Question

class A
{
   static int iterator;
   class iterator
   {
      [...]
   };
   [...]
};

I (think I) understand the reason why typename is needed here:

template <class T>
void foo() {
   typename T::iterator* iter;
   [...]
}

but I don’t understand the reason why typename is not needed here:

void foo() {
   A::iterator* iter;
   [...]
}

Can anyone explain?

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EDIT:

The reason why the compiler does not have a problem with the latter, I found to be answered well in a comment:

in the case of A::iterator I don’t see why the compiler wouldn’t confuse it with the static int iterator ? – xcrypt

@xcrypt because it knows what both A::iterators are and can pick which one depending on how it is used – Seth Carnegie

---

The reason why the compiler needs typename before the qualified dependent names, is in my opinion answered very well in the accepted answer by Kerrek SB. Be sure to also read the comments on that answer, especially this one by iammilind:

“T::A * x;, this expression can be true for both cases where T::A is a type and T::A is a value. If A is a type, then it will result in pointer declaration; if A is a value, then it will result in multiplication. Thus a single template will have different meaning for 2 different types, which is not acceptable.”

Answer 1

A name in C++ can pertain to three different tiers of entities: Types, values, and templates.

struct Foo
{
    typedef int A;                   // type
    static double B;                 // value
    template <typename T> struct C;  // template
};

The three names Foo::A, Foo::B and Foo::C are examples of all three different tiers.

In the above example, Foo is a complete type, and so the compiler knows already what Foo::A etc. refer to. But now imagine this:

template <typename T> struct Bar
{
    T::A x;
};

Now we are in trouble: what is T::A? if T = Foo, then T::A = int, which is a type, and all is well. But when T = struct { static char A; };, then T::A is a value, which doesn’t make sense.

Therefore, the compiler demands that you tell it what T::A and T::B and T::C are supposed to be. If you say nothing, it is assumed to be a value. If you say typename, it is a typename, and if you say template, it is a template:

template <typename T> struct Bar
{
    typename T::A x;    // ah, good, decreed typename

    void foo()
    {
        int a = T::B;   // assumed value, OK

        T::template C<int> z;  // decreed template
        z.gobble(a * x);
    }
};

Secondary checks such as whether T::B is convertible to int, whether a and x can be multiplied, and whether C<int> really has a member function gobble are all postponed until you actually instantiate the template. But the specification whether a name denotes a value, a type or a template is fundamental to the syntactic correctness of the code and must be provided right there during the template definition.