c++ - 为什么 char 既没有符号也没有符号，而 wchar_t 是？

Question

下面的 C++ 程序编译没有错误:

void f(char){}
void f(signed char){}
void f(unsigned char){}
int main(){}  

同一程序的 wchar_t 版本不会:

void f(wchar_t){}
void f(signed wchar_t){}
void f(unsigned wchar_t){}
int main(){}

错误:‘void f(wchar_t)’的重新定义
void f(signed wchar_t){}

wchar_t 似乎是unsigned。
为什么重载会出现不一致？

Answer 1

char 都是不同的类型，可以重载

[基础.基础]/1

[...] Plain char, signed char, and unsigned char are three distinct types, collectively called narrow character types. [...]

wchar_t也是一个distinct类型，但是不能用signed或者unsigned来限定，只能和标准整数一起使用类型。

[dcl.类型]/2

As a general rule, at most one type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a type-specifier-seq or trailing-type-specifier-seq. The only exceptions to this rule are the following:

[...]

signed or unsigned can be combined with char, long, short, or int.

[dcl.type.simple]/2

[...] Table 9 summarizes the valid combinations of simple-type-specifiers and the types they specify.

wchar_t 的符号是实现定义的:

[基础.基础]/5

[...] Type wchar_t shall have the same size, signedness, and alignment requirements (3.11) as one of the other integral types, called its underlying type.