使用类成员的声明应该是成员声明（C ++ 2003） [英] Using declaration for a class member shall be a member declaration (C++2003)

问题描述

第7.3.3段。的C ++ 2003标准规定

Paragraph 7.3.3. of C++2003 standard states that

使用类成员的声明是成员声明。

Using declaration for a class member shall be a member declaration.

这意味着以下语法错误：

This means that the following gives a syntax error:

struct S1
{
    static int var1;
};

using S1::var1;

以下编译正确：

namespace N2
{
    int var2;
}

using N2::var2;

有没有人知道背后的理由（如果有）？

Does anybody know the rationale (if any) behind that?

甚至，标准给出了struct的静态数据成员的显式示例，并指出它应该导致语法错误。 MS C ++给出此错误：

Even more, the standard gives explicit example with static data member of the struct and tells that it should cause syntax error. MS C++ gives this error:

cpptest1.cxx（9）：error C2885：'S1 :: var1'：not-在非类范围的声明

cpptest1.cxx(9) : error C2885: 'S1::var1': not a valid using-declaration at non-class scope

仍然不清楚为什么应该禁止。

It is still not clear why this should be banned.

推荐答案

它不工作的原因是因为标准禁止它。请特别注意示例中的 s ：

The reason it doesn't work is simply because the Standard forbids it. Pay special attention to s in the example:

作为解决方法，您可以说：

As a workaround, you can say:

auto& var = S1::var;

使标准（因此所有编译器）更加复杂，以处理具有方便的解决方法的异常情况只是不通过成本效益测试。

Making the Standard (and therefore all compilers) more complicated to handle an unusual case that has a convenient workaround just doesn't pass the cost-benefit test.

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