Currently, I can compose ranges-v3 views like this:
auto v = ranges::view::reverse | ranges::view::filter([](int l){return l>5;});
But if I wanted to return v from a function I'd need to know its type. What is the type of a ranges-v3 view?
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1 Answer
Since C++14 you can use auto as the return type of functions and it will get deduced:
auto f() {
return ranges::view::reverse | ranges::view::filter([](int l){return l>5;});
}
// f's return type is the type of the return expression, exactly as is I had:
// auto returnValue = return-expression;
// where f's type is decltype(returnValue)
The only downside is that the definition of f has to appear in the same TU where you are using it.
2 Comments
Patrick Fromberg
I would say
auto const f() { ... because the return value does not change. The only changing thing here is what the return value calculates lazily.
Rakete1111
@PatrickFromberg No no no no you don't want that. You are right it doesn't change, but marking it as
const inhibits move semantics and so the range will get copied instead of moved. I don't know how expensive that is, but a move is cheaper :)