I have vector of vectors of T:
std::vector<std::vector<T>> vector_of_vectors_of_T;
I want to merge all of them into single vector of T:
std::vector<T> vector_of_T;
I am currently using this method:
size_t total_size{ 0 };
for (auto const& items: vector_of_vectors_of_T){
total_size += items.size();
}
vector_of_T.reserve(total_size);
for (auto const& items: vector_of_vectors_of_T){
vector_of_T.insert(end(vector_of_T), begin(items), end(items));
}
Is there more straightforward method? Like a ready std function? If not, is there more efficient way to do it manually?
It's a nice exercise to try and write up a generic join. The code below takes a nested container R1<R2<T> and returns a joined container R1<T>. Note that because of the allocator parameters in the Standard Library, this is a bit cumbersome. No attempt is being made to check for allocator compatibility etc.
Fortunately, there's action::join function in the upcoming range-v3 library by Eric Niebler, that is quite robust already and works today on Clang:
#include <range/v3/all.hpp>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
// quick prototype
template<template<class, class...> class R1, template<class, class...> class R2, class T, class... A1, class... A2>
auto join(R1<R2<T, A2...>, A1...> const& outer)
{
R1<T, A2...> joined;
joined.reserve(std::accumulate(outer.begin(), outer.end(), std::size_t{}, [](auto size, auto const& inner) {
return size + inner.size();
}));
for (auto const& inner : outer)
joined.insert(joined.end(), inner.begin(), inner.end());
return joined;
}
int main()
{
std::vector<std::vector<int>> v = { { 1, 2 }, { 3, 4 } };
// quick prototype
std::vector<int> w = join(v);
std::copy(w.begin(), w.end(), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
// Eric Niebler's range-v3
std::vector<int> u = ranges::action::join(v);
std::copy(u.begin(), u.end(), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
}
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