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Question: Maybe is impossible to make std::begin (and friends) as friend function (lemmy.ml)

submitted 7 months ago by [email protected] to c/[email protected]

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Yes, it is probably a weird question, but I tried a lot, and I started to think that maybe is impossible to overload this template function properly:

#include <iterator>

class Foo
{
private:
    const int arr[5] = {10, 20, 30, 40, 50};
public:
    const int* begin() const { return arr; }

friend auto std::begin<>(const Foo &f) -> decltype(f.begin());
}

It always throw the same error (in GCC 12.2.0):

main.cxx:10:13: error: template-id ‘begin<>’ for ‘const int* std::begin<>(const Foo&)’ does not match any template declaration

I just wanna know if is possible do things like this. Thanks.

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[–] [email protected] 2 points 7 months ago* (last edited 7 months ago) (2 children)

I think the issue is that Foo is incomplete when you're declaring the friend, so I think it's impossible. I just tried it and g++ ignores the target candidate due to "member access into incomplete type", which makes sense since std::begin is already defined and calls .begin(). The closest you can get is to use another friend to expose arr and overload std::begin manually, but that's a bit silly 😅

[–] [email protected] 2 points 7 months ago (1 children)

Apparently it is impossible for this kind of functions to be defined as friends of classes:

template <typename T>
auto do_something(T &t) -> decltype(t.private_msg);

class Foo
{
private:
    const char *private_msg = "You can't touch me!";
    
friend auto do_something<>(Foo &f) -> decltype(f.private_msg); // Error!
};

template <>
auto do_something<Foo>(Foo &f) -> decltype(f.private_msg) // Error!
{
    return f.private_msg; // Error!
}

After trying different combinations, it seems that I managed to get it working with the condition the whole template are considered friends of the class. I don't know if I should consider it a language problem, but it seems that way, since the template restrictions (in this case) are minor.

template <typename T>
auto do_something(T &t) -> decltype(t.private_msg);

class Foo
{
private:
    const char *private_msg = "You can't touch me!";

template <typename T>
friend auto do_something(T &t) -> decltype(t.private_msg); // This works fine!
};

template <>
auto do_something<Foo>(Foo &f) -> decltype(f.private_msg)
{
    return f.private_msg;
}

Do you think I found an error in the language?

[–] [email protected] 2 points 7 months ago* (last edited 7 months ago)

Ah, nice idea. I've tried a few different ways of doing this, and I think what you're seeing is a discrepancy in how the compiler handles member access into incomplete types. It seems that, in your examples, the compiler is allowing -> decltype(f.private_msg) within the class, but I think it's not selecting do_something outside of it because it uses decltype(t.private_msg). In my case, I'm not even able to do that within the class.

For example, since I'm not able to use decltype(f.private_msg) inside the class, I'm using decltype(private_msg) instead, which causes an error at the do_something declaration related to incomplete type (presumably because of the t.private_msg usage):

// candidate template ignored; member access into incomplete type
template 〈class T〉 auto do_something(T ＆t) -> decltype(t.private_msg);
class Foo {
        const char *private_msg = "You can't touch me!";
        friend auto do_something〈〉(Foo ＆f) -> decltype(private_msg);
};
template 〈〉 auto do_something(Foo ＆f) -> decltype(f.private_msg) {
        return f.private_msg;
}

My reasoning is that removing the t.private_msg from the declaration works:

template 〈class Ret, class T〉 auto do_something(T ＆t) -> Ret;
class Foo {
        const char *private_msg = "You can't touch me!";
        friend auto do_something〈〉(Foo ＆f) -> decltype(private_msg);
};
template 〈〉 auto do_something(Foo ＆f) -> decltype(f.private_msg) {
        return f.private_msg;
}
static Foo foo{};
// this works, but Ret cannot be deduced and must be specified somehow:
static auto something = do_something〈const char*〉(foo);

The reason your second example works is because the friend template inside the class acts as a template declaration rather than a specialization, which isn't specialized until after Foo is complete:

// the do_something inside Foo is a declaration, meaning this isn't used
// template 〈class T〉
// auto do_something(T ＆t) -> decltype(t.private_msg);
class Foo {
        const char *private_msg = "You can't touch me!";
        template 〈class T〉 // t.private_msg is allowed because T is not Foo yet
        friend auto do_something(T ＆t) -> decltype(t.private_msg);
};
template 〈〉 auto do_something(Foo ＆f) -> decltype(f.private_msg) {
        return f.private_msg;
}