How to befriend method from other class that handles this class (incomplete types problem)?

Question

This sounds like a very obvious question but I can't find anything on the internet. I have a class other that handles a message in one of its methods. To do so, it has to access a private data member of the message, hence it should be friend of it. But I can't get it to compile. No matter the declaration order, it always complains about types being incomplete. How else would I accomplish such a thing?

Demo

#include <cstdio>

struct other;

class message
{
    friend auto other::handle_message(message) -> void;
    int private_ = 10;
};

struct other
{
    auto handle_message(message msg) -> void {
        printf("Private part of msg is %d!\n", msg.private_);
    }
};

int main() {}

Answer 1

What you need is the following

class message;

struct other
{
    auto handle_message(message msg) -> void;
};

/* or just
    struct other
    {
        auto handle_message( class  message msg) -> void;
    };
*/


class message
{
    friend auto other::handle_message(message) -> void;
    int private_ = 10;
};

auto other::handle_message(message msg) -> void {
    printf("Private part of msg is %d!\n", msg.private_);
}

That is the class other must be defined before the class message and its member function handle_message must be defined after the definition of the class message.

Here is a demonstration program.

#include <cstdlib>

class message;

struct other
{
    auto handle_message( message msg ) -> void;
};

class message
{
    friend auto other::handle_message( message ) -> void;
    int private_ = 10;
};

auto other::handle_message( message msg ) -> void {
    std::printf( "Private part of msg is %d!\n", msg.private_ );
}

int main()
{
    other().handle_message( message() );
}

The program output is

Private part of msg is 10!