Composability of STL algorithms

Back in 2000, the problem was already noted. Gary Powell and Martin Weiser came up with a "view" concept, and coined the name "View Template Library". It didn't take off then but the idea makes sense. A "view" adaptor essentially applies an on-the-fly transform. For instance, it can adapt the value_type.

The concept probably should be readdressed now we have C++0x. We've made quite some progress in generic programming since 2000.

For example, let's use the vector<pair<int, int>> to vector<int> example. That could be quite simple:

std::vector<std::pair<int, int>> values = GetValues();
vtl2::view v (values, [](std::pair<int, int> p) { return p.first }); 
std::vector<int> result(view.begin(), view.end());

Or, using the boost::bind techniques, even simpler:

std::vector<std::pair<int, int>> values = GetValues();
vtl2::view v (values, &std::pair<int, int>::first); 
std::vector<int> result(view.begin(), view.end());

I think the problem is unfortunately structural

1. C++ uses two iterators to represent a sequence
2. C++ functions are single-valued

so you cannot chain them because a function cannot return "a sequence".

An option would have been to use single-object sequences instead (like the range approach from boost). This way you could have combined the result of one processing as the input of another... (one object -> one object).

In the standard C++ library instead the processing is (two objects -> one object) and it's clear that this cannot be chained without naming the temporary object.

Since C++20 you can use std::ranges::copy together with the range adaptors std::views::filter and std::views::values from the Ranges library as follows:

int main() {
    std::vector<std::pair<int, int>> values = { {1,2}, {4,5}, {6,7}, {9,10} };
    std::vector<int> result;

    auto even = [](const auto& p) { return (p.first % 2) == 0; };
    std::ranges::copy(values | std::views::filter(even) | std::views::values,
                      std::back_inserter(result));

    for (int i : result)
        std::cout << i << std::endl;

    return 0;
}

Output:

5
7

In the solution above, no temporary vector is created for an intermediate result, because the view adaptors create ranges that don't contain elements. These ranges are just views over the input vector, but with a customized iteration behavior.

Code on Wandbox

You're right. You can use Boost.Range adaptors to achieve composition.