Merge vector of vectors into a single vector

Question

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?

Answer 1

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"; } 

Live Example