std::vector::resize() vs. std::vector::reserve()

There are two different methods for a reason:

std::vector::reserve will allocate the memory but will not resize your vector, which will have a logical size the same as it was before.

std::vector::resize will actually modify the size of your vector and will fill any space with objects in their default state. If they are ints, they will all be zero.

After reserve, in your case, you will need a lot of push_backs to write to element 5. If you don't wish to do that then in your case you should use resize.

One thing about reserve: if you then add elements with push_back, until you reach the capacity you have reserved, any existing references, iterators or pointers to data in your vector will remain valid. So if I reserve 1000 and my size is 5, the &vec[4] will remain the same until the vector has 1000 elements. After that, I can call push_back() and it will work, but the stored pointer of &vec[4] earlier may no longer be valid.

Answered here by Jan Hudec : Choice between vector::resize() and vector::reserve()

The two functions do vastly different things.

The resize() method (and passing argument to constructor is equivalent to that) will insert given number of elements to the vector (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 dont'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, than reserve() that estimate and if it turns out to be not enough, just let the vector do it's thing.

It depends on what you want to do. reserve does not add any elements to the vector; it only changes the capacity(), which guarantees that adding elements will not reallocate (and e.g. invalidate iterators). resize adds elements immediately. If you want to add elements later (insert(), push_back()), use reserve. If you want to access elements later (using [] or at()), use resize. So youre MyClass::my_method can be either:

void MyClass::my_method()
{
    my_member.clear();
    my_member.reserve( n_dim );
    for ( int k = 0; k < n_dim; ++ k ) {
        my_member.push_back( k );
    }
}

or

void MyClass::my_method()
{
    my_member.resize( n_dim );
    for ( int k = 0; k < n_dim; ++ k ) {
        my_member[k] = k;
    }
}

Which one you chose is a question of taste, but the code you quote is clearly incorrect.