you lose the guarantee that when you look at a class's interface, you see its entire interface. Adding friends from the outside (and, accordingly, methods) is a very bad idea, because. encapsulation and invariants become empty zilch

вс, 17 июл. 2022 г. в 23:24, Anoop Rana via Std-Proposals <std-proposals@lists.isocpp.org>:
I don't think that this is a good idea because friendship is granted by the class itself and not the user of the class. That is, you're proposing that a user should be able to access different fields which is nonsensical. It is the class who decides which members are public or private etc and to whome to grant friendship.



On Sun, 17 Jul 2022, 23:31 Frederick Virchanza Gotham via Std-Proposals, <std-proposals@lists.isocpp.org> wrote:
Recently I was working with an RS232 library when programming a microcontroller in C++. I needed a little finer control over the library's inner workings, more control than the library provided. I had to edit the library's header file to change the visibility of one of the class's member variables, I changed it from 'private' to 'public', and also I made one of the member functions 'public'. Alternatively I could have added one line at the end of the class definition:

    friend class MyOwnClass;

and then I could have accessed all the private members through static member functions belonging to MyOwnClass.

I propose that we should be able to "impose friendship" without editing the library's header files. For example:

    class Foo { int monkey; };

    int main(void)
    {
        Foo obj;

        using friend Foo;

        obj.monkey = 5;
    }

The statement "using friend Foo" only has an effect within 'main', similar to how "using namespace std" would have limited scope here.

Alternatively, the syntax could be on an access-by-access basis, something like as follows:

    int main(void)
    {
        Foo obj;

        friend<obj>.monkey = 5;
    }

or the syntax could be:

    friend<obj.monkey> = 5;

or maybe:

    std::friend(obj, monkey) = 5;

or:

    friend >> obj.monkey = 5;

Some of you might be thinking: "Why affirm such subterfuge when the author of the library obviously didn't want you messing around in there?", well computer programming isn't an exact science. Indeed we have paradigms and design patterns, but really at the end of the day we're trying to get things done, and this might mean following the instructions from Step 1 to Step 9 but then deviating at Step 10.

Being able to manually impose friendship when and where you need it, would mean not needing to edit the 3rd party library's header file.
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