C++ visibility of privately inherited typedefs to nested classes
- by beldaz
First time on StackOverflow, so please be tolerant.
In the following example (apologies for the length) I have tried to isolate some unexpected behaviour I've encountered when using nested classes within a class that privately inherits from another. I've often seen statements to the effect that there is nothing special about a nested class compared to an unnested class, but in this example one can see that a nested class (at least according to GCC 4.4) can see the public typedefs of a class that is privately inherited by the closing class.
I appreciate that typdefs are not the same as member data, but I found this behaviour surprising, and I imagine many others would, too. So my question is threefold:
Is this standard behaviour? (a decent explanation of why would be very helpful)
Can one expect it to work on most modern compilers (i.e., how portable is it)?
#include <iostream>
class Base {
typedef int priv_t;
priv_t priv;
public:
typedef int pub_t;
pub_t pub;
Base() : priv(0), pub(1) {}
};
class PubDerived : public Base {
public:
// Not allowed since Base::priv is private
// void foo() {std::cout << priv << "\n";}
class Nested {
// Not allowed since Nested has no access to PubDerived member data
// void foo() {std::cout << pub << "\n";}
// Not allowed since typedef Base::priv_t is private
// void bar() {priv_t x=0; std::cout << x << "\n";}
};
};
class PrivDerived : private Base {
public:
// Allowed since Base::pub is public
void foo() {std::cout << pub << "\n";}
class Nested {
public:
// Works (gcc 4.4 - see below)
void fred() {pub_t x=0; std::cout << x << "\n";}
};
};
int main() {
// Not allowed since typedef Base::priv_t private
// std::cout << PubDerived::priv_t(0) << "\n";
// Allowed since typedef Base::pub_t is inaccessible
std::cout << PubDerived::pub_t(0) << "\n"; // Prints 0
// Not allowed since typedef Base::pub_t is inaccessible
//std::cout << PrivDerived::pub_t(0) << "\n";
// Works (gcc 4.4)
PrivDerived::Nested o;
o.fred(); // Prints 0
return 0;
}
