Tag: required

For example, consider the following example:

template
struct foo
{
template
    Friend foo cast (foo const &) {return {}; }
Friend foo bar (foo const &) {return {}; }
};

int main (int, char **)
{
foo on;
car b = bar (a);
car c = cast(on);
return 0;
}

occupation is not visible to the argument-dependent search (ADL) while bar is. This is due to the fact that cast is a friend function with a required template parameter. What is the cleanest way to do it occupation visible to ADL?

The shortest is likely

template Cancel cast ();

which can be extended by adding Clear To make it clear that it can not be called at compile time but at connection time:

template void cast () = delete;

In addition, any argument could be added to make the expected number of parameters visible, such as:

template null cast (void *) = delete;

The type here, however, does not match the actual final type. In this simple case I could use

template void occupation (foo const &) = delete;

But if I add another class later, e.g. quxthat also occupies a friend, this could actually be more misleading.
A template template parameter may seem possible, but too complex.

Which version would you prefer and why, or do you have better suggestions?

The actual code has a special cast function (with a longer, more meaningful name).