Question

Let v1 be the target vector, v2 needs to be appended to the back of it.

I’m now doing:

v1.reserve(v1.size() + v2.size()); 
copy(v2.begin(), v2.end(), back_inserter(v1));

Is this the most efficient way? Or can it maybe be done just via copying a chunk of memory?
Thanks!

Answer 1

After a lot of arguing (and a reasonable comment from Matthieu M. and villintehaspam), I’ll change my suggestion to

v1.insert( v1.end(), v2.begin(), v2.end() );

I’ll keep the former suggestion here:

v1.reserve( v1.size() + v2.size() ); 
v1.insert( v1.end(), v2.begin(), v2.end() );

There are some reasons to do it the latter way, although none of them enough strong:

• there is no guarantee on to what size will the vector be reallocated — e.g. if the sum size is 1025, it may get reallocated to 2048 — dependant on implementation. There is no such guarantee for reserve either, but for a specific implementation it might be true. If hunting for a bottleneck it might be rasonable to check that.
• reserve states our intentions clear — optimization may be more efficient in this case (reserve could prepare the cache in some top-notch implementation).
• also, with reserve we have a C++ Standard guarantee that there will be only a single reallocation, while insert might be implemented inefficiently and do several reallocations (also something to test with a particular implementation).