How recursive variadic templates is work?

With this code:

static unsigned count = 0;
template<typename... T>
auto sum(T... args)
{
    ++count;
    return (... + args);
}
int main (void)
{
    std::cout << sum(12, 32, 32, 12, 4, 3, 43, 432) << std::endl;
    std::cout << "TIME: " << count << std::endl;
}

Output is:

$> ./program.out
570
TIME: 1

Why is count equals to 1? I expected count to be 8. Is sum template function called only once?

> Is sum template function call once ?

Yes, it won't be called recursively. Instead, the expression is expanded for fold expression.

> The instantiation of a fold expression expands the expression e as
> follows:
>
> ...
> 2) Unary left fold (... op E) becomes (((E₁ op
> E₂) op ...) op E_N)
> ...
>
> (where N is the number of elements in the pack expansion)

You might want to put ++count into the fold expression, e.g.

template<typename... T>
auto sum(T... args)
{
    return (... + (++count, args));
}

As @Xatyrian pointed, its value is just same as the number of elements in the pack expansion, which could be taken by sizeof... too.

If you wanted to call sum multiple times, you could do so recursively:

static unsigned count = 0;
template <typename T>
auto sum(T t)
{
	++count;
	return t;
}
template <typename T, typename... Ts>
auto sum(T t, Ts... ts)
{
	++count;
	return t + sum(ts...);
}