a const reference class member prolong the life of a temporary?

编辑： 你也应该返回 Matrix&:

Matrix& Matrix::operator =(const Matrix & m)