If you would like to add vector to itself both popular solutions will fail:
std::vector<std::string> v, orig;
orig.push_back("first");
orig.push_back("second");
// BAD:
v = orig;
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "", "" }
// BAD:
v = orig;
std::copy(v.begin(), v.end(), std::back_inserter(v));
// std::bad_alloc exception is generated
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "first", "second" }
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
std::copy(v.begin(), v.end(), std::back_inserter(v));
// Now v contains: { "first", "second", "first", "second" }
// GOOD (best):
v = orig;
v.insert(v.end(), orig.begin(), orig.end()); // note: we use different vectors here
// Now v contains: { "first", "second", "first", "second" }