有两个矩阵a和b，均为2行3列。求两个矩阵之和。重载运算符“+”，使之能用于矩阵相加。

可以通过重载运算符“+”来实现矩阵相加。具体实现如下：

#include <iostream>
using namespace std;

class Matrix {
private:
    int a[2][3];
public:
    Matrix() {
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 3; j++) {
                a[i][j] = 0;
            }
        }
    }

    Matrix operator+(const Matrix& other) {
        Matrix result;
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 3; j++) {
                result.a[i][j] = a[i][j] + other.a[i][j];
            }
        }
        return result;
    }

    friend istream& operator>>(istream& in, Matrix& m) {
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 3; j++) {
                in >> m.a[i][j];
            }
        }
        return in;
    }

    friend ostream& operator<<(ostream& out, const Matrix& m) {
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 3; j++) {
                out << m.a[i][j] << " ";
            }
            out << endl;
        }
        return out;
    }
};

int main() {
    Matrix a, b;
    cout << "请输入矩阵a：" << endl;
    cin >> a;
    cout << "请输入矩阵b：" << endl;
    cin >> b;
    Matrix c = a + b;
    cout << "矩阵a加b的结果为：" << endl;
    cout << c;
    return 0;
}

在上面的代码中，我们通过重载运算符“+”来实现矩阵相加。同时，我们也重载了输入和输出运算符“>>”和“<<”，使得我们可以方便地输入和输出矩阵。