std :: variant和boost :: variant之间有什么区别？ [英] What are the differences between std::variant and boost::variant?

问题描述

在此问题的答案中：

C ++标准库中的boost :: variant等效于什么？

有人提到 boost :: variant 和 std :: variant 有所不同。

• 就使用这些类的人而言，有什么区别？


• 有什么动机委员会是否表示采用具有这些差异的 std :: variant ？


• 使用这两个代码进行编码时应注意什么？ ，以保持与切换到另一个的最大兼容性？

（动机是使用 boost :: variant 在C ++ 17之前的代码中）

解决方案

• 分配/安置行为：

  
  
  
  
  

  • boost :: variant 可能< a href = http://www.boost.org/doc/libs/1_62_0/doc/html/variant/design.html#variant.design.never-empty.heap-backup-solution rel = noreferrer>在执行赋值时将其分配给实时的变量。有 a控制何时发生的规则数量，因此 boost :: variant 是否分配内存取决于 Ts 实例化。

  
  

  • std :: variant 将从不动态分配内存。但是，作为对C ++对象复杂规则的让步，如果抛出分配/放置，则变体 可以进入 valueless_by_exception状态。在这种状态下，不能访问变体，也不能访问用于访问特定成员作品的任何其他功能。

    
    
    

    只有在分配/安置抛出时，您才能进入此状态。

  
  



• 升压。变体包括 recursive_variant ，其中允许变体包含自身。实际上，它们是指向 boost :: variant 的指针的特殊包装，但它们与访问机制绑定在一起。

  
  
  

  std :: variant 没有此类帮助程序类型。




• std :: variant 提供了更多使用C ++ 11后功能的功能。例如：

  
  
  
  
  

  • 它将转发特殊成员的 noexcept 状态

  
  

  • 它具有基于可变参数模板的就地构造函数和放置功能。<​​/ p>

  
  

  • 缺陷解决方案应用于C ++ 17可能意味着它还将转发其类型的普通可复制性。也就是说，如果所有类型都是微不足道的，那么 variant< Ts> 也是如此。

  
  

In an answer to this SO question:

What is the equivalent of boost::variant in the C++ standard library?

it is mentioned that boost::variant and std::variant differ somewhat.

• What are the differences, as far as someone using these classes is concerned?
• What motivation did the committee express to adopt std::variant with these differences?
• What should I watch out for when coding with either of these, to maintain maximum compatibility with switching to the other one?

(the motivation is using boost::variant in pre-C++17 code)

解决方案

• Assignment/emplacement behavior:

  • boost::variant may allocate memory when performing assignment into a live variant. There are a number of rules that govern when this can happen, so whether a boost::variant will allocate memory depends on the Ts it is instantiated with.

  • std::variant will never dynamically allocate memory. However, as a concession to the complex rules of C++ objects, if an assignment/emplacement throws, then the variant may enter the "valueless_by_exception" state. In this state, the variant cannot be visited, nor will any of the other functions for accessing a specific member work.

    You can only enter this state if assignment/emplacement throws.

• Boost.Variant includes recursive_variant, which allows a variant to contain itself. They're essentially special wrappers around a pointer to a boost::variant, but they are tied into the visitation machinery.

  std::variant has no such helper type.

• std::variant offers more use of post-C++11 features. For example:

  • It forwards the noexcept status of the special member functions of its constituent types.

  • It has variadic template-based in-place constructors and emplacement functions.

  • Defect resolutions applied to C++17 may mean that it will also forward trivial copyability of its types. That is, if all of the types are trivially copyable, then so too will variant<Ts>.

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