c++ - Choice between vector::resize() and vector::reserve()

Question

I am pre-allocating some memory to my a vector member variable. Below code is minimal part

class A {
  vector<string> t_Names;
public:
  A () : t_Names(1000) {}
};

Now at some point of time, if the t_Names.size() equals 1000. I am intending to increase the size by 100. Then if it reaches 1100, again increase by 100 and so on.

My question is, what to choose between vector::resize() and vector::reserve(). Is there any better choice in this kind of scenario ?

Edit: I have sort of precise estimate for the t_Names. I estimate it to be around 700 to 800. However in certain (seldom) situations, it can grow more than 1000.

Answer 1

The two functions do vastly different things!

The resize() method (and passing argument to constructor is equivalent to that) will insert or delete appropriate number of elements to the vector to make it given size (it has optional second argument to specify their value). It will affect the size(), iteration will go over all those elements, push_back will insert after them and you can directly access them using the operator[].

The reserve() method only allocates memory, but leaves it uninitialized. It only affects capacity(), but size() will be unchanged. There is no value for the objects, because nothing is added to the vector. If you then insert the elements, no reallocation will happen, because it was done in advance, but that's the only effect.

So it depends on what you want. If you want an array of 1000 default items, use resize(). If you want an array to which you expect to insert 1000 items and want to avoid a couple of allocations, use reserve().

EDIT: Blastfurnace's comment made me read the question again and realize, that in your case the correct answer is don't preallocate manually. Just keep inserting the elements at the end as you need. The vector will automatically reallocate as needed and will do it more efficiently than the manual way mentioned. The only case where reserve() makes sense is when you have reasonably precise estimate of the total size you'll need easily available in advance.

EDIT2: Ad question edit: If you have initial estimate, then reserve() that estimate. If it turns out to be not enough, just let the vector do it's thing.