const auto std :: initializer_list Clang和GCC之间的区别

问题描述

限时删除！！

我想了解当结合初始化列表和 const auto 时，C ++ 11应该是正确的行为。对于下面的代码，我在GCC和Clang之间得到不同的行为，并想知道哪个是正确的：

  #include < iostream> 
 #include< typeinfo> 
 #include< vector> 
 
 int main（）
 {
 const std :: initializer_list< int> l1 = {1，2，3}; 
 const auto l2 = {1，2，3}; 
 
 std :: cout<< explicit：<< typeid（l1）.name（）<< std :: endl; 
 std :: cout<< auto：<< typeid（l2）.name（）<< std :: endl; 
} 
  

使用g ++编译输出为：

  explicit：St16initializer_listIiE 
 auto：St16initializer_listIKiE 
  

虽然clang ++编译版本产生：

  explicit：St16initializer_listIiE 
 auto：St16initializer_listIiE 
  

似乎GCC正在将 auto c $ c> std :: initializer_list< const int> ，而Clang生成 std :: initializer_list< int> 。 GCC版本创建一个问题，当我使用它来初始化 std :: vector 。所以下面的工作在Clang下，但为GCC产生一个编译器错误。

  //在clang下编译，但对于GCC失败，因为l4 
 std :: vector< int& v2 {l2}; 
  

如果GCC生成正确的版本，那么似乎建议将各种STL容器扩展到对这些情况包括另一个列表初始化器重载。

注意：这种行为似乎在多个版本的GCC（4.8,4.9,5.2）和Clang（3.4和3.6）之间是一致的。

解决方案

GCC错误。 [dcl.spec.auto] / p7（引用N4527）：

/ code>，如同对于功能模板

 模板< class U> void meow（const std :: initializer_list< U>）; 
  

给予呼叫 meow（{1，2，3} code>。

现在考虑一下const auto l3 = {1，2，3}; （其中GCC正确地推导为 std :: initializer_list< int> ）。在这种情况下的扣除被执行，如同对于功能模板

  template< class U> void purr（std :: initializer_list< U>）; 
  

给予调用 purr（{1，2，3} code>。

由于函数参数的顶级cv限定被忽略，所以显然应该产生相同的类型。 p>

---

[temp.deduct.call] / p1：

Deducing <$对 1 ，<$ c> P'（ U $ c> 2 或 3 ，所有类型 int 的文本， code> int 。

I am trying to understand what the correct behavior of C++11 should be when combining initialization lists and const auto. I am getting different behavior between GCC and Clang for the following code and would like to know which is the correct one:

#include <iostream>
#include <typeinfo>
#include <vector>

int main()
{
    const std::initializer_list<int> l1 = { 1, 2, 3 };
    const auto l2 = { 1, 2, 3 };

    std::cout << "explicit: " << typeid(l1).name() << std::endl;
    std::cout << "auto:     " << typeid(l2).name() << std::endl;
}

Compiled with g++ the output is:

explicit: St16initializer_listIiE
auto:     St16initializer_listIKiE

While the clang++ compiled version produces:

explicit: St16initializer_listIiE
auto:     St16initializer_listIiE

It seems that GCC is turning the auto line into a std::initializer_list<const int> while Clang produces std::initializer_list<int>. The GCC version creates a problem when I use it to initialize a std::vector. So the following works under Clang but produces a compiler error for GCC.

// Compiles under clang but fails for GCC because l4
std::vector<int> v2 { l2 };

If GCC is producing the correct version then it seems to suggest that the various STL containers should be extended to include another list initializer overload for these cases.

Note: this behavior seems to be consistent across multiple versions of GCC (4.8, 4.9, 5.2) and Clang (3.4 and 3.6).

解决方案

GCC bug. [dcl.spec.auto]/p7 (quoting N4527):

Thus, in const auto l2 = { 1, 2, 3 };, deduction is performed as if for the function template

template<class U> void meow(const std::initializer_list<U>);

given the call meow({1, 2, 3}).

Now consider the const-less case auto l3 = { 1, 2, 3 }; (which GCC correctly deduces as std::initializer_list<int>). Deduction in this case is performed as if for the function template

template<class U> void purr(std::initializer_list<U>);

given the call purr({1, 2, 3}).

Since top-level cv-qualification of function parameters are ignored, it should be obvious that the two deduction should yield the same type.

---

[temp.deduct.call]/p1:

Deducing P' (which is U) against 1, 2, or 3, all literals of type int, obviously yields int.

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