GCC和Clang对constexpr构造函数的不同行为 [英] GCC and Clang different behaviors on constexpr constructor

问题描述

对于此结构：

  struct Wrapper {
 int value; 
 
 constexpr explicit Wrapper（int v）noexcept：value（v）{} 
 Wrapper（const Wrapper& that）noexcept：value（that.value）{} 
}; 
  

此功能：

  constexpr Wrapper makeWrapper（int v）
 {
 return Wrapper（v）; 
} 
  

以下代码无法为Clang（Apple LLVM 7.3.0版） ，但是对于 -Wall -Wextra -Werror -pedantic-errors ：<$ p

>编译GCC（4.9+） pre> constexpr auto x = makeWrapper（123）;


Clang抱怨非constexpr构造函数'Wrapper'不能用于常量表达式。哪个编译器是正确的？

解决方案

虽然复制或移动时返回 Wrapper 从 makeWrapper（）可以省略，它需要与C ++ 14存在。现有的拷贝构造函数是非 - constexpr ，它的存​​在禁止创建一个隐式移动构造函数。因此，我认为clang是正确的：你需要使复制构造函数 constexpr 。

注意使用C ++ 17代码可能会正确：在一些上下文中有一个建议使复制限制是强制的： P0135r0 。然而，似乎这种变化还没有落在工作文件中。它可能会在本周登陆，虽然（感谢@NicolBolas指出，它不存在，但）。我没有在邮件中看到更新的文章。

For this struct:

struct Wrapper {
    int value;

    constexpr explicit Wrapper(int v) noexcept : value(v) {}
    Wrapper(const Wrapper& that) noexcept : value(that.value) {}
};

And this function:

constexpr Wrapper makeWrapper(int v)
{
    return Wrapper(v);
}

The following code fails to compile for Clang (Apple LLVM version 7.3.0), but compiles fine for GCC (4.9+), both with -Wall -Wextra -Werror -pedantic-errors:

constexpr auto x = makeWrapper(123);

Clang complains that "non-constexpr constructor 'Wrapper' cannot be used in a constant expression." Which compiler is right?

解决方案

Although the copy or move when returning Wrapper from makeWrapper() can be elided, it is required to exist with C++14. The existing copy constructor is non-constexpr and its existence inhibits creation of an implicit move constructor. As a result I think clang is right: you'd need to make the copy constructor a constexpr.

Note that with C++17 the code might become correct: there is a proposal to make copy-elision mandatory in some contexts: P0135r0. However, it seems this change hasn't landed in the working paper, yet. It may land this week, though (thanks to @NicolBolas for pointing out that it isn't there, yet). I haven't seen an updated paper in the mailing.

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