这是什么语法 - new（this）T（）; [英] what is this syntax - new (this) T();

问题描述

我遇到这样的C ++代码：

  T :: operator =（const T& t）
 {
 ... 
 new（this）T 
 ... 
} 
  

这行对我来说很陌生： code> new（this）T（t）;

我可以看到它是调用复制构造函数填充this ，但不知何故我只是不能理解的语法。猜猜我这么习惯 this = new T（t）;

你能帮我吗？ p>

解决方案

这就是所谓的新的布局运算符。它在括号中的表达式指定的地址处构造对象。例如，可以用以下方式定义复制赋值运算符。

  const C& C :: operator =（const C& other）{
 if（this！=& other）{
 this->〜C //生存期*这结束
 new（this）C（other）; // C类创建的新对象
} 
 return * this; 
} 
  

在本例中，首先使用显式调用析构函数，然后在这个地址使用复制构造函数创建一个新对象。

这是新的操作符不分配新的内存区。它使用已分配的内存。

此示例取自C ++标准。至于我，我不会返回一个const对象。将操作符声明为

更正确。 $ b

  C& C :: operator =（const C& other）; 
  

I run into such C++ code:

T& T::operator=(const T&t) 
{
   ...
   new (this) T(t);
   ...
}

This line looks so foreign to me:new (this) T(t);

I can see it is calling the copy constructor to populate "this", but somehow I just cannot make sense out of the syntax. Guess I am so used to this = new T(t);

Could you help me out?

解决方案

It is so-called the new placement operator. it constructs an object at address specified by the expression in parentheses. For example the copy assignment operator can be defined the following way

const C& C::operator=( const C& other) {
   if ( this != &other ) {
      this->~C(); // lifetime of *this ends
      new (this) C(other); // new object of type C created
   }
   return *this;
}

In this example at first the current object is destroyed using an explicit call of the destructor and then at this address a new object is created using the copy constructor.

That is this new operator does not allocate a new extent of memory. It uses the memory that was already allocated.

This example is taken from the C++ Standard. As for me I would not return a const object. It would be more correctly to declare the operator as

C& C::operator=( const C& other);

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