Best way to append vector to vector [duplicate]

Answer 1

Assuming you want to copy and not move, this would be the best way:

a.reserve(a.size()+b.size()+c.size()); // Reserve space first
a.insert(a.end(),b.begin(),b.end());
a.insert(a.end(),c.begin(),c.end());

If you want to move:

a.reserve(a.size()+b.size()+c.size()); // Reserve space first
a.insert(a.end(),std::make_move_iterator(b.begin()),
         std::make_move_iterator(b.end()));
a.insert(a.end(),std::make_move_iterator(c.begin()),
         std::make_move_iterator(c.end()));
b.swap(std::vector()); // Clear and deallocate space
c.swap(std::vector()); // Clear and deallocate space

Update: You've edited your question several times now making it somewhat of a moving target. Your first option is now very similar to my first suggestion.

Update 2: As of C++11, you may no longer have to use the "swap with empty vector" trick to clear and deallocate space, depending on your library's implementation of vector. The following may do the job in a more intuitive way:

// Empty the vectors of objects
b.clear(); 
c.clear();

// Deallocate the memory allocated by the vectors 
// Note: Unlike the swap trick, this is non-binding and any space reduction
//       depends on the implementation of std::vector
b.shrink_to_fit();
c.shrink_to_fit();