C++:继承字段的初始化 [英] C++: Initialization of inherited field

问题描述

我有一个关于在派生类的构造函数中初始化继承成员的问题.示例代码:

I've a question about initialization of inherited members in constructor of derived class. Example code:

class A
    {
public:
    int m_int;
    };

class B: public A
    {
public:
    B():m_int(0){}
    };

这段代码给了我以下输出:

This code gives me the following output:

在构造函数'B::B()'中:第 10 行:错误:B"类没有任何名为m_int"的字段

(参见 http://codepad.org/tn1weFFP)

我猜为什么会这样?m_int应该是B的成员，并且m_int在中初始化时父类A应该已经被初始化>B 发生(因为父构造函数在继承类的成员初始化之前运行).我的推理哪里出错了?这段代码到底发生了什么?

I'm guessing why this happens? m_int should be member of B, and parent class A should already be initialized when initialization of m_int in B happens (because parent constructors run before member initialization of inherited class). Where is a mistake in my reasoning? What is really happens in this code?

EDIT:我知道初始化此成员的其他可能性(基构造函数或派生构造函数中的赋值)，但我想了解为什么我尝试它的方式是非法的?一些特定的 C++ 语言功能之类的?如果可能，请指出 C++ 标准中的一段.

EDIT: I'm aware of other possibilities to initialize this member (base constructor or assignment in derived constructor), but I want to understand why is it illegal in the way I try it? Some specific C++ language feature or such? Please point me to a paragraph in C++ standard if possible.

推荐答案

你需要为 A 创建一个构造函数(它可以被保护，所以只有 B 可以调用它)，它像你一样初始化 m_int，然后你调用 :A(0) 你有 :m_int(0)

You need to make a constructor for A (it can be protected so only B can call it) which initializes m_int just as you have, then you invoke :A(0) where you have :m_int(0)

您也可以在 B 的构造函数的主体中设置 m_int = 0.它是可访问的(正如您所描述的)它只是在特殊的构造函数语法中不可用.

You could also just set m_int = 0 in the body of B's constructor. It is accessible (as you describe) it's just not available in the special constructor syntax.

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