Why do struct & class templates need explicit angle brackets syntax on empty specialization?

Asked Nov 01 '18 at 22:36

Active Nov 01 '18 at 22:36

Viewed 71 times

template<class T=int>
void foo() {}

template<class T=int>
struct bar {};

int main() {
    foo<long>();
    foo<>();
    foo();

    bar<long>();
    bar<>();
    //bar(); // compilation failure, 'bar' no appropriate default constructor available
}

I am not understanding why structs and classes (such as bar) require angle brackets if a default specialization is provided. For foo, I am able to call it without the angle brackets, whereas bar requires empty angle brackets.

If I try create bar without angle brackets I get a seemlingly (to me) unrelated error.

Any insight would be helpful.

Edit: Compiled on Visual C++ 2015

asked Nov 01 '18 at 22:36

Michael Choi

1

please include the full error message – 463035818_is_not_an_ai Nov 01 '18 at 22:56
@user463035818 there was the full error message. I'll reiterate it below though "''bar' no appropriate default constructor available". I was confused because the constructor seems somewhat unrelated to the issue I'm having. – Michael Choi Nov 01 '18 at 22:59
1

hm... its different [here](https://godbolt.org/z/obvKWJ) – 463035818_is_not_an_ai Nov 01 '18 at 23:03
@user463035818 My mistake, that was only the first error, the next one is the same as the online compiler you displayed. In any case, would you know why C++ complains here? – Michael Choi Nov 01 '18 at 23:09
the only reason is: this is how the rules are ;). Since c++17 it is allowed (see one of the answers on the duplicate) – 463035818_is_not_an_ai Nov 01 '18 at 23:12

Why do struct & class templates need explicit angle brackets syntax on empty specialization?

0 Answers0