Populating a vector with known number of elements: specify its size in constructor or by using reserve method?

Question

I would like to create a vector of some complex type, by reading individual elements from a stream. I know the vector size in advance. Is it better to specify the number of elements in the vector constructor or by using reserve method? Which one of these two is better?

int myElementCount = stream.ReadInt(); vector<MyElement> myVector(myElementCount); for (int i = 0; i < myElementCount; i++) { myVector[i] = stream.ReadMyElement(); }

or

int myElementCount = stream.ReadInt(); vector<MyElement> myVector; myVector.reserve(myElementCount); for (int i = 0; i < myElementCount; i++) { myVector.push_back(stream.ReadMyElement()); }

What about the case where I just create a vector of ints or some other simple type.

Use resize() instead of reserve(), it's a common confusion for newbs. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Nov 12, 2015 at 0:28
@πάνταῥεῖ Why? Doesn't resize also default construct the objects? OP is doing the push_back anyway. Maybe I'm confused too :) — vsoftco
– vsoftco, Commented Nov 12, 2015 at 0:31

Adam · Accepted Answer · 2015-11-12 00:28:01Z

6

It depends on what MyElement is, especially what its operator= does, so it's largely the usual "try both and use the faster one for you". There is a third choice, use c++11 and emplace_back, especially if MyElement is heavy.

As a datapoint, for int or double I found that using the constructor (or resize()) and [] is faster. Specifically, this way the loop is much easier for the compiler to vectorize.

answered Nov 12, 2015 at 0:28

Adam

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2 Comments

vsoftco Over a year ago

Side comment: emplace_back and push_back are equivalent on rvalues (which is what the OP is using), the latter also moves the object. But I agree that emplace_back clearly states the intention.

Tony Delroy Over a year ago

"especially what its operator= does" - the push_back approach involves (possibly elided) copy construction of elements (with the default allocator, that's basically a placement new), while the constructor-sized approach involve default construction then assignment, so the relative costs of all those operations is important, as is whether the compiler can optimise away repeated capacity checks and size increments effectively then do as much loop unrolling or other optimisations....

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