初始化顺序问题 [英] Initialization order issues

问题描述

给定代码示例：

class B {
    //Some contents.
};

class C {
    private:
        B& b;
};

class A {
    private:
        B b;
        C c;
};

类C具有对b的引用，因此需要使用它进行初始化。 A类包含B的实例和C的实例。

Class C has a reference to a b, so it needs to be initialized with it. Class A contains an instance of B and an instance of C.

我的问题是：我可以用A中的B实例初始化A中的C实例打扰构造函数）？第二，我需要在A中执行B的任何显式初始化，或者它是默认初始化的，因为它是类中的类类型。

My question is: Can I initialize the C instance in A with the B instance in A (assuming I did bother to put the constructors in)? Secondly, do I need to perform any explicit initialization of the B in A, or is it default initialized since its a class type within a class?

推荐答案

成员变量按它们在类声明中声明的顺序进行初始化（，即使您在构造函数的初始化列表中具有不同的顺序），因此是，正在初始化时间 c ，将初始化 b ，您可以使用 b 来初始化 c 。

Member variables are initialised in the order that they are declared in the class declaration (even if you have them in a different order in the initialisation list of the constructor), so yes, by the time c is being initialised, b will be initialised, and you can use b to initialise c.

如Ricardo Cardenes所说，在类定义中声明 c 之前的 b （这意味着您将传递 C :: C 对未初始化的 B 的引用），但如果您使用 C :: C 。首先声明 b 更安全，因为虽然你不能在 C ::里面使用 b C 现在，您可能会在未来忘记该引用指向未初始化的 B ，并导致UB。

As Ricardo Cardenes notes, it will still work even if you declare c before b in the class definition (which means you will pass C::C a reference to an uninitialised B) however you cause undefined behaviour if you use the object inside C::C. It's safer to declare b first, because although you may not use b inside C::C now, you might in the future and forget that the reference refers to an uninitialised B, and cause UB.

没有，你不必显式初始化 b （除非 POD ），除非你不希望它是默认构造的。所以这段代码是你想要的（再次，如果 B 不是POD）：

And no, you do not have to explicitly initialise b (unless it is POD) unless you don't want it to be default-constructed. So this code would be what you want (again, if B isn't POD):

A::A() : c(b) { }

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