C++ push_back vs Insert vs emplace

c++ vector insert push-back
Darkalfx picture Darkalfx · Feb 21, 2013 · Viewed 8.3k times · Source

I'm currently making an application using vectors with C++.

I know how pre-optimization is the root of all evil.

But I really can't help being curious.

I'm adding parts of other vectors into another vector.
We'll say the vector will have a size that never changes of 300.

Since I always append to the end of the vector

Is it faster to do:
a.reserve(300);
a.insert(a.end(), b.begin(), b.end());

or would it be faster to loop through the vector I want to append and add each items individually(while still reserving beforehand) with push_back or emplace. (unsure which is faster)

Anyone can help me on this?

Answer

Fred Foo picture Fred Foo · Feb 21, 2013

Here's a general principle: when a library provides both do_x_once and do_x_in_batch, then the latter should be at least as fast as calling do_x_once in a simple loop. If it isn't, then the library is very badly implemented since a simple loop is enough to get a faster version. Often, such batch functions/methods can perform additional optimizations because they have knowledge of data structure internals.

So, insert should be at least as fast as push_back in a loop. In this particular case, a smart implementation of insert can do a single reserve for all the elements you want to insert. push_back would have to check the vector's capacity every time. Don't try to outsmart the library :)

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