See the example from cplusplus.com for std::copy_if:
std::vector<int> foo = {25,15,5,-5,-15};
std::vector<int> bar;
// copy only positive numbers:
std::copy_if (foo.begin(), foo.end(), std::back_inserter(bar), [](int i){return i>=0;} );
std::copy_if evaluates the lambda expression for every element in foo here and if it returns true it copies the value to bar.
The std::back_inserter allows us to actually insert new elements at the end of bar (using push_back()) with an iterator without having to resize it to the required size first.