Question

Short story:

struct A{};
struct B:private A{};

void f(void *){}

void f(A*){}

int main(){
 B* b;
 f(b);
}

and GCC complains
error: ‘A’ is an inaccessible base of ‘B’

Long story:
To see if a class is a subclass (or the same) of another without using boost, I do

template<typename B,typename D> struct is_base_or_same_of{

 typedef char (&yes)[2] ;  

 static yes test(const B* b);
 static char test(const void* p);

 static const D* d();

 static const bool value=sizeof(test(d()))==sizeof(yes);

};

and the situation is the same

How could I make the compiler “prefer” the void* version?

Answer 1

This has to do with the process of overload resolution and how it goes. When you pass a pointer to the derived object, the compiler will try to find the best match for the function and determines that it is the overload that takes a pointer to A. Only then access specifiers are checked and the compiler complains that A is not an accessible base from that context.

The compiler is not allowed to go back, discard that overload and try with the rest of them.