C ++如何初始化成员对象？ [英] C++ How to Initialize member object?

问题描述

在下面的例子的第一部分有一些方法来初始化（？）类型C的成员变量c？或者必须使用示例第二部分中所示的new（）方法？

Is there some way to initialize(?) member variable c of type C in the first part of the below example? Or must I use the new() method shown in the second part of the example?

B类将A类视为注入依赖。 B类也是由C类组成的。

Class B takes class A as an injected dependency. So does class C. Class B is additionally composed of class C.

如何获取注入的A到B的成员C？

How do I get the injected A to B's member C?

第1部分

class A { // ...; };

class C {
public:
    C(A &a) : a(a) {}   // constructor
};


// Does not work as is.  Want to make compiler manage C lifetime.
class B {
public:
    B(A &a);    // constructor

    C c(a);     // member variable
};

// constructor
B::B(A &a) : a(a) {
}

第2部分

// Works, but requires programmer to manage C's lifetime.
class B {
public:
    B(A &a);    // constructor

    C *c;       // member variable
};

// constructor
B::B(A &a) : a(a) {
    c = new C(a);
}

下面几个好的答案！我的道歉为混乱的例子。我已经投票了所有好的答案和问题。不幸的是，我只能将一个答案标记为接受的答案，所以我选择第一个给我的啊哈时刻，我看到解决我的真正的问题，这比我的跛脚示例更复杂。

Several good answers below! My apologies for the confusing example. I have up-voted all of the good answers and questions. Unfortunately I can only mark one answer as the accepted answer, so I am choosing the first one which gave me the "ah-ha" moment in which I saw the solution to my real problem, which was more complex than my lame example here.

推荐答案

成员变量在构造函数的初始化列表中初始化（在body之前），因此您需要这样做：

Member variables are initialized in constructor's initialization list (before body) so you need to do that:

B::B(A &a)
    : c(a) // Calls constructor C(a) on member c
{}

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