templates - declaring a C++ set iterator

The
following code will not
compile:

#include
            
#include 
using namespace
            std;

template
void printSet(set
            s){

 set::iterator
            it;
}

int main(int argc, char** argv){

            set s;
 printSet(s);
 return
            0;
}

I
get an error saying:

set.cpp: In
            function ‘void printSet(std::set,
            std::allocator<_CharT> >)’:
set.cpp:7: error: expected `;' before
            ‘it’
set.cpp: In function ‘void printSet(std::set,
            std::allocator<_CharT> >) [with T = int]’:
set.cpp:12: instantiated
            from here
set.cpp:7: error: dependent-name
            ‘std::set,std::allocator<_CharT> >::iterator’ is
            parsed as a non-type, but instantiation yields a type
set.cpp:7: note: say
            ‘typename std::set,std::allocator<_CharT>
            >::iterator’ if a type is
            meant

What
did I do wrong? I feel like I've hardly written anything, and already C++ gives me this
scary message.

In case it is helpful, it looks
like, if I comment out the line with the iterator, there are no errors. However, all the
examples I've seen online so far seem to declare iterators this way. I
think.

Answer

Precede your declaration with the keyword
typename,
thusly:

typename
            set::iterator
            it;

You need to do
this whenever you refer to a type that is dependent upon a template argument. In this
case, iterator is dependent on T. Otherwise, the compiler has
difficulty trying to figure out what iterator is, whether it's
a type, or a static member or something else. (well, technically, it doesn't try to
figure it out, its decision is already made, it just happens to be
wrong)

Note that if you have a
concrete type, for
example:

set::iterator
            it;

The
typename is not necessary.