使用声明和访问修饰符的嵌套类

Question

编译一些代码,我遇到了编译器错误,这对我来说似乎很奇怪,并且与继承,嵌套类,使用声明和访问修饰符有关.

基本上,目的是使派生类型公开基类型的嵌套受保护类.

以下简短示例来说明问题:

class Base
{
protected:

    struct Nested
    {
        enum value_enum
        {
            val = 0,
            val2,
            val3
        };
    };

};

class Derived : public Base
{
public:

    using Base::Nested;
};


int main(int argc, char** argv)
{
    //Base::Nested aa; // error, as (I) expected
    //Base::Nested::value_enum ab; // error, as (I) expected

    Derived::Nested ba; // works, as (I) expected
    Derived::Nested::value_enum bb; // MSVC error, as (I) did not expect


    return 0;
}

看到现场.

MSVC11(v11.00.61030)对此代码进行了阻塞,并出现以下错误:

错误C2248:'Base :: Nested':无法访问类'Base'中声明的受保护结构

GCC和Clang都正确地编译了这个,因此如果没有引用标准中相关部分的能力,我认为这是一个MSVC错误.

有没有办法解决这个与MSVC？

Answer 1

对于 MSVC，以下解决方法有效：

class Derived : public Base
{
    public:

    using Base::Nested;
    typedef Base::Nested::value_enum value_enum; // add this
};

int main(int argc, char** argv)
{
    //Base::Nested aa; // error, as (I) expected
    //Base::Nested::value_enum ab; // error, as (I) expected

    Derived::Nested ba; // works, as (I) expected
    Derived::value_enum bb = Derived::value_enum::val; // now works in MSVC


    return 0;
}