c++ - C++：如何创建两个相互使用的类

Question

这个问题只在类相互作为成员类型时给出答案，而不是在它们相互使用函数时给出答案。如何在下面的例子中做到这一点？Bar是一个单例容器，它处理对所有Bars 完成的操作。

// Foo.h

#include "Bar.h"

class Foo {
public:
    Foo() { Bar::get_instance()->push_back(this); }

    void do_something() {}
};

// Bar.h

#include "Foo.h"
#include <vector>

class Bar {
public:
    Bar(const Bar &other) = delete;

    void operator=(const Bar &) = delete;

    static Bar *get_instance()
    {
        static Bar bar;
        return &bar;
    }

    void push_back(Foo *foo) { foos_.push_back(foo); }

    void do_something_all() { for (Foo *f : foos_) f->do_something(); }

private:
    Bar() = default;

    std::vector<Foo *> foos_;
};

标签： c++c++14

Answer 1

你把它分开。定义类，但将实现部分放在类定义之后。

class Foo {
public:
    Foo(); // declare but do not implement

    // this can be implemented here since it doesn't need Bar:
    void do_something() { std::cout << "Foo::do_something()\n"; }
};

class Bar {
public:
    Bar(const Bar&) = delete;
    void operator=(const Bar&) = delete;

    static Bar& get_instance() { // return a reference instead
        static Bar bar;
        return bar;
    }

    // you can store a pointer - but require a reference:
    void push_back(Foo& foo) { foos_.push_back(&foo); }

    // or else you need to check for nullptr in here:
    void do_something_all() { for(Foo* f : foos_) f->do_something(); }

private:
    Bar() = default;

    std::vector<Foo*> foos_;
};

// Implementation part:
// Here Foo can use Bar

Foo::Foo() {
    Bar::get_instance().push_back(*this);
}

演示