使用声明不能在函数范围内重复。这是为什么？ [英] A using-declaration can not be repeated in function scope. Why is that?

问题描述

在[namespace.udecl] / 10中，您有以下示例：

In [namespace.udecl]/10 you have the following example:

namespace A {
    int i;
}
namespace A1 {
    using A::i;
    using A::i; // OK: double declaration
}
void f() {
    using A::i;
    using A::i; // error: double declaration
}

这段代码在clang中编译。

This snippet compiles in clang.

推荐答案

第一个是在命名空间内的声明，多个using语句可能会频繁使用#includes。第二个是在一个函数的定义内，你永远不会这样做，除非你犯了一个错误。例如，你不能定义相同的符号两次，但你可以声明几次。

The first is a declaration inside a namespace, and the multiple using statements could happen frequently using #includes. The second is inside a definition of a function, and you would never do that unless you made a mistake. You can't define the same symbol twice either, for example, but you can declare several times.

using语句也不仅仅是一个声明。它有点强大，因为它从一个命名空间导入一个函数到另一个。例如，它可以将受保护的基类成员函数拉入派生类中，使其成为公共的。这几乎是一个链接的定义。

The using statement is also more than just a declaration. It is a bit stronger, as it imports a function from one namespace to another. For example, it can pull a protected base class member function into a derived class, making it public. It's almost a definition by linkage.

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