g ++可变参数模板问题 [英] g++ variadic template issue

问题描述

因此，我将此程序提供给了g ++和clang(在Linux和x86_64上均如此):

So I gave this program to g++ and clang (both on Linux, x86_64):

#include <iostream>

using namespace std;

template<char... Cs>
struct A {
  static const string s;
  static A a;
  ~A() {
    cout << "s = " << s << "\n";
  }
};

template<char... Cs>
const string A<Cs...>::s = {{Cs...}};

template<char... Cs>
A<Cs...> A<Cs...>::a;

int main(void)
{
  (void)A<'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a'>::a;

  return 0;
}

Clang输出s = aaaaaaaaaaaaaaaa(按预期).

Clang outputs s = aaaaaaaaaaaaaaaa (as expected).

g ++(版本5到8)输出s = s = aaaaaaaa(非常意外).

g++ (versions 5 until 8) outputs s = s = aaaaaaaa (pretty unexpected).

如果不使用可变参数模板，则不会发生这种情况(如果删除所有<>代码并内联字符列表以初始化A::s.

This doesn't happen if you don't use the variadic template (if you remove all the <> code and inline the character list to initialize A::s.

如果将std::string替换为字符数组(并改为使用A<Cs...>::s = {Cs...})，也不会发生这种情况.

It also doesn't happen if you replace the std::string by a character array (and use A<Cs...>::s = {Cs...} instead).

这不是代码，还是编译器错误?

Is this code not meant to be, or is it a compiler bug?

推荐答案

您的代码不正确.该标准的重要部分是6.6.3/1 [basic.start.dynamic] 在N4659中:

Your code is incorrect. The important part of the standard is 6.6.3/1 [basic.start.dynamic] in N4659:

具有静态存储持续时间的非局部变量的动态初始化是无序的，如果该变量是 隐式或显式实例化的专业化[...]

Dynamic initialization of a non-local variable with static storage duration is unordered if the variable is an implicitly or explicitly instantiated specialization [...]

因为初始化没有顺序，所以您不能依赖销毁顺序.任何命令都是合法的，无论构造顺序如何.参见6.6.4/3 [basic.start.term]

Because the initialization is not ordered, you cannot rely on the order of destruction. Any order is legal, regardless of order of construction. See 6.6.4/3 [basic.start.term]

gcc在销毁a之前销毁s， 这是发生的并导致奇怪的输出. 实时.

gcc is thus allowed to destroy s before it destroys a, which is what happens and causes the weird output. Live.

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