#include <iostream> using namespace std; class A { public: A(int a = 0, int b = 0) : x(a), y(b) { } friend A operator + (const A &x1, const A &x2); // ①请根据程序修改其中错误 friend ostream&operator << ( const A&out, const A &c); // ②请根据程序修改其中错误 private: int x, y; }; A operator + (const A &x1, const A &x2) { return A(x1.x + x2.x, x1.y + x2.y); } ostream&operator << (const A &out, const A &c) { // ③请根据程序修改其中错误 out << "(" << c.x << ", " << c.y << ")"; return out; // ④请根据程序修改其中错误 } int main() { A x1(5, 4), x2(2, 10), x3; x3 = x1 + x2; cout << "x3 = x1 + x2 = " << x3 << endl; return 0; }怎么改错

在这段程序中，有以下错误需要修改：

① `operator +` 函数的定义缺少类名 `A` 的限定符，应改为：

A A::operator + (const A &x1, const A &x2) {
    return A(x1.x + x2.x, x1.y + x2.y);
}

② `operator<<` 函数的定义缺少类名 `A` 的限定符，应改为：

ostream& operator << (ostream &out, const A &c) {
    out << "(" << c.x << ", " << c.y << ")";
    return out;
}

③ 在 `operator<<` 函数的实现中，参数名 `out` 和函数体内的 `out` 发生了重名，应将其中一个修改为其他名称，例如：

ostream& operator << (ostream &os, const A &c) {
    os << "(" << c.x << ", " << c.y << ")";
    return os;
}

④ 在 `operator<<` 函数的实现中，应该返回 `ostream&` 类型的对象，而不是 `A&` 类型，应改为：

ostream& operator << (ostream &out, const A &c) {
    out << "(" << c.x << ", " << c.y << ")";
    return out;
}

修改后的完整程序如下：

#include <iostream>
using namespace std;

class A {
public:
    A(int a = 0, int b = 0) : x(a), y(b) { }
    friend A operator + (const A &x1, const A &x2);
    friend ostream& operator << (ostream &out, const A &c);
private:
    int x, y;
};

A A::operator + (const A &x1, const A &x2) {
    return A(x1.x + x2.x, x1.y + x2.y);
}

ostream& operator << (ostream &out, const A &c) {
    out << "(" << c.x << ", " << c.y << ")";
    return out;
}

int main() {
    A x1(5, 4), x2(2, 10), x3;
    x3 = x1 + x2;
    cout << "x3 = x1 + x2 = " << x3 << endl;
    return 0;
}