Question

Is the following code legal? MSVC 9 and g++ 4.4 disagree:

struct base
{
  struct derived {};
};

struct derived : base {};

int main()
{
  typedef derived::derived type;
  return 0;
}

MSVC complains, confusing the nested name for the type’s constructor:

c:\dev>cl test.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
test.cpp(10) : error C2146: syntax error : missing ';' before identifier 'type'
test.cpp(10) : error C2761: '{ctor}' : member function redeclaration not allowed

test.cpp(10) : error C2065: 'type' : undeclared identifier

While g++ does not:

$ g++ --version test.cpp
g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

For context, my code contains an iterator called pointer. To provide the iterator interface, it provides the nested type pointer, which is a synonym for itself.

Answer 1

Comeau thinks that your code is incorrect, so I think that the constructor-interpretation shadowing the type-interpretation is what the standard mandates¹.

Still, your code happily compiles if you clear up the ambiguity and tell the compiler that you’re trying to access the derived member of the base class:

struct base
{
  struct derived {};
};

struct derived : base {};

int main()
{
  typedef derived::base::derived type;
  return 0;
}

By the way, the fact that the constructor-interpretation prevails kinda makes sense: you have a well-known way to tell the compiler that you want to refer to stuff of the base class (via the scope resolution operator), but you wouldn’t have a syntax to do the reverse (forcing the compiler to understand that you’re referring to the constructor). So the “constructor-by-default” behavior seems quite sensible.

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1. I may look it up later, but I don’t guarantee that I actually will, this kind of pesky name issues are always a mess to look up.