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?
question from:
https://stackoverflow.com/questions/6743093/composability-of-stl-algorithms 
