Does a range convert to bool via emtpy() member function when used as a if condition?

Question

The following code compiles, runs, and gives the result as if range is being converted to bool via range.empty(). Is it actually what happens? I'm really not comfortable enough in navigating Range-v3 headers, so I'm asking this question here.

#include <iostream>
#include <range/v3/view/take.hpp>
#include <range/v3/view/drop.hpp>
#include <vector>

int main() {

    std::vector<int> v{1};
    if (auto range = v | ranges::views::take(1)) {
        std::cout << "take" << std::endl;
    }
    if (auto range = v | ranges::views::drop(1)) {
        std::cout << "drop" << std::endl;
    }
}

score 1 · Accepted Answer · edited Feb 15 '21 at 18:18

The only operations required for a type to fit the std::ranges::range concept (or its Range-v3 equivalent) is being able to perform std::ranges::begin/end on it (or its Range-v3 equivalent). But this does not mean that a range type only has those operations.

A type that fits a concept can implement anything it wants on top of that. So while the range concept does not specify the functionality you're seeing, it would be valid for any particular type that satisfies range to provide it.

So the question you're really asking is whether these particular range view types provide such an operator. The answer is yes, according to the documentation. The view_interface type provides an explicit operator bool if the underlying range is a type for which empty testing is allowed. And for what it's worth, so does the C++20 equivalent.

Does a range convert to bool via emtpy() member function when used as a if condition?

1 Answers1