is_trivially_copyable和is_trivially_copy_constructible有什么区别？ [英] What is the difference between is_trivially_copyable and is_trivially_copy_constructible?

问题描述

这些给出不同的答案，何时会有用，如果有的话。

解决方案

测试 属性。

一个简单的可复制类是一个类：

- 没有非平凡的复制构造函数（12.8） p>

- 没有非平凡的移动构造函数（12.8），

- 没有非平凡的复制赋值运算符13.5.3，12.8），

- 没有非平凡的移动分配运算符（13.5.3,12.8）和

- 有一个简单的析构函数（12.4）。

一个简单的类是一个有一个琐碎的默认构造函数（12.1） / p>

[注意：特别是，一个简单的可复制或简单的类没有虚函数
或虚拟基类。 -end note ]

后者测试是否存在这顺带地是平凡可复制属性的要求。

类X的复制/移动构造函数是微不足道的，如果它是不是用户提供的，如果

- 类X没有虚拟函数（10.3），没有虚拟基类（10.1）和

- 为每个非静态数据成员选择复制/移动每个直接基类子对象的构造函数是微不足道的，

（或其数组），选择复制/移动该成员的构造函数
是微不足道的;

否则复制/移动构造函数是不平凡的。

很容易制作出能为这些特征提供不同结果的类型：

  struct foo {
 foo（foo const&）= default; //这是一个平凡的复制构造函数
〜foo（）; // this is a non-trivial destructor 
}; 
  

When would these give a different answer, and when would this difference be useful, if at all?

解决方案

The former tests for the trivially copyable property, which in few words means that the type is memcpy-safe.

A trivially copyable class is a class that:

— has no non-trivial copy constructors (12.8),

— has no non-trivial move constructors (12.8),

— has no non-trivial copy assignment operators (13.5.3, 12.8),

— has no non-trivial move assignment operators (13.5.3, 12.8), and

— has a trivial destructor (12.4).

A trivial class is a class that has a trivial default constructor (12.1) and is trivially copyable.

[ Note: In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes.—end note ]

The latter tests for the presence of a trivial copy constructor, which incidentally is a requirement for the trivially copyable property. It basically implies that the copy constructor for the type performs a bitwise copy.

A copy/move constructor for class X is trivial if it is not user-provided and if

— class X has no virtual functions (10.3) and no virtual base classes (10.1), and

— the constructor selected to copy/move each direct base class subobject is trivial, and

— for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial;

otherwise the copy/move constructor is non-trivial.

It is easy to fabricate a type that provides different results for these traits:

struct foo {
    foo(foo const&) = default; // this is a trivial copy constructor
    ~foo(); // this is a non-trivial destructor
};

这篇关于is_trivially_copyable和is_trivially_copy_constructible有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！