inserter iterator to insert in different container depending on predicate - c++

Question

Lets say I have a vector of numbers and I want to create two vectors to separate the numbers on odd and even ones. Easily with a simple for:
std::vector<int> odds;
std::vector<int> evens;
std::vector<int> numbers;
for (int number : numbers) {
if isOdd(number)
odds.push_back(number);
else
evens.push_back(number);
}
I would like to know if there any kind of inserter_iterator that can manage to do this so i can write something like
std::vector<int> odds;
std::vector<int> evens;
std::vector<int> numbers;
auto pred = [](int i) { return isOdd(i) ? True : False;};
auto identity = [](int i) {return i;};
std::transform(std::begin(numbers), std::end(numbers), some_inserter(odd, evens, pred), identity);
This is just out of curiosity, I'm trying to learn how algorithms and iterators work. A solution based on ranges is also valid.

Answer 1

You can use std::partition_copy algorithm, as follows https://en.cppreference.com/w/cpp/algorithm/partition_copy
int main() {
std::vector vec = {1, 2, 3, 4, 5, 6};//the vector to be partitioned
std::vector<int> odds(vec.size());//will hold odds (to have the sam size as the original (the maximum case) )
std::vector<int> evens(vec.size());//will hold evens
auto [oddsEnd, evensEnd] =
std::partition_copy(vec.begin(), vec.end(), odds.begin(), evens.begin(), [](int i){return i%2!=0;});//will copy at the front (won't insert at the back)
odds.erase(oddsEnd, odds.end());//to erase the undesired excess
evens.erase(evensEnd, evens.end());//to erase the undesired excess
}
OR use std::back_inserter https://en.cppreference.com/w/cpp/iterator/back_inserter (See the comments)
int main() {
std::vector vec = {1, 2, 3, 4, 5, 6};//the vector to be partitioned
std::vector<int> odds;//will hold odds
std::vector<int> evens;//will hold evens
std::partition_copy(vec.begin(), vec.end(), std::back_inserter(odds), std::back_inserter(evens), [](int i){return i%2!=0;});//will insert
}

Answer 2

You can simply use for_each which can be used almost like you described:
std::vector<int> odds;
std::vector<int> evens;
std::vector<int> numbers;
auto pred = [](int i) { return i % 2;};
auto identity = [](int i) {return i;};
std::for_each(std::begin(numbers), std::end(numbers), [&](int number) mutable {
int iden = identity(number);
if (pred(iden)) odds.push_back(iden);
else evens.push_back(iden);
});
// Or with range-based loop:
for (auto &elem : numbers) {
if (pred(iden)) odds.push_back(iden);
else evens.push_back(iden);
}