甚至在创建该类之前声明对象 [英] Declare an object even before that class is created

问题描述

在C ++中创建类之前，有没有声明一个类的对象？我问，因为我试图使用两个类，第一个需要有一个其中的第二个类的实例，但第二个类也包含第一个类的实例。我意识到你可能认为我可能进入一个无限循环，但我实际上需要在第一个类之前创建和实例的第二个类。

Is there anyway to declare an object of a class before the class is created in C++? I ask because I am trying to use two classes, the first needs to have an instance of the second class within it, but the second class also contains an instance of the first class. I realize that you may think I might get into an infinite loop, but I actually need to create and instance of the second class before the first class.

推荐答案

你不能这样做：

class A {
    B b;
};
class B {
    A a;
};

最明显的问题是编译器不知道如何大型需要做类A，因为B的大小取决于A的大小！

The most obvious problem is the compiler doesn't know how to large it needs to make class A, because the size of B depends on the size of A!

但是，你可以这样做：

class B; // this is a "forward declaration"
class A {
    B *b;
};
class B {
    A a;
};

将B类声明为前向声明允许您使用指针（和引用）具有整个类定义。

Declaring class B as a forward declaration allows you to use pointers (and references) to that class without yet having the whole class definition.

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