The STL algorithms are a pretty useful thing in C++. But one thing that kind of irks me is that they seem to lack composability.
For example, let's say I have a vector<pair<int, int>> and want to transform that to a vector<int> containing only the second member of the pair. That's simple enough:
std::vector<std::pair<int, int>> values = GetValues();
std::vector<int> result;
std::transform(values.begin(), values.end(), std::back_inserter(result),
[] (std::pair<int, int> p) { return p.second; });
Or maybe I want to filter the vector for only those pairs whose first member is even. Also pretty simple:
std::vector<std::pair<int, int>> values = GetValues();
std::vector<std::pair<int, int>> result;
std::copy_if(values.begin(), values.end(), std::back_inserter(result),
[] (std::pair<int, int> p) { return (p.first % 2) == 0; });
But what if I want to do both? There is no transform_if algorithm, and using both transform and copy_if seems to require allocating a temporary vector to hold the intermediate result:
std::vector<std::pair<int, int>> values = GetValues();
std::vector<std::pair<int, int>> temp;
std::vector<int> result;
std::copy_if(values.begin(), values.end(), std::back_inserter(temp),
[] (std::pair<int, int> p) { return (p.first % 2) == 0; });
std::transform(values.begin(), values.end(), std::back_inserter(result),
[] (std::pair<int, int> p) { return p.second; });
This seems rather wasteful to me. The only way I can think of to avoid the temporary vector is to abandon transform and copy_if and simply use for_each (or a regular for loop, whichever suits your fancy):
std::vector<std::pair<int, int>> values = GetValues();
std::vector<int> result;
std::for_each(values.begin(), values.end(),
[&result] (std::pair<int, int> p)
{ if( (p.first % 2) == 0 ) result.push_back(p.second); });
Am I missing something here? Is there a good way to compose two existing STL algorithms into a new one without needing temporary storage?
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.