为什么g ++ 5在自动类型推导中推导对象而不是initializer_list [英] Why does g++5 deduces object instead of initializer_list in auto type deduction

问题描述

我最近看到这段代码：

struct Foo{};

int main() 
{
    Foo a;
    // clang++ deduces std::initializer_list
    // g++5.1 deduces Foo
    auto b{a}; 
    a = b;
}

它使用g ++ 5.1编译正常，但在clang ++ -std = c ++ 11 和 -std = c ++ 14 ，结果相同）。原因是 clang ++会将 b 的类型推断为 std :: initializer_list< Foo> ，而 g ++ 5.1 推断为 Foo 。 AFAIK，类型确实应该是（反直觉的） std :: initializer_list 这里。为什么g ++ 5推导类型为 Foo ？

It compiles fine with g++5.1, but fails in clang++ (used both -std=c++11 and -std=c++14, same results). The reason is that clang++ deduces the type of b as std::initializer_list<Foo>, whereas g++5.1 deduces as Foo. AFAIK, the type should indeed be (counter-intuitive indeed) std::initializer_list here. Why does g++5 deduces the type as Foo?

推荐答案

有一个建议C ++ 1z实现括号初始化的新类型扣除规则（ N3922 ），我想gcc实现了它们：

There is a proposal for C++1z that implements new type deduction rules for brace initialization (N3922), and I guess gcc implemented them:

对于直接列表初始化：

1.对于只有一个元素的braced-init列表，自动扣除将从该条目中推导出;

2.对于具有多个元素的支撑初始列表，自动扣除将无效。

For direct list-initialization:
1. For a braced-init-list with only a single element, auto deduction will deduce from that entry;
2. For a braced-init-list with more than one element, auto deduction will be ill-formed.

[示例：

auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
auto x3{ 1, 2 }; // error: not a single element
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
auto x5{ 3 }; // decltype(x5) is int. 

- 结束示例]

-- end example]

这里是 gcc patch 关于Unicorn初始化的新更改。

Here is the gcc patch concerning the new changes with regards to "Unicorn initialization."

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