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Showing content from https://cplusplus.com/map::value_comp below:

public member function

<map>

std::map::value_comp 
value_compare value_comp() const;

Return value comparison object

Returns a comparison object that can be used to compare two elements to get whether the key of the first one goes before the second.

The arguments taken by this function object are of member type value_type (defined in map as an alias of pair<const key_type,mapped_type>), but the mapped_type part of the value is not taken into consideration in this comparison.

The comparison object returned is an object of the member type map::value_compare, which is a nested class that uses the internal comparison object to generate the appropriate comparison functional class. It is defined with the same behavior as:

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template <class Key, class T, class Compare, class Alloc>
class map<Key,T,Compare,Alloc>::value_compare
{   // in C++98, it is required to inherit binary_function<value_type,value_type,bool>
  friend class map;
protected:
  Compare comp;
  value_compare (Compare c) : comp(c) {}  // constructed with map's comparison object
public:
  typedef bool result_type;
  typedef value_type first_argument_type;
  typedef value_type second_argument_type;
  bool operator() (const value_type& x, const value_type& y) const
  {
    return comp(x.first, y.first);
  }
}


The public member of this comparison class returns true if the key of the first argument is considered to go before that of the second (according to the strict weak ordering specified by the container's comparison object, key_comp), and false otherwise.

Notice that value_compare has no public constructor, therefore no objects can be directly created from this nested class outside map members.


Parameters none

Return value The comparison object for element values.
Member type value_compare is a nested class type (described above).

Example 
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// map::value_comp
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> mymap;

  mymap['x']=1001;
  mymap['y']=2002;
  mymap['z']=3003;

  std::cout << "mymap contains:\n";

  std::pair<char,int> highest = *mymap.rbegin();          // last element

  std::map<char,int>::iterator it = mymap.begin();
  do {
    std::cout << it->first << " => " << it->second << '\n';
  } while ( mymap.value_comp()(*it++, highest) );

  return 0;
}

Output: 
mymap contains:
x => 1001
y => 2002
z => 3003


Complexity Constant.

Iterator validity No changes.

Data races The container is accessed.
No contained elements are accessed: concurrently accessing or modifying them is safe.

Exception safetyStrong guarantee: if an exception is thrown, there are no changes in the container.

See also
map::key_comp
Return key comparison object (public member function)
map::find
Get iterator to element (public member function)
map::count
Count elements with a specific key (public member function)
map::lower_bound
Return iterator to lower bound (public member function)
map::upper_bound
Return iterator to upper bound (public member function)

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