Concatenating two std::vectors

Question

How do I concatenate two std::vectors?

Answer 1

This solution might be a bit complicated, but boost-range has also some other nice things to offer.

#include 
#include 
#include 

int main(int, char**) {
    std::vector a = { 1,2,3 };
    std::vector b = { 4,5,6 };
    boost::copy(b, std::back_inserter(a));
    for (auto& iter : a) {
        std::cout << iter << " ";
    }
    return EXIT_SUCCESS;
}

Often ones intention is to combine vector a and b just iterate over it doing some operation. In this case, there is the ridiculous simple join function.

#include 
#include 
#include 
#include 

int main(int, char**) {
    std::vector a = { 1,2,3 };
    std::vector b = { 4,5,6 };
    std::vector c = { 7,8,9 };
    // Just creates an iterator
    for (auto& iter : boost::join(a, boost::join(b, c))) {
        std::cout << iter << " ";
    }
    std::cout << "\n";
    // Can also be used to create a copy
    std::vector d;
    boost::copy(boost::join(a, boost::join(b, c)), std::back_inserter(d));
    for (auto& iter : d) {
        std::cout << iter << " ";
    }
    return EXIT_SUCCESS;
}

For large vectors this might be an advantage, as there is no copying. It can be also used for copying an generalizes easily to more than one container.

For some reason there is nothing like boost::join(a,b,c), which could be reasonable.