为什么 auto x{3} 会推导出一个 initializer_list? [英] Why does auto x{3} deduce an initializer_list?

问题描述

我喜欢 C++11 中的 auto.太棒了.但它有一个不一致的地方让我很紧张，因为我总是被它绊倒:

I love auto in C++11. It's wonderful. But it has one inconsistency that really gets on my nerves, because I trip over it all the time:

int i = 3;       // i is an int with value 3
int i = int{3};  // i is an int with value 3
int i(3);        // i is an int with value 3 (possibly narrowing, not in this case)
int i{3};        // i is an int with value 3

auto i = 3;      // i is an int with value 3
auto i = int{3}; // i is an int with value 3
auto i(3);       // i is an int with value 3
auto i{3};       // wtf, i is a std::initializer_list<int>?!

这种奇怪的行为让新手感到困惑，对有经验的用户来说很烦人——C++ 有足够多的小不一致和极端情况，人们必须牢记它.谁能解释为什么标准委员会决定在这种情况下引入一个新的标准?

This strange behaviour is confusing for newcomers, and annoying for experienced users -- C++ has enough little inconsistencies and corner cases that one has to keep in mind as it is. Can anybody explain why standards committee decided to introduce a new one in this case?

如果声明 std::initializer_list 类型的变量是有用的或经常做的事情，我可以理解，但根据我的经验，这几乎从来不是故意的——在极少数情况下，你确实想做，任何一个

I could understand it if declaring a variable of type std::initializer_list was something that was useful or done frequently, but in my experience it's almost never deliberate -- and in the rare cases where you did want to do it, any of

std::initializer_list<int> l{3};
auto l = std::initializer_list<int>{3};
auto l = {3}; // No need to specify the type

会很好用.那么 auto x{i} 特殊情况背后的原因是什么?

would work just fine. So what's the reason behind the special case for auto x{i}?

推荐答案

长话短说:

• 花括号初始化表达式 {} 本身没有类型
• auto 必须推断类型信息
• int{3} 显然意味着创建一个 int 变量，其值取自初始化列表"，因此它的类型只是 int 和可以在任何更广泛的上下文中使用(int i = int{3} 可以工作并且 auto i = int{3} 可以推断类型，因为右侧显然是类型int)
• {3} 本身没有类型(它不能是int，因为它不是值 但是一个 初始化列表)，所以 auto 不会工作——但是，因为委员会认为 auto 在这种情况下应该仍然有效，他们决定(是的，定义为无类型)初始化列表的最佳"类型是... std::initializer_list，正如您可能已经猜到的那样.

• a braced initializer expression {} has no type by itself
• auto has to infer type information
• int{3} obviously means "create an int var with value taken from initializer list", thus its type is just int and can be used in any wider context (int i = int{3} will work and auto i = int{3} can deduce type, because right side is obviously of type int)
• {3} by itself has no type (it can't be int, because it's not a value but an initializer list), so auto wouldn't work — but, because committee considered that auto should still work in this case, they decided that the "best" type for (yeah, typeless by definition) initializer list would be... std::initializer_list, as you already probably guessed.

但是，正如您所指出的，这使得 auto 的整个行为在语义上非常不一致.这就是为什么向委员会提交了更改它的提案 - 即 N3681、N3912 和 N3922.由于委员会就此事没有达成共识，之前的提案被 FI3 拒绝，http://isocpp.org/files/papers/n3852.html#FI3 ，当前 (N3922) 获得 采用 ca.2015年第一季度;

But, as you pointed out, this made the whole behaviour of auto quite semantically inconsistent. That's why there were proposals to change it — namely N3681, N3912 and N3922 — submitted to the committee. Former proposal was REJECTED as FI3 due to no committee consensus on this matter, http://isocpp.org/files/papers/n3852.html#FI3 , current (N3922) got adopted ca. Q1 of 2015;

tl;dr 你可以假设 符合标准的编译器¹ 带有前沿 C++support² 要么已经有了新的、更合理的语义，要么很快就会有.

tl;dr you may assume that standards-compliant compilers¹ with bleeding-edge C++ support² either have the new, more sane-ish semantics already in place, or will have it shortly.

标准化委员会通过在 C++17 草案中采用 N3922 承认了这个问题.

The Standardization Committee acknowledged the problem by adopting N3922 into draft C++17.

——原来是

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

现在，无论好坏.

_{进一步阅读:}

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3681.html

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3912.html

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3922.html

http://scottmeyers.blogspot.com/2014/03/if-braced-initializers-have-no-type-why.html

http://herbsutter.com/2014/11/24/updates-to-my-trip-report/

¹GCC 5.1 (& up) 即使在 C++11/C++14 模式下也明显使用 N3922

¹GCC 5.1 (& up) apparently uses N3922 even in C++11/C++14 mode

²Clang 3.8，有警告

²Clang 3.8, with the caveat

这是一个向后不兼容的更改，适用于所有允许从 auto 进行类型推导的语言版本(根据 C++ 委员会的要求).

This is a backwards-incompatible change that is applied to all language versions that allow type deduction from auto (per the request of the C++ committee).

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