Implicit use of initializer_list

Question

§[dcl.init.list] 8.5.4/2:

The template std::initializer_list is not predefined; if the header <initializer_list> is not included prior to a use of std::initializer_list — even an implicit use in which the type is not named (7.1.6.4) — the program is ill-formed.

Does that mean this program is ill-formed?

#include <vector>
int main() {
    // uses vector::vector(initializer_list<T>, const Allocator&) constructor
    std::vector<int> v = {1, 2, 3};
}

In practice it's not ill-formed, because `std::vector` constructor uses `initializer_list`, so that must be included in ``. But unless that dependency is stated somewhere, the formal could be ill-formed. And the reason for that is that the standard library is allowed to do any kinds of magic, including relying on non-standard behavior of the compiler, so that one can't reason that the standard library must be implemented in the same kind of way as one's own code. — Cheers and hth. - Alf, Jun 22 '15 at 02:56

score 5 · Accepted Answer · answered Jun 22 '15 at 03:20

5

Your program is not ill-formed because <vector> is guaranteed to include <initializer_list> (the same is true for all standard library containers)

§23.3.1 [sequences.general]

Header <vector> synopsis

#include <initializer_list>
...

Searching the standard for #include <initializer_list> reveals the header is included along with the following headers

<utility>
<string>
<array>
<deque>
<forward_list>
<list>
<vector>
<map>
<set>
<unordered_map>
<unordered_set>
<queue>
<stack>
<algorithm>
<random>
<valarray>
<regex>

answered Jun 22 '15 at 03:20

Praetorian

106,671
19
240
328

1

cf. http://stackoverflow.com/questions/26614983/which-headers-in-the-c-standard-library-are-guaranteed-to-include-another-head – T.C. Jun 22 '15 at 03:23
@T.C. Ha! Turns out I even upvoted you on that answer :). I'll leave it to the OP to decide if it's a dupe, since this one is specifically about `` – Praetorian Jun 22 '15 at 03:27

Implicit use of initializer_list

1 Answers1