CODEWITHSUNDEEP
c++vectorstd
mk33
mk33

Reputation: 351

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

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.

Upvotes: 7

Views: 255

Answers (1)

Adam
Adam

Reputation: 17369

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.

Upvotes: 6

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