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c++vector

Vector: initialization or reserve?

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

Option 1:

vector<int> vec(3); //in .h
vec.at(0)=var1;     //in .cpp
vec.at(1)=var2;     //in .cpp
vec.at(2)=var3;     //in .cpp

Option 2:

vector<int> vec;     //in .h
vec.reserve(3);      //in .cpp
vec.push_back(var1); //in .cpp
vec.push_back(var2); //in .cpp
vec.push_back(var3); //in .cpp

I guess, Option2 is better than Option1. Is it? Any other options?

Solution

  • 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.