括号初始化列表和无符号类型 [英] braced-init-list and unsigned types

问题描述

gcc 8和clang 7不接受以下代码，这些代码应默认构造 unsigned int 类型的临时代码:

gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int:

unsigned int ui = unsigned int{};

clang 7报告错误，例如

clang 7 reports an error such as

<source>:6:22: error: expected primary-expression before 'unsigned'

Visual C ++ 2015和2017接受此要求.

Visual C++ 2015 and 2017 accept this.

很明显，这适用于 int 之类的类型或任何默认可构造的类类型.

Obviously, this works with a type such as int, or any default-constructible class type.

这是正确的C ++ 14代码(在这种情况下是clang和gcc的错误)?如果没有，为什么不呢?除无符号类型外，还有哪些类型会受到相同的限制?

Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than unsigned types would suffer from the same restriction?

推荐答案

new_type {expression-list(可选)} (如 unsigned int {} )符合<一个href ="https://en.cppreference.com/w/cpp/language/explicit_cast" rel ="noreferrer">显式类型转换，该转换只允许使用单个单词类型名称.

new_type { expression-list(optional) } like unsigned int{} fits the syntax of explicit type conversion, which allows only single-word type name.

单字类型名称后跟一个括号初始列表，是指定类型的prvalue.指定一个临时对象(直到C ++ 17) 其结果对象是(因为C ++ 17)直接用指定的braced-init-list初始化列表.

A single-word type name followed by a braced-init-list is a prvalue of the specified type designating a temporary (until C++17) whose result object is (since C++17) direct-list-initialized with the specified braced-init-list.

请注意， unsigned int 不是单字类型名称，而 int 是.因此 int {} 可以正常工作.

Note that unsigned int is not a single-word type name, while int is. So int {} works fine.

对于功能强制转换表达式，

函数强制转换表达式由一个简单的类型说明符或一个typedef说明符组成(换句话说，一个单词类型名称: unsigned int(expression)或 int *(expression)无效)，

作为解决方法，您可以应用类型别名，例如

As a workaround, you can apply type alias, e.g.

using type = unsigned int;
type ui = type{};

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