为什么类的初始化器只使用=或{}？ [英] Why can in-class initializers only use = or {}?

问题描述

课程初始设定（C ++ 11功能）必须以卷曲形式大括号或遵循a =符号。

这是什么原因？

解决方案

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我不是100％积极的这一点，但这可能是为了防止语法歧义。例如，考虑以下类：

  class BadTimes {
 struct Overloaded; 
 int重载; //法律，但一个很奇怪的想法。 
 
 int confusing（Overloaded）; //<  -  this line 
}; 
  

指示的行是什么意思？如所写的，这是一个名为的成员函数的声明，该函数接受 Overloaded （<其名称未在函数声明中指定），并返回 int 。如果C ++ 11允许初始化器使用括号，这将是不明确的，因为它也可以是 int 类型的成员的定义混淆，它被初始化为数据成员 Overloaded 的值。 （这与最常见解析解析的当前问题相关。）

通过要求花括号，这个歧义被删除：

  class BadTimes {
 struct Overloaded; 
 int重载; //法律，但一个很奇怪的想法。 
 
 int confusing {Overloaded}; //<  -  this line 
}; 
  

现在，很清楚，混淆一个 int 初始化为重载的值，因为没有办法读取它作为函数声明。 >

希望这有助于！

In-class initializers (C++11 feature) must be enclosed in curly braces or follow a = sign. They may not be specified inside parenthesis.

What is the reason for this?

解决方案

I am not 100% positive about this, but this might be to prevent a syntax ambiguity. For example, consider the following class:

class BadTimes {
    struct Overloaded;
    int Overloaded;            // Legal, but a very strange idea.

    int confusing(Overloaded); // <-- This line
};

What does the indicated line mean? As written, this is a declaration of a member function named confusing that accepts as a parameter an object of type Overloaded (whose name isn't specified in the function declaration) and returns an int. If C++11 were to allow initializers to use parentheses, this would be ambiguous, because it could also be a definition of a member of type int named confusing that is initialized to the value of the data member Overloaded. (This is related to the current issue with the Most Vexing Parse.)

By requiring curly braces, this ambiguity is removed:

class BadTimes {
    struct Overloaded;
    int Overloaded;            // Legal, but a very strange idea.

    int confusing{Overloaded}; // <-- This line
};

Now, it's clear that confusing is actually an int initialized to the value of Overloaded, because there's no way to read it as a function declaration.

Hope this helps!

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