c – 使用默认参数解析虚函数

#include <iostream>
using namespace std;
class A
{
    public:
        virtual void display(int i=5) { cout<< "Base::" << i << endl; }
};

class B : public A
{
    public:
        void display(int i=9) { cout<< "Derived::" << i << endl; }
};

#include <iostream>
#include "header.h"
using namespace std;
int main()
{
    A * a = new B();
    a->display();

    A* aa = new A();
    aa->display();

    B* bb = new B();
    bb->display();
}

Derived::5
Base::5
Derived::9

您的代码实际上是由编译器看到的,如下所示：
(display()方法实际上并不存在,但解析的工作方式类似)

class A
{
public:
    virtual void display(int i) { cout<< "Base::" << i << endl; }
    void display() { display(5); }
};

class B : public A
{
public:
    void display(int i) override { cout<< "Derived::" << i << endl; }
    void display() { display(9); }
};

现在您应该了解会发生什么.您正在调用调用虚函数的非虚拟显示().用更严格的话说：默认参数的解析就像没有arg非虚方法那样 – 通过变量的类型(不是由对象的实际类型),但是代码是根据真实对象执行的类型,因为它是虚方法：

int main()
{
    A * a = new B(); // type of a is A*   real type is B
    a->display();    // calls A::display() which calls B::display(5)

    A* aa = new A(); // type of aa is A*  real type is A
    aa->display();   // calls A::display() which calls A::display(5)

    B* bb = new B(); // type of bb is B*  real type is B
    bb->display();   // calls B::display() which calls B::display(9)
}