具有auto的initializer_list包含多个表达式 [英] initializer_list with auto contains multiple expressions

问题描述

一个简单的问题，

auto x11 {1,2,3,4};
auto x1 = {1,2,3,4};
auto x22 {1.0, 2.25, 3.5};
auto x2 = {1.0, 2.25, 3.5};

据我了解，此处是否具有=应该没有区别.但是，使用llvm/clang 6.0.0(带有--std = c ++ 17)，我得到了:

main1.cpp:35:17: error: initializer for variable 'x11' with type 'auto' contains multiple
  expressions
auto x11 {1,2,3,4};
~~~~~~~~    ^

main1.cpp:37:20: error: initializer for variable 'x22' with type 'auto' contains multiple
  expressions
auto x22 {1.0, 2.25, 3.5};

摘自Stroustroup的C ++书，第162页:

auto x1 {1,2,3,4}; // x1 is an initializer_list<int>
auto x2 {1.0, 2.25, 3.5 }; // x2 is an initializer_list of<double>

那么，那里没有 = 真的有问题吗?

解决方案

自动的规则类型推论自C ++ 17以来已更改.

(自C ++ 17起)
在直接列表初始化中(但不在复制列表初始化中)， 当从大括号初始化列表中推导auto的含义时， braced-init-list必须仅包含一个元素，并且auto的类型 将是该元素的类型:

auto x1 = {3}; // x1 is std::initializer_list<int>
auto x2{1, 2}; // error: not a single element
auto x3{3};    // x3 is int
               // (before C++17 x2 and x3 were both std::initializer_list<int>)

因此，在C ++ 17之前，示例中的所有变量都可以正常工作，并且类型为std::initializer_list<int>.但是从C ++ 17开始，对于直接初始化(即x11和x22)，支撑初始化器必须只包含一个元素(它们的类型将是元素的类型)，然后成为格式错误的代码. /p>

请参见 N3922 和 N3681 了解更多信息.

Rather simple question,

auto x11 {1,2,3,4};
auto x1 = {1,2,3,4};
auto x22 {1.0, 2.25, 3.5};
auto x2 = {1.0, 2.25, 3.5};

As far as I understand, there should be no difference here with respect to having = or not. However, using llvm/clang 6.0.0 (with --std=c++17), I get :

main1.cpp:35:17: error: initializer for variable 'x11' with type 'auto' contains multiple
  expressions
auto x11 {1,2,3,4};
~~~~~~~~    ^

main1.cpp:37:20: error: initializer for variable 'x22' with type 'auto' contains multiple
  expressions
auto x22 {1.0, 2.25, 3.5};

From Stroustroup's C++ book, page.162:

auto x1 {1,2,3,4}; // x1 is an initializer_list<int>
auto x2 {1.0, 2.25, 3.5 }; // x2 is an initializer_list of<double>

So, is there really a problem in not having = in there?

解决方案

The rule of auto type deduction changed since C++17.

(since C++17)
In direct-list-initialization (but not in copy-list-initalization), when deducing the meaning of the auto from a braced-init-list, the braced-init-list must contain only one element, and the type of auto will be the type of that element:

auto x1 = {3}; // x1 is std::initializer_list<int>
auto x2{1, 2}; // error: not a single element
auto x3{3};    // x3 is int
               // (before C++17 x2 and x3 were both std::initializer_list<int>)

So before C++17, all the variables in your sample work fine and have type std::initializer_list<int>. But since C++17, for direct initialization (i.e. for x11 and x22) the braced-initializer must contain only one element (and their type would be the type of the element) then become ill-formed code.

See N3922 and N3681 for more.

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