• friend complex operator+(const complex &A, const complex &B) const; 非成员函数不能是const，会报错
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    class A {
    public:
        void fun1() {
            cout << a << endl;
            cout << "基类函数 fun1" << endl;
        }
    private:
        int a = 10;
    };
    class B :public A {
    public:
    //    void fun1() {
    //        cout << a << endl;
    //        cout << "派生类函数 fun1" << endl;
    //    }
    private:
        int a = 20;
    };
    int main(){
        B b;
        b.fun1();
    }

    10
    基类函数 fun1

输出的a是基类名空间下的a，而不是派生类名空间下的a

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class A {
public:
    void fun1() {
        cout << a << endl;
        cout << "基类函数 fun1" << endl;
    }
private:
    int a = 10;
};
class B :public A {
public:
    void fun1() {
        cout << a << endl;
        cout << "派生类函数 fun1" << endl;
    }
private:
    int a = 20;
};
int main(){
    B b;
    b.fun1();
}

20
派生类函数 fun1

因为在派生类中定义了相同名字的函数和变量，所以基类名空间下的被隐藏了

demo3

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//
// Created by 13467 on 2023/2/20.
//

#include <iostream>

using namespace std;

class complex {
public:
    complex();

    complex(double real, double imag);

    ~complex(){
        cout << "deconstruct" << endl;
    }
public:
    //声明运算符重载
    complex operator+(const complex &A) const;

    void display() const;

private:
    double m_real;  //实部
    double m_imag;  //虚部
};

complex::complex() : m_real(0.0), m_imag(0.0) {}

complex::complex(double real, double imag) : m_real(real), m_imag(imag) {
    cout << "construct" << endl;

}

complex complex::operator+(const complex &A) const {
    complex B;
    B.m_imag = this->m_imag + A.m_imag;
    B.m_real = this->m_real + A.m_real;
    return B;
}

void complex::display() const {
    cout << m_real << " + " << m_imag << "i" << endl;
}

int main() {
    complex c1(4.3, 5.8);
    complex c2(2.4, 3.7);
    complex c3;
    c3 = c1 + c2;
    c3.display();

    return 0;
}
// construct
// construct
// deconstruct
// 6.7 + 9.5i
// deconstruct
// deconstruct
// deconstruct

为什么

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//
// Created by 13467 on 2023/2/20.
//

#include <iostream>

using namespace std;

class complex {
public:
    complex();

    complex(double real, double imag);

    ~complex() {
        cout << "deconstruct" << endl;
    }

public:
    //声明运算符重载
//    complex operator+(const complex &A) const;
    friend complex operator+(const complex &A, const complex &B);

    complex operator *(const complex &other)const;
    void display() const;

private:
    double m_real;  //实部
    double m_imag;  //虚部
};

complex::complex() : m_real(0.0), m_imag(0.0) {}

complex::complex(double real, double imag) : m_real(real), m_imag(imag) {
    cout << "construct" << endl;
}

//complex complex::operator+(const complex &A) const {
//    complex B;
//    B.m_imag = this->m_imag + A.m_imag;
//    B.m_real = this->m_real + A.m_real;
//    return B;
//}
complex complex::operator*(const complex &A) const{
    return complex(this->m_real * A.m_real, this->m_imag * A.m_imag);
}

complex operator+(const complex &A, const complex &B) {
    complex C;
    C.m_real = A.m_real + B.m_real;
    C.m_imag = A.m_imag + B.m_imag;
    return C;
}

void complex::display() const {
    cout << m_real << " + " << m_imag << "i" << endl;
}

int main() {
    complex c1(4.3, 5.8);
    complex c2(1.0, 3.7);
    complex c3;
    c3 = c1 + c2 * c2;
    c3.display();
    return 0;
}

construct
construct
construct
deconstruct
deconstruct
5.3 + 19.49i
deconstruct
deconstruct
deconstruct