Matthias Kretz | VirWhat's the constexpr-if equivalent to `condition ? x : y`? As far as I know we only have `[&] { if constexpr (condition) return x; else return y; }()`. I hate it.
Dani (
Antifascista)@mkretz
Correct.
AFAIK, nobody ever proposed an equivalent.
What would your envisioned semantic look like?
* no search for a 'common type'?
* one of the two 'sub-expressions' is 'discarded', like in https://eel.is/c++draft/stmt.if#2 ?
@DanielaKEngert The semantics would be as-if it's the lambda expression I used. So yes, no common type and one sub-expression is discarded. I need the latter for `has_subscript_operator ? x[0] : x`: This does not compile because if `has_subscript_operator == false` the `x[0]` expression is ill-formed.
@mkretz
So, a simple named function would do in this case because it would depend only on the type of 'x'?
So, a simple named function would do in this case because it would depend only on the type of 'x'?
@DanielaKEngert You're thinking of `select<condition>(x[0], x)`? That would not work because `x[0]` can be ill-formed.
@mkretz
No, similar to a templated function like this
template <typename T>
auto first_in(const T &x) {
if constexpr (some_cond_based_on_T)
return x[0];
else
return x;
}
@DanielaKEngert In the case that triggered my post, yes I could work around it via such a function. But the condition is actually a trait of the class where it is used to initialize a data member. If there's a mismatch with the type then I want it to be ill-formed. This is pedantic, since the trait of the class basically is derived from that type again. But you know ... refactorings in the future ... who knows.
I have many of such lambdas. It's just annoying.
I have many of such lambdas. It's just annoying.
@mkretz
Propose "do expressions" ðŸ¤
Oh wait, Barry already did that AFAIK. They are an important building block for 'pattern matching' as proposed. Otherwise it would become as annoying as in your use-case.
Jonathan Müller@DanielaKEngert @mkretz do expressions as proposed don't help me in many situations because they destroy temporaries eagerly and not deferred until the end of the full expression.
Oh shoot. 💩
OTOH, this would open another venue for Nico to give highly oninionated talks 😂