c++ - 使用声明和访问修饰符的嵌套类

Question

在编译一些代码时，我遇到了一个编译器错误，这个错误对我来说很奇怪，并且与继承、嵌套类、使用声明和访问修饰符有关。

基本上，派生类型的目的是公开基类型的嵌套 protected 类。

以下简短示例演示了该问题:

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;
}

参见 live .

MSVC11 (v11.00.61030) 在此代码上阻塞并出现以下错误:

error C2248: 'Base::Nested' : cannot access protected struct declared in class 'Base'

GCC 和 Clang 都能正确编译它，因此无法从标准中引用相关部分，我认为这是一个 MSVC 错误。

MSVC 有办法解决这个问题吗？

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;
}