operator<<(ostream&, X) for class X nested in a class template

Question

This one compiles and works like it should (non-nested template):

#include 

template class Z;

template          


        

Answer 1

This is a general problem for functions:

template 
void func(typename C::iterator i);

Now, if I call func(int*), which value of C should I use ?

In general, you cannot work backward ! Many different C could have defined an internal type iterator that happens to be a int* for some set of parameters.

In your case, you are complicating the situation a bit:

template 
void func(typename Z::ZZ const&);

But fundamentally this is the same issue, Z is a template, not a full class, and you are asking to create a function for the inner type ZZ of this template.

Suppose I do:

template 
struct Z { typedef T ZZ; };

template 
struct Z { typedef T ZZ; };

Note: typical of iterators, the value_type is not const-qualified

Then, when invoking func(int), should I use Z or Z ?

It is non-deducible.

And thus the whole thing is referred to as a non-deducible context, and the Standard forbids it because there is no sensible answer.

Rule of Thumb: be suspicious of typename in the parameters of a function.

Note: they are OK if another argument already pinned down the type, example typename C::iterator find(C&, typename C::const_reference); because once C is deduced, then C::const_reference may be used without trouble