Vector: initialization or reserve?

Question

I know the size of a vector, which is the best way to initialize it?

Option 1:

vector vec(3); //in .h
vec.at(0)=var1;             


        

Answer 1

Since it seems 5 years have passed and a wrong answer is still the accepted one, and the most-upvoted answer is completely useless (missed the forest for the trees), I will add a real response.

Method #1: we pass an initial size parameter into the vector, let's call it n. That means the vector is filled with n elements, which will be initialized to their default value. For example, if the vector holds ints, it will be filled with n zeros.

Method #2: we first create an empty vector. Then we reserve space for n elements. In this case, we never create the n elements and thus we never perform any initialization of the elements in the vector. Since we plan to overwrite the values of every element immediately, the lack of initialization will do us no harm. On the other hand, since we have done less overall, this would be the better* option.

* better - real definition: never worse. It's always possible a smart compiler will figure out what you're trying to do and optimize it for you.

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Conclusion: use method #2.