c++ - 使用简单类型说明符的显式类型转换(功能符号)

Question

移植一些代码我发现了那一行

unsigned char uc = unsigned char(c);

被 MSVC 接受但被 GCC 拒绝。这个语法正确吗？
标准说

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

是不是意味着MS是对的？ unsigned char 是“简单类型说明符”吗？

Answer 1

GCC和 CLANG是正确的，代码无效。

Simple type specifier是单字类型名称:

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 不是简单类型说明符，它是简单类型说明符的组合，如Table 9 所示。从标准。

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来自 cppreference.com 的解释:

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.