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

Both variants have different semantics, i.e. you are comparing apples and oranges.

The first gives you a vector of n default-initialized values, the second variant reserves the memory, but does not initialize them.

Choose what better fits your needs, i.e. what is "better" in a certain situation.