Strange declaration(templates). C++

Question

How can I understand what is declared here: (this is taken from another post on this forum)

template<typename C> static char (&f(ChT<int Fallback::*, &C::x>*))[1];

Here's how I read:

template of static function f called with (ChT<int Fallback::*, &C::x>*), but then I can't make sense why is there an address-of operator and why is there an array?

I'm still learning how to understand C++ declarations, so please explain this slowly and carefully.

Are you sure this is a valid declaration? I can't get it to compile. — Jay Walker, Mar 17 '10 at 16:05
@Jay Walker - yes it is valid, now after guys explained that to me I just remember some similiar example, long time ago which was something like this declare f with some args wich returns pointer to f1 with some args and f1 return type is another pointer to f2. Back then to do that I've used typedefs otherwise you have return type at the end of the declaration. — There is nothing we can do, Mar 17 '10 at 16:11

score 7 · Answer 1 · answered Mar 17 '10 at 16:04

7

Using some typedefs:

typedef char (&arrayref_t)[1];

This is a reference to an array of characters. The array has one element.

typedef ChT<int Fallback::*, &C::x> tmpl_t;

This is a template class, instantiated with the type "pointer to an int member of the Fallback class", and a member pointer to x in class C.

static arrayref_t f(tmpl_t*);

The function now takes a pointer to a tmpl_t and returns an arrayref_t.

answered Mar 17 '10 at 16:04

sth

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I've just observed VERY interesting thing (yes, I'm still munching it). Where you're saying "This is ..., and a member pointer to x in class C", it should be mentioned that this member pointer in class C is actually member of class Fallback due to declaration of class ChT. So in a fact they are connected with each other. – There is nothing we can do Mar 17 '10 at 17:05

score 3 · Accepted Answer · answered Mar 17 '10 at 16:03

It's important to see return type. So, return type of this function is reference to char[1]; Imagine that f returns something like reference to the following:

char ret[1];

For example

template<typename C> static char (&f(ChT<int Fallback::*, &C::x>*))[1]
{

   static char xx[1] = {'F'};
   return xx;
}

Strange declaration(templates). C++

2 Answers2