问题描述

在这个 SO 问题的答案中:

In an answer to this SO question:

C++ 标准库中 boost::variant 的等价物是什么?

提到boost::variant和std::variant有些不同.

• 就使用这些类的人而言，有什么区别?
• 委员会表示采用具有这些差异的 std::variant 的动机是什么?
• 使用其中任何一个进行编码时我应该注意什么，以保持切换到另一个的最大兼容性?

(动机是在 C++17 之前的代码中使用 boost::variant)

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

推荐答案

• 分配/安置行为:

  • Assignment/emplacement behavior:

    • boost::variant 可以在执行赋值时分配内存到实时variant.有 a控制何时发生这种情况的规则数量，所以 boost::variant 是否会分配内存取决于它被实例化的 Ts.

    • 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 将永远动态分配内存.但是，作为对 C++ 对象复杂规则的让步，如果赋值/定位抛出，则 variant 可能 进入valueless_by_exception"状态.在这种状态下，不能访问 variant，也不能访问任何其他访问特定成员的函数.

    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.

    您只能在分配/安置抛出时进入此状态.

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

    Boost.Variant 包括 recursive_variant，其中 允许 variant 包含自身.它们本质上是对 boost::variant 指针的特殊包装，但它们与访问机制相关联.

    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 没有这样的辅助类型.

    std::variant has no such helper type.

    std::variant 提供了对 C++11 后特性的更多使用.例如:

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

    • 它转发其组成类型的特殊成员函数的noexcept状态.

    它具有基于可变参数模板的就地构造函数和定位函数.

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

    缺陷解决方案 应用于 C++17 可能意味着它也将转发其类型的微不足道的可复制性.也就是说，如果所有类型都可以简单地复制，那么 variant 也是如此.

    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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