braced-init-list and unsigned types

问题

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 reports an error such as

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

Visual C++ 2015 and 2017 accept this.

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

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?

回答1:

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

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.

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

This is same for functional cast expression,

The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name: unsigned int(expression) or int*(expression) are not valid),

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

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

回答2:

"Int" is optional in "unsigned int".

Try simply:

unsigned ui = unsigned{};

It works.

Edit: I found this answer: Standard behavior for direct initialization of unsigned short

It's just because only single-word type name could be used in functional cast expression, while unsigned short is not a single-word type name; short is.

来源：https://stackoverflow.com/questions/53336344/braced-init-list-and-unsigned-types