将成员函数声明为友元：Lippman第5版相关内容是否有误？

Yep, there's a critical compile-time issue with the code snippet you shared from Lippman's C++ Primer 5th Edition (ISBN-13: 978-0321714114) pages 280-281. Let’s walk through why and how to fix it:

The Core Problem: Missing Required Forward Declarations #

When you write friend void Window_mgr::clear(ScreenIndex); inside the Screen class, the compiler has no prior knowledge of:

• What the Window_mgr class even is (it hasn’t been declared yet)
• That Window_mgr has a member function named clear that takes a ScreenIndex parameter

Without these pieces of information, the compiler will throw errors like 'Window_mgr' has not been declared or invalid use of incomplete type 'class Window_mgr'.

The Corrected Code Structure #

To make this work, you need to add forward declarations in the right order:

// Forward declare the Screen class first (since Window_mgr's clear function references it)
class Screen;

// Declare the Window_mgr class and its clear member function
class Window_mgr {
public:
    using ScreenIndex = size_t; // Assume ScreenIndex is defined here (adjust if it's a Screen member type)
    void clear(ScreenIndex);
};

// Now define the Screen class with the valid friend declaration
class Screen {
    // The compiler now knows Window_mgr exists and has a clear function, so this is legal
    friend void Window_mgr::clear(Window_mgr::ScreenIndex);
    
    // Screen's member variables and functions go here
    // ...
};

// Finally, implement the Window_mgr::clear function (now that Screen is fully defined)
void Window_mgr::clear(ScreenIndex idx) {
    // Access Screen's private members here (allowed by the friend declaration)
    // ...
}

If ScreenIndex is a member type of Screen (e.g., using ScreenIndex = size_t; inside Screen), you’ll need to adjust the order slightly—you’d declare Screen first, then Window_mgr with a function that uses Screen::ScreenIndex (though defining ScreenIndex outside both classes is often simpler to avoid circular dependency headaches).

Why the Book’s Snippet Might Look "Wrong" #

Lippman’s example is likely focusing on the concept of declaring a specific member function as a friend (rather than the entire class), so it omits the boilerplate forward declarations to keep the example concise. In real-world code, though, those declarations are mandatory for the code to compile.

内容的提问来源于stack exchange，提问作者user2865249