Question

Seen it just now here.

Never met such construction before and I do not understand, what it means! And how it works in specialization, as typedefs do not generate new types:

Wrong:

template <typename T>
void a();
typedef int a_t;
typedef int b_t;
template<> void a<a_t>(){}
template<> void a<b_t>(){}

---

Compiles with warning: 'typedef' was ignored in this declaration, works as expected:

template <typename T>
void a();
typedef class a_t;
typedef class b_t;
template<> void a<a_t>(){}
template<> void a<b_t>(){}

Answer 1

I finally unsserstood, how it works.

typedef class a_t;

is not like

typedef int a_t;

but like

typedef int;

So, I forward declared class a_t and then typedef‘d it to nothing. It also explains warning `typedef' was ignored in this declaration and all templates related stuff.