c++设计一个3行4列的矩阵类，重载运算符“+”和“-”，能实现矩阵类对象的加减运算；重载流插入运算符“《”和流提取运算符“》”，使之能用于矩阵的输入和输出

好的，下面是代码实现：

#include <iostream>

using namespace std;

class Matrix {
private:
    int data[3][4];

public:
    Matrix() {}

    Matrix(int arr[][4]) {
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 4; j++) {
                data[i][j] = arr[i][j];
            }
        }
    }

    friend Matrix operator+(const Matrix &a, const Matrix &b) {
        Matrix c;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 4; j++) {
                c.data[i][j] = a.data[i][j] + b.data[i][j];
            }
        }
        return c;
    }

    friend Matrix operator-(const Matrix &a, const Matrix &b) {
        Matrix c;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 4; j++) {
                c.data[i][j] = a.data[i][j] - b.data[i][j];
            }
        }
        return c;
    }

    friend ostream &operator<<(ostream &os, const Matrix &m) {
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 4; j++) {
                os << m.data[i][j] << " ";
            }
            os << endl;
        }
        return os;
    }

    friend istream &operator>>(istream &is, Matrix &m) {
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 4; j++) {
                is >> m.data[i][j];
            }
        }
        return is;
    }
};

int main() {
    int a1[3][4] = {{1, 2, 3, 4},
                    {5, 6, 7, 8},
                    {9, 10, 11, 12}};
    int a2[3][4] = {{2, 3, 4, 5},
                    {6, 7, 8, 9},
                    {10, 11, 12, 13}};

    Matrix m1(a1), m2(a2), m3, m4;
    m3 = m1 + m2;
    m4 = m1 - m2;
    cout << "m1 + m2 =" << endl << m3 << endl;
    cout << "m1 - m2 =" << endl << m4 << endl;

    cout << "请输入一个3行4列的矩阵：" << endl;
    cin >> m1;
    cout << "你输入的矩阵为：" << endl << m1 << endl;

    return 0;
}

这个矩阵类中重载了加减运算符和流插入提取运算符，可以进行矩阵的加减运算和输入输出。注意，这里重载的输出操作符是 `<<`，输入操作符是 `>>`。另外，为了方便起见，这里的矩阵大小是固定的，即3行4列。如果需要更一般化的矩阵类，可以考虑使用模板。