Hey so new to cpp. I don't understand why this does't output anything-CodePudding

#include <iostream>
#include <vector>


using std::vector;
using MATRIX = vector<vector<int>>;


class Matrix {
public:
    MATRIX matrix;
    int row = matrix.size();
    int col = matrix[0].size();

    void repr() {
        for (const vector<int> &sub_arr: matrix) {
            for (int element: sub_arr) {
                std::cout << element << ' ';
            }
            std::cout << '\n';
        }
    }

    Matrix operator (const Matrix &other) {
        std::cout << row << col;
        Matrix new_matrix;

        if (row != other.matrix.size() || col != other.matrix[0].size()) { return {{}}; }

        for (int i = 0; i < row;   i) {
            vector<int> temp_arr;
            for (int j = 0; j < col;   j) {
                temp_arr.push_back(matrix[i][j]   other.matrix[i][j]);
            }
            new_matrix.matrix.push_back(temp_arr);
        }
        return new_matrix;
    }

    Matrix operator-(const Matrix &other) {
        Matrix new_matrix;
        if (row != other.matrix.size() || col != other.matrix[0].size()) { return {{}}; }

        for (int i = 0; i < row;   i) {
            vector<int> temp_arr;
            for (int j = 0; j < col;   j) {
                temp_arr.push_back(matrix[i][j] - matrix[i][j]);
            }
            new_matrix.matrix.push_back(temp_arr);
        }
        return new_matrix;
    }

//    Matrix operator*(const Matrix &other) {}
};

int main() {
    Matrix matrix = {{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}};
    Matrix addition = matrix   matrix;
    addition.repr();
}

So when I define row and col at the class level, it doesn't produce any output. But when I define it in the function, it works fine.

I don't understand why it won't work when I define it in the class level. I inserted some debug print statements, and row and col seem to be correct.

CodePudding user response：

The most "natural" solution I can see is to have a constructor taking the matrix data as an argument, and use that as the base for initializations:

class Matrix {
    size_t row;
    size_t col;
    MATRIX matrix;

public:
    Matrix()
        : row{ }, col{ }, matrix{ }
    {
    }

    Matrix(MATRIX const& data)
        : row{ data.size() }, col{ data.empty() ? 0 : data[0].size() }, matrix{ data }
    {
    }

    // ...
};

Note the check that the passed data isn't empty.

CodePudding user response：

When you instantiate your Matrix object like this:

Matrix matrix = {{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}};

the aggregate initialization takes place, first your MATRIX matrix; is initialized, then you assign values 3 and 3 to your row and col. However, in your second line of main and inside Matrix operator (const Matrix &other) you create default constructed Matrix and here you have undefined behavior, because:

    int row = matrix.size();  // 0
    int col = matrix[0].size();  // UB, out of bounds access

So, for you maybe sometimes it prints something, on other machines it might crash before the addition.

CodePudding user response：

When row and col are initialized, matrix is still empty. To initialize them after matrix is known, write a constructor for your class.

#include <vector>

using std::vector;
using MATRIX = vector<vector<int>>;

class Matrix {
public:
    MATRIX matrix;
    int row, col;

    // Constructor
    Matrix(const MATRIX& m)
        : matrix(m), row(m.size()), col(m[0].size())
    {}
};

int main() {
    Matrix matrix = {{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}};
}