c++ - 不完整类的成员函数作为 friend = 正式有效？

Question

下面的代码，其中嵌套类 Info 将外部类 Impl 的两个成员函数指定为友元，用 Visual C++ 和 g++ 编译得很好，代码如下如下所示。

但是，如果成员函数没有在 Info 之前声明，那么 g++ 会提示不完整类而不是缺少声明:

In file included from ../console_io.hpp:6:0,                 from console_io.cpp:1:../console_io.impl.windows.hpp:69:47: error: invalid use of incomplete type 'class console::Display::Impl'         friend auto Impl::api_info() const -> Ref_<const Api_info>;                                               ^../console_io.impl.windows.hpp:62:20: note: definition of 'class console::Display::Impl' is not complete until the closing brace     class Display::Impl                    ^../console_io.impl.windows.hpp:70:47: error: invalid use of incomplete type 'class console::Display::Impl'

Minimal example:

struct Foo
{
    class Bar
    {
        friend void Foo::m();
    };

    void m(){}
};

foo.cpp:5:28: error: invalid use of incomplete type 'struct Foo'         friend void Foo::m();                            ^foo.cpp:1:8: note: definition of 'struct Foo' is not complete until the closing brace struct Foo        ^

And so by association I wonder about the formal validity of making a member function of T a friend, at a point where T is not yet complete.

---

I present the actual original code that compiles, so that, if it's formally invalid, reasonable alternatives for this use case can be suggested. I think that I, as the one who's asking, am possibly the least competent to decide what's relevant or not for answers. Notation: Ref_<T> means T&.

class Display::Impl
{
private:
    using Api_info = impl::winapi::Console_screen_buffer_info;

    inline auto api_info() const -> Ref_<const Api_info>;   // Def. after class.
    inline void invalidate_api_info();                      // Def. after class.

    class Info
    {
    friend auto Impl::api_info() const -> Ref_<const Api_info>;
    friend void Impl::invalidate_api_info();
    private:
        bool        is_valid_   = false;
        Api_info    api_info_   = {};
    };

    // State:
    impl::winapi::Handle    text_buffer_;
    mutable Info            info_;

    Impl( Ref_<const Impl> ) = delete;
    auto operator=( Ref_<const Impl> ) -> Ref_<Impl> = delete;

public:
    auto size() const
        -> Point
    {
        auto const api_size = api_info().dwSize;
        return Point{ api_size.x, api_size.y };
    }

    ~Impl()
    {}      // TODO:

    Impl( const Point size )
        : text_buffer_( impl::winapi::CreateConsoleScreenBuffer(
            impl::winapi::flag_GENERIC_READ | impl::winapi::flag_GENERIC_WRITE,
            0,              // No sharing
            nullptr,        // Default security.
            impl::winapi::flag_CONSOLE_TEXTMODE_BUFFER, // The allowed value.
            nullptr         // Reserved.
            ) )
    {
        hopefully( text_buffer_ != impl::winapi::invalid_handle_value )
            || fail( "console::Display: CreateConsoleScreenBuffer failed" );
    }
};

auto Display::Impl::api_info() const
    -> Ref_<const Api_info>
{
    if( not info_.is_valid_ )
    {
        impl::winapi::GetConsoleScreenBufferInfo( text_buffer_, &info_.api_info_ )
            || fail( "GetConsoleScreenBufferInfo failed" );
        info_.is_valid_ = true;
    }
    return info_.api_info_;
}

void Display::Impl::invalidate_api_info()
{ info_.is_valid_ = false; }

Answer 1

经过一番挖掘，我认为这就是您要找的东西:

§9.3 [class.mfct]:

⁷ Previously declared member functions may be mentioned in friend declarations.

因此(据我了解标准)，当您在嵌套类之前声明成员函数时，您的代码是有效的。