c++ - Why are we allowed to take the address of an incomplete type? -

consider code:

class addressable; class class1  { void foo(addressable &a) { (void) &a; } };  // ok class addressable { void *operator &() { return this; } }; class class2  { void foo(addressable &a) { (void) &a; } };  // error: operator & private 

why c++ allow taking address of incomplete reference type?

couldn't potentially illegal, shown above? intentional?

yes, that's intentional, , possibility of breakage if operator& overloaded known.

taking address of incomplete types has been possible since long before c++. in c, there absolutely no risk of breakage, because & cannot overloaded.

c++ chose not unnecessarily break valid programs, , specified if incomplete type turn out have overloaded & operator, it's unspecified whether overloaded operator gets used.

quoting n4140:

5.3.1 unary operators [expr.unary.op]

if & applied lvalue of incomplete class type , complete type declares operator&(), unspecified whether operator has built-in meaning or operator function called.

this can interpreted apply class being declared, , when declaration of operator& has been seen:

extern struct a; struct {   int operator&();   decltype(&a) m; // int, or *? }; int main() {   return a().m; // valid if m int } 

here, gcc gives m type a * , rejects program, clang gives type int , accepts it.