具有简单类型说明符的显式类型转换（功能表示法）

Question

Porting some code I have discovered that line

unsigned char uc = unsigned char(c);

is accepted by MSVC but rejected by GCC. Is this syntax correct?
Standard says that

A simple-type-specifier (7.1.7.2) ... followed by a parenthesized optional expressionlist or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer

Does it mean that MS is right? Is unsigned char a 'simple-type-specifier'?

Answer 1

GCC and CLANG are correct, the code is not valid.

Simple type specifier is single-word type name:

The simple type specifiers are

 simple-type-specifier: nested-name-specifieropt type-name nested-name-specifier template simple-template-id nested-name-specifieropt template-name char char16_t char32_t wchar_t bool short int long signed unsigned float double void auto decltype-specifier type-name: class-name enum-name typedef-name simple-template-id decltype-specifier: decltype ( expression ) decltype ( auto ) 

unsigned char is not a simple-type-specifier, it's a combination of simple-type-specifiers, as shown in Table 9 from standard.

Table [tab:simple.type.specifiers] summarizes the valid combinations of simple-type-specifiers and the types they specify.

Table 9 — simple-type-specifiers and the types they specify

 Specifier(s) Type ... unsigned char “unsigned char” ... 

Here 's an explanation from cppreference.com:

2) 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), followed by a single expression in parentheses.

Answer 2

或者您可以使用：

unsigned char uc = static_cast<unsigned char>(c);