I recently discovered that friend declarations scoping follows extremely peculiar rules - if you have a friend
declaration (definition) for a function or a class th
Well, for answering that, you have to look at another major feature of C++: Templates.
Consider a template such as this:
template <class T>
struct magic {
friend bool do_magic(T*) { return true; }
};
Used in code like this:
bool do_magic(void*) { return false; }
int main() {
return do_magic((int*)0);
}
Will the exit-code be 0
or 1
?
Well, it depends on whether magic
was ever instantiated with int
anywhere observable.
At least it would, if friend
-functions only declared inline would be found by ordinary lookup-rules.
And you can't break that conundrum by just injecting everything possible, as templates can be specialized.
That was the case for a time, but was outlawed as "too magic", and "too ill-defined".
There were additional problems with name injection, as it wasn't nearly as well-defined as hoped for. See N0777: An Alternative to Name Injection from Templates for more.