初始化：括号对等号 [英] initialization: parenthesis vs. equals sign

问题描述

可能重复：

之间有什么区别

  ; 
  

和

  T a = b; 
  

和

  T a = T（b）; 
  

？

解决方案

  T a（b）; 
  

是直接初始化，除非它解析为函数声明，其中

  T a = b; 
  

是复制初始化，这意味着它的工作原理就像一个临时对象在右边构造，并且 a 然后被复制构造，或者在C ++ 11和更高版本中，可能从该临时构造移动构造。

编译器可以随时随地删除（删除）临时+复制/移动，但复制或移动构造函数（无论哪一个逻辑使用）都必须可访问， code>显式。

例如，在C ++ 03中，不能复制初始化 std :: ostringstream ，因为它没有复制构造函数。在C ++ 11中，如果初始化器是临时的，那么可以复制初始化 ostringstream ，这将导致逻辑​​移动构造（但是通常会被省略， ）。例如，此副本初始化声明

  ostringstream s = ostringstream（blah）; 
  

…不编译为C ++ 03，因为在C ++ 03中，复制初始化调用类的复制构造函数，它不存在。然而它编译为C ++ 11，因为在C + + 11复制初始化调用移动构造函数。而且（为了保持其作为流的幻觉），不能直接复制 std :: ostringstream ，它可以移动。 / p>

另一个这样的区别：在C ++ 03中只有复制初始化语法支持 initializer，在C ++ 03中， T 是聚合类型，例如原始数组。在C ++ 11中，大括号符号已经被扩展和推广为一个统一的初始化语法，因此它也可以用于直接初始化。因此，以下直接初始化声明，

  int v [] {3,1,4,1,5,9,2 ，6，5，4}; 
  

…不会编译为C ++ 03，但编译为C ++ 11和更高版本。

= 复制初始化语法是C的原始初始化语法。

在C ++ 11和更高版本中，由于移动语义，它可以在更宽的范围比C ++ 03中的情况，如使用 std :: ostringstream 。

Possible Duplicate:
Is there a difference in C++ between copy initialization and assignment initialization?

What's the difference between

T a(b);

and

T a = b;

and

T a = T(b);

?

解决方案

T a( b );

is direct initialization, unless it parses as a function declaration, in which case it's a function declaration.

T a = b;

is copy initialization, which means that it works as if a temporary object is constructed on the right hand side, and that a is then copy constructed or, in C++11 and later, possibly move constructed, from that temporary.

The compiler is free to elide (remove) the temporary+copying/moving whenever it can, but a copy or move constructor, whichever would be logically used, must still be accessible and not explicit.

For example, in C++03 you cannot copy-initialize a std::ostringstream, because it doesn't have a copy constructor. In C++11 you can copy-initialize an ostringstream if the initializer is a temporary, which then results in a logical move construction (which however will usually be elided, optimized away). For example, this copy initialization declaration,

ostringstream s = ostringstream( "blah" );

… doesn't compile as C++03, because in C++03 the copy initialization invokes the class' copy constructor, which doesn't exist. It does however compile as C++11, because in C++11 the copy initialization invokes the move constructor. And while (to maintain its illusion of being a stream) a std::ostringstream can't be directly copied, it can be moved.

Another such difference: in C++03 only the copy initialization syntax supports curly braces initializer, which in C++03 you can use when T is an aggregate type such as a raw array. In C++11 the curly braces notation has been extended and generalized as a uniform initialization syntax, so it can be used also with direct initialization. And so the following direct initialization declaration,

int v[]{ 3, 1, 4, 1, 5, 9, 2, 6, 5, 4 };

… does not compile as C++03, but does compile as C++11 and later.

The = copy initialization syntax is the original initialization syntax from C.

And in C++11 and later, due to move semantics, it can be used in a much wider range of cases than in C++03, such as with a std::ostringstream.

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