Question

What I read in the C++ standard about injected class names contradicts (as I see it) with the behavior of a sample program I will present shortly. Here’s what I read:

• From 3.4 (paragraph 3)

  The injected-class-name of a class (clause 9) is also considered to be
  a member of that class for the purposes of name hiding and lookup.

• From 9 (paragraph 2)

  A class-name is inserted into the scope in which it is declared
  immediately after the class-name is seen. The class-name is also
  inserted into the scope of the class itself; this is known as the
  injected-class-name. For purposes of access checking, the
  injected-class-name is treated as if it were a public member name.

From these I understand that the following is a well-formed translation unit and it compiles successfully.

#include <vector>
class X: std::vector<int>
{
   vector mem;
};

However, I would suppose that the following should have produced an error, but it doesn’t

#include <vector>
class X: std::vector<int>, std::vector<char>
{
   vector mem; //compiles OK... mem is apparently std::vector<int>
};

Since the name vector is injected into both std::vector<int> and std::vector<char> as as if a public member name, then it should be inherited by X and therefore the name vector in X should be ambiguous. Am I missing something?

P.S. I am using MSVC9.0

Answer 1

I found it! It’s right there in the standard! I was right! It should be ambiguous!

Clause 14.6.1 Paragraph

A lookup that finds an injected-class-name (10.2) can result in an
ambiguity in certain cases (for example, if it is found in more than
one base class). If all of the injected-class-names that are found
refer to specializations of the same class template, and if the name
is followed by a template-argument-list, the reference refers to the
class template itself and not a specialization thereof, and is not
ambiguous. [Example:

template <class T> struct Base { };
template <class T> struct Derived: Base<int>, Base<char> 
{ 
    typename Derived::Base b; // error: ambiguous typename 
    Derived::Base<double> d;  // OK 
};

—end example]

Bottom line: This is yet another Microsoft compiler BUG. Disabling language extensions doesn’t help either.