c++ - Concatenating two std::vector -- which method is more efficient and how/why? -

c++ - Concatenating two std::vector -- which method is more efficient and how/why? -

consider next scenario:

std::vector<int> a; std::vector<int> b; std::vector<int> ab;

i want ab have contents of a , contents of b in same order.

approach 1:

ab.reserve( a.size() + b.size() ); // preallocate memory ab.insert( ab.end(), a.begin(), a.end() ); ab.insert( ab.end(), b.begin(), b.end() );

approach 2:

std::vector<int> ab ( a.begin(), a.end() ); // calling constructor ab.insert ( ab.end(), b.begin(), b.end() );

which 1 of above methods more efficient? why? there different method more efficient?

i think first 1 faster sec 1 because perform 1 memory allocation , sec 1 have reallocate @ to the lowest degree once. my measurements seem indicate sizes less 100,000 faster:

std::vector<int> ab(a.size() + b.size()); std::copy(a.begin(), a.end(), ab.begin()); std::copy(b.begin(), b.end(), ab.begin() + b.size());

i'm guessing because, not only perform 1 allocation , no reallocation, doesn't need check if should reallocation. downside may have 0 out memory initially, think cpus @ that.

c++ performance concatenation stdvector