参考班级成员琐事 [英] Reference Class Member Triviality

问题描述

让我们说，出于我自己的原因，我希望一个类具有一个非静态引用成员.我认为应该从使用它的大多数代码中轻松优化此类型.因此，我在单元测试中断言了它的微不足道.

Let's say, for reasons of my own, I want a class to have a non-static reference member. I would think that this type should be easily optimized out of most code that uses it. Therefore, I assert its triviality in a unit test.

Clang和GCC同意该类是微不足道的，但MSVC则不同意.根据标准，谁是正确的，为什么是正确的?

Clang and GCC agree that the class is trivial, but MSVC disagrees. Who is right, and why, according to the standard?

#include <type_traits>

struct example {
    int& r;
};


// Clang and GCC let this pass
// MSVC fires this assertion
static_assert(
    std::is_trivial<example>::value,
    "example is not trivial"
);

推荐答案

根据C ++ 17 [class]/6，对于一个琐碎的类，除其他要求外，它至少必须具有一个未删除的类默认构造函数. example类的默认构造函数被删除，因此example并不是一个简单的类.

According to C++17 [class]/6, for a class to be trivial, among other requirements, it has to have at least one non-deleted default constructor. The example class's default constructor is deleted, so example is not a trivial class.

在C ++ 17之前，情况还不太清楚.平凡的类需要具有平凡的默认构造函数，并且尚不清楚是否删除的默认构造函数符合条件.理查德·史密斯(Richard Smith)在 CWG 1928 中询问，默认和隐式删除的特殊成员函数是否微不足道.委员会的答复是:

Before C++17, the situation is somewhat less clear. It was required for a trivial class to have a trivial default constructor, and it was not clear whether a deleted default constructor qualifies as trivial. Richard Smith asked in CWG 1928 whether special member functions that are defaulted and implicitly deleted are trivial. The Committee's response was:

CWG认为删除功能的琐碎无关紧要.任何可以观察到删除功能的琐碎性的情况都应进行修改，以消除该依赖关系.

CWG feels that the triviality of a deleted function should be irrelevant. Any cases in which the triviality of a deleted function is observable should be amended to remove that dependency.

随后， CWG 1496 的分辨率解决了普通类的问题:不再重要删除的默认构造函数可以认为是微不足道的，因为在C ++ 17中，定义表明 all 所有默认构造函数(如果有多个)必须琐碎或删除，并且至少一个必须被删除.

Subsequently, the resolution of CWG 1496 resolved this issue in the case of trivial classes: it no longer matters whether a deleted default constructor qualifies as trivial, because in C++17, the definition says that all default constructors (in case there are multiple) must be either trivial or deleted and at least one must be non-deleted.

似乎GCC和Clang可能一直在考虑将删除的默认构造函数视为琐碎的事情，并且尚未使用琐碎的类"的C ++ 17定义进行更新.您应该提交错误报告.

It seems that GCC and Clang may have been considering deleted default constructors as trivial, and haven't been updated with the C++17 definition of "trivial class". You should file bug reports.

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