How to filter a std::integer_sequence

Question

If I theoretically have a sequence of integers like

std::integer_sequence

How can I filter it with

Answer 1

Filtering a sequence is equivalent to transforming a sequence of values into a sequence of sequences of at most one value and then concatenating them. That is, filtering the even values from <0,1,2,3> would be the same as transforming that into the sequence <<0>,<>,<2>,<>> and concatenating to yield <0,2>.

With C++17, this takes remarkably little code. We'll start with our own value and sequence type (you can easily convert a std::integer_sequence to a value_sequence):

template 
struct value { };

template 
struct value_sequence { };

The reason we use our own is so we can add operators to it. Like +:

template 
constexpr value_sequence operator+(value_sequence,
                                                 value_sequence )
{
    return {};
}

We'll use that for concatenation. Next, we add a function to transform a single value into a sequence of zero or one element:

template 
constexpr auto filter_single(value, F predicate) {
    if constexpr (predicate(Val)) {
        return value_sequence{};
    }
    else {
        return value_sequence<>{};
    }
}

And lastly, we just need our top-level filter to put it all together:

template 
constexpr auto filter(value_sequence, F predicate) {
    return (filter_single(value{}, predicate) + ...);
}

Usage from the original example:

constexpr auto evens = filter(
    value_sequence<0, 1, 2, 3, 4, 5, 6, 7, 8, 9>{},
    [](int i) constexpr { return i%2 == 0; });

How cool is C++17!