c++ - 在 typedef 中使用相同的名称并从命名空间中使用

Question

示例代码:

struct X { void f() {} };
typedef X A;

namespace N { 
    struct A { void g() {} };
};

using N::A;

int main()
{
    A a;
    a.f();
}

此代码编译正确，A a; 创建一个 X，而不是 N::A。

标准中的哪条规则涵盖了这种行为？我原以为 A a; 会产生歧义。如果我将第一个结构命名为 A 而不是 X 并删除 typedef，那么我确实会收到这样的错误。 (g++ 8.3)

Answer 1

根据 [namespace.udecl]/1 using N::A 将未限定的 A 引入出现 using 声明的声明区域。

... the unqualified-id is declared in the declarative region in which the using-declaration appears as a synonym for each declaration introduced by the using-declarator.

所以歧义由[namespace.udecl]/13共同覆盖

Since a using-declaration is a declaration, the restrictions on declarations of the same name in the same declarative region also apply to using-declarations.

和[basic.scope.declarative]/4

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,

• they shall all refer to the same entity, or all refer to functions and function templates; or
• exactly one declaration shall declare a class name or enumeration name that is not a typedef name and the other declarations shall all refer to the same variable, non-static data member, or enumerator, or all refer to functions and function templates; in this case the class name or enumeration name is hidden.

OP 中的 typedef 声明和 using 声明都没有填写任何一个项目符号，因此同一声明区域中的这对声明格式错误。