ostream不打印添加两个valarray的结果（使用运算符重载）

Question

I am still learning about operator overloading and wanted to add two matrices using valarrays, but when i print their addition nothing happens, so here is the code.

// A structure to store a matrix
struct matrix
{
    valarray<int> data; //valarray that will simulate matrix
    int row, col;
};

matrix operator+(matrix mat1, matrix mat2);

int main() {
    int data1 [] = {1, 2, 3, 4, 5, 6, 7, 8};
    int data2 [] = {13, 233, 3, 4, 5, 6, 7, 8};

    matrix mat1, mat2,ans;
    createMatrix(4, 2, data1, mat1);
    createMatrix(4, 2, data2, mat2);
    cout << mat1 + mat2;
    return 0;
}

//Creating the matrix
void createMatrix(int row, int col, int num[], matrix& mat) {
    mat.row = row;
    mat.col = col;
    mat.data.resize (row * col);
    for (int i = 0; i < row * col; i++)
        mat.data [i] = num [i];
}

ostream& operator<<(ostream& out, matrix mat) {
    for (int i = 0; i < mat.col * mat.row; ++i) {
        out << mat.data[i] << " ";
        if ((i + 1) % mat.col == 0)
            cout << endl;
    }
    return out;
}
// Adding them
matrix operator+(matrix mat1, matrix mat2) {
    matrix ans;
    ans.data.resize(mat1.row * mat1.col);
    for(int i = 0; i < mat1.row * mat1.col; ++i)
        ans.data[i] = (mat1.data[i] + mat2.data[i]);
    return ans;
}

There seem to be no error but when I run this it prints nothing.

Answer 1

In operator+ definition you are not setting row and col members for ans object so change your code as follows

matrix operator+(matrix mat1, matrix mat2)
{
    matrix ans;
    ans.data.resize(mat1.row*mat1.col);
    ans.row = mat1.row; // <---
    ans.col = mat1.col; // <---
    for(int i=0;i<mat1.row*mat1.col;++i)
    {
        ans.data[i]=(mat1.data[i]+mat2.data[i]);
    }
    return ans;
}

Without setting these members their values are indeterminate, and probably

for(int i=0;i<mat.col*mat.row;++i)

condition i<mat.col*mat.row in operator<< returns false in first iteration, that is why you didn't see any output.