Resizing std::vector without destroying elements

Question

I am using all the time the same std::vector<int> in order to try to avoid allocating an deallocating all the time. In a few lines, my code is as follows:

std::vector<int> myVector;
myVector.reserve(4);

for (int i = 0; i < 100; ++i) {
    fillVector(myVector);
    //use of myVector
    //....
    myVector.resize(0);
}

In each for iteration, myVector will be filled with up to 4 elements. In order to make efficient code, I want to use always myVector. However, in myVector.resize() the elements in myVector are being destroyed. I understand that myVector.clear() will have the same effect.

I think if I could just overwrite the existing elements in myVector I could save some time. However I think the std::vector is not capable of doing this.

Is there any way of doing this? Does it make sense to create a home-grown implementation which overwrites elements ?

Answer 1

Your code is already valid (myVector.clear() has better style than myVector.resize(0) though).

'int destructor' does nothing.
So resize(0) just sets the size to 0, capacity is untouched.

Answer 2

Simply don't keep resizing myVector. Instead, initialise it with 4 elements (with std::vector<int> myVector(4)) and just assign to the elements instead (e.g. myVector[0] = 5).

However, if it's always going to be fixed size, then you might prefer to use a std::array<int, 4>.