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

Question

There is a thread in the comments section in this post about using std::vector::reserve() vs. std::vector::resize().

Here is the original c

Answer 1

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.