初始化标量用大括号 [英] Initializing scalars with braces

问题描述

在C和C ++，可以初始化数组，并使用大括号结构：

  int类型的[] = {2，3，5，7};
进入E = {答案，42};
 

不过，在从2007年谈话，Bjarne的提到这个语法也适用标量。我试了一下：

  INT I = {7};
 

和它的实际工作！什么是落后，允许标量的初始化用大括号什么道理呢？

请注意：我是专门的不的谈论C ++ 11均匀初始化。这是好老C89和C ++ 98。

解决方案

  

什么是落后，允许标量的初始化用大括号什么道理呢？

INT 是POD。因此，支架初始化是允许在 INT 的情况下（以及所有内建类型），因为它使初始化语法的相一致的其他豆荚。

另外，我想任何的理的背后C ++ 11的统一初始化语法的，也是（部分），适用于该语法由C ++ 03允许的。这仅仅是C ++ 03没有延伸，以包括非POD类型，如标准集装箱。

我可以看到一个地方，这个初始化是有用的C ++ 03。

 模板＆LT; typename的T＆GT;
无效F（）
{
    ŧ的obj = {大小（）}; // T是POD：内置型或POD结构
    // code
}
 

现在这可以用结构被实例化其用合适的成员，以及任何算术类型开始的：

 结构头
{
    为size_t的大小; //它是第一构件
    // ...
};F＆LT;报头GT;（）; //身体变得：头OBJ = {大小（）; };这是罚款
F＆LT;为size_t＆GT;（）; //身体变得：为size_t的obj = {大小（）; };这是罚款
 

另外请注意，POD，是否结构或内置类型，也可以均匀地初始化

 头H =头（）; //值初始化
INT I = INT（）; //值初始化
 

所以我相信的有一个的原因是一致！

In C and C++, one can initialize arrays and structs using braces:

int a[] = {2, 3, 5, 7};
entry e = {"answer", 42};

However, in a talk from 2007, Bjarne mentions that this syntax also works for scalars. I tried it:

int i = {7};

And it actually works! What is the rationale behind allowing the initialization of scalars with braces?

Note: I am specifically not talking about C++11 uniform initialization. This is good old C89 and C++98.

解决方案

What is the rationale behind allowing the initialization of scalars with braces?

int is POD. So the brace initialization is allowed in case of int (and for all build-in types), as it makes the initialization-syntax consistent with other PODs.

Also, I guess whatever rationale behind C++11 Uniform Initialization Syntax are, are also (partially) applicable to this syntax allowed by C++03. It is just C++03 didn't extend this to include non-pod types such as the standard containers.

I can see one place where this initialization is helpful in C++03.

template<typename T>
void f()
{
    T  obj = { size() } ; //T is POD: built-in type or pod-struct
    //code
}

Now this can be instantiated with struct which begins with a suitable member, as well as any arithmetic type:

struct header
{ 
    size_t size; //it is the first member
    //...
};

f<header>(); //body becomes : header obj = { size(); }; which is fine
f<size_t>(); //body becomes : size_t obj = { size(); }; which is fine

Also note that POD, whether struct or built-in types, can also be initialized uniformly as:

header h = header(); //value-initialized
int    i = int();    //value-initialized

So I believe one reason is consistency!

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