Question

I was looking into std::set code. I see insert signature as _Pairib insert(const value_type& _Val). Why is the input parameter passed by reference? I know that standardcContainers copy their elements into memory of the container. Does anybody know how this is achieved? Where do allocators enter the picture? Any small code/pseudocode which explains how the elements are stored/inserted would be appreciated. I am interested in understanding how the copy is done.

Answer 1

The allocator is a template parameter. Look at the definition here:

template < class Key, class Compare = less<Key>,
           class Allocator = allocator<Key> > class set;

If you don’t specify allocator of your own, it will take the default allocator (which would probably be just a new).

You can use STL containors on classes with public copy constructors, destructors and assignment operators. See here:

Elements inserted into an STL container can be of any object type that
supplies a public copy constructor, a public destructor, and a public
assignment operator. The destructor may not throw an exception.
Furthermore, associative containers such as set and map must have a
public comparison operator defined, which is operator< by default.
Some operations on containers might also require a public default
constructor and a public equivalence operator.

So basically the copy is done by using the above public member functions that you implement in your classes.