Question

我想实现一个复合模式，我可以使用std :: cout

打印内容

当我打印时，使用基类插入运算符（运算符＆lt;＆lt ;;）而不是最派生类型。如何使用派生类型最多的运算符？

实施例：

#include <iostream>
#include <memory>

using namespace std;

class Base {
    int i;
public:
    Base(int _i) : i(_i) {}
    virtual ~Base() {}

    friend inline ostream& operator<<(ostream& os, Base& value) { return os << "i: " <<  value.i; }
    friend inline ostream& operator<<(ostream& os, shared_ptr<Base> value) { return os << "i: " <<  value->i; }
};

class Derived : public Base {
    int x;
public:
    Derived(int _x) : Base(2*_x), x(_x) {}

    friend inline ostream& operator<<(ostream& os, Derived& value) { return os << "x: " << value.x; }
    friend inline ostream& operator<<(ostream& os, shared_ptr<Derived> value) { return os << "x: " << value->x; }
};

int main () {
    Base* a = new Base(1);
    Derived* d = new Derived(6);
    Base* b = new Derived(7);

    shared_ptr<Base> ba = make_shared<Base>(3);
    shared_ptr<Derived> de = make_shared<Derived>(4);
    shared_ptr<Base> bd = make_shared<Derived>(5);

    cout << "a is: " << a << endl;
    cout << "*a is: " << *a << endl;
    cout << "d is: " << d << endl;
    cout << "*d is: " << *d << endl;
    cout << "b is: " << b << endl;
    cout << "*b is: " << *b << endl << endl;

    cout << "ba is: " << ba << endl;
    cout << "*ba is: " << *ba << endl;
    cout << "de is: " << de << endl;
    cout << "*de is: " << *de << endl;
    cout << "bd is: " << bd << endl;
    cout << "*bd is: " << *bd << endl;

    delete a;
    delete d;
    delete b;
    return 0;
}

live code

这吐出

a is: 0x1fe2bb0
*a is: i: 1
d is: 0x1fe2bd0
*d is: x: 6
b is: 0x1fe2bf0
*b is: i: 14

ba is: i: 3
*ba is: i: 3
de is: x: 4
*de is: x: 4
bd is: i: 10
*bd is: i: 10

但我希望看到* b print 7和bd print 5（即使用Derived类＆＃39;插入运算符）

Answer 1

好的，在搜索了很多之后，我找到了this回答。在打了我的脑袋并说“当然＆＃39;”后，我总结道：

不要重载插入操作符，因为你不能使它成为虚拟的（它不是成员函数），而是在子类中声明一个在子类中重写的虚函数。插入运算符使用此函数：

class Base {
    ...
    friend inline ostream& operator<<(ostream& os, shared_ptr<Base> value) { return value->display(os, value); }
    virtual ostream& display(ostream& os) { os << "i: " << i << endl; }
};

class Derived : public Base {
    ...
    ostream& display(ostream& os) { os << "x: " << x << endl; }
};

live code

如何使用具有多态性

1 个答案: