stl set

TType of the elements. Each element in a  set container is also uniquely identified by this value (each value is itself also the element's key).
Aliased as member types  set::key_type and  set::value_type. CompareA binary predicate that takes two arguments of the same type as the elements and returns a  bool. The expression  comp(a,b), where  comp is an object of this type and  a and  b are key values, shall return  true if  a is considered to go before  b in the  strict weak ordering the function defines.
The  set object uses this expression to determine both the order the elements follow in the container and whether two element keys are equivalent (by comparing them reflexively: they are equivalent if  !comp(a,b) && !comp(b,a)). No two elements in a  set container can be equivalent.
This can be a function pointer or a function object (see  constructor for an example). This defaults to  less, which returns the same as applying the  less-than operator ( a).
Aliased as member types  set::key_compare and  set::value_compare. AllocType of the allocator object used to define the storage allocation model. By default, the  allocator class template is used, which defines the simplest memory allocation model and is value-independent.
Aliased as member type  set::allocator_type.