C++ std::set

std::set

template < class T,                        // set::key_type/value_type
           class Compare = less<T>,        // set::key_compare/value_compare
           class Alloc = allocator<T>      // set::allocator_type
           > class set;

Set

Sets are containers that store unique elements following a specific order.

In a set, the value of an element also identifies it (the value is itself the key, of type T), and each value must be unique. The value of the elements in a set cannot be modified once in the container (the elements are always const), but they can be inserted or removed from the container.

Internally, the elements in a set are always sorted following a specific strict weak ordering criterion indicated by its internal comparison object (of type Compare).

set containers are generally slower than unordered set containers to access individual elements by their key, but they allow the direct iteration on subsets based on their order.

Sets are typically implemented as binary search trees.

Container properties

• Associative Elements in associative containers are referenced by their key and not by their absolute position in the container.
• Ordered The elements in the container follow a strict order at all times. All inserted elements are given a position in this order.
• Set The value of an element is also the key used to identify it.
• Unique keys No two elements in the container can have equivalent keys.
• Allocator-aware The container uses an allocator object to dynamically handle its storage needs.

Template parameters

• T
  Type 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.

• Compare
  A binary predicate(断言) 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<b).
  Aliased as member types set::key_compare and set::value_compare.

• Alloc
  Type 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.

Member types

member type	definition	notes
key_type	The first template parameter (T)
value_type	The first template parameter (T)
key_compare	The second template parameter (Compare)	defaults to: less<key_type>
value_compare	The second template parameter (Compare)	defaults to: less<value_type>
allocator_type	The third template parameter (Alloc)	defaults to: allocator<value_type>
reference	allocator_type::reference	for the default allocator: value_type&
const_reference	allocator_type::const_reference	for the default allocator: const value_type&
pointer	allocator_type::pointer	for the default allocator: value_type*
const_pointer	allocator_type::const_pointer	for the default allocator: const value_type*
iterator	a bidirectional iterator to value_type	convertible to const_iterator
const_iterator	a bidirectional iterator to const value_type
reverse_iterator	reverse_iterator
const_reverse_iterator	reverse_iterator<const_iterator>
difference_type	a signed integral type, identical to: iterator_traits::difference_type	usually the same as ptrdiff_t
size_type	an unsigned integral type that can represent any non-negative value of difference_type	usually the same as size_t

Member functions

• (constructor) Construct set (public member function )
• (destructor) Set destructor (public member function )
• operator= Copy container content (public member function )

Iterators:

• begin Return iterator to beginning (public member function )
• end Return iterator to end (public member function )
• rbegin Return reverse iterator to reverse beginning (public member function )
• rend Return reverse iterator to reverse end (public member function )
• cbegin Return const_iterator to beginning (public member function )
• cend Return const_iterator to end (public member function )
• crbegin Return const_reverse_iterator to reverse beginning (public member function )
• crend Return const_reverse_iterator to reverse end (public member function )

Capacity:

• empty Test whether container is empty (public member function )
• size Return container size (public member function )
• max_size Return maximum size (public member function )

Modifiers:

• insert Insert element (public member function )
• erase Erase elements (public member function )
• swap Swap content (public member function )
• clear Clear content (public member function )
• emplace Construct and insert element (public member function )
• emplace_hint Construct and insert element with hint (public member function )

Observers:

• key_comp Return comparison object (public member function )
• value_comp Return comparison object (public member function )

Operations:

• find Get iterator to element (public member function )
• count Count elements with a specific value (public member function )
• lower_bound Return iterator to lower bound (public member function )
• upper_bound Return iterator to upper bound (public member function )
• equal_range Get range of equal elements (public member function )

Allocator:

• get_allocator Get allocator (public member function )

Code Example

#include <iostream>
#include <set>

using namespace std;

bool fncomp(int lhs, int rhs)
{ return lhs < rhs; }

struct classcomp
{
    bool operator() (const int& lhs, const int& rhs) const
    { return lhs<rhs; }
};

int main(int argc, char **argv)
{
    set<int> first;

    int myints[] = {10,20,30,40,50};

    set<int> first1(myints, myints+5);
    set<int> first2(first1);

    set<int> first3(first2.begin(),first2.end());

    set<int, classcomp> first4;

    bool (*fn_pt)(int,int) = fncomp;
    set<int, bool(*)(int,int)> first5(fn_pt);
    /** other function please to reference other container */

    set<int> second;
    for(int i=0; i < 10; i++)
    {
        second.insert(i);
    }

    cout << '\n';
    set<int>::key_compare comp = second.key_comp();
    int nNum = *(second.rbegin());
    auto it = second.begin();
    do{
        cout << *it << '\t';
    }while( comp(*(++it), nNum) );
    /** outpur:0 1 2 3 4 5 6 7 8 */

    set<int> third;
    for(int i=0; i < 5; i++)
    {
        third.insert(i);
    }
    set<int>::value_compare valComp = third.value_comp();
    nNum = *(third.rbegin());
    it = third.begin();
    cout << '\n';
    do
    {
        cout << *it << '\t';
    }while( valComp( *(++it), nNum ) );

    return 0;
}

Reference

cplusplus