public member function
<map>

std::map::key_comp

key_compare key_comp() const;
Return key comparison object
Returns a copy of the comparison object used by the container to compare keys.

The comparison object of a map object is set on construction. Its type (member key_compare) is the third template parameter of the map template. By default, this is a less object, which returns the same as operator<.

This object determines the order of the elements in the container: it is a function pointer or a function object that takes two arguments of the same type as the element keys, and returns true if the first argument is considered to go before the second in the strict weak ordering it defines, and false otherwise.

Two keys are considered equivalent if key_comp returns false reflexively (i.e., no matter the order in which the keys are passed as arguments).

Parameters

none

Return value

The comparison object.
Member type key_compare is the type of the comparison object associated to the container, defined in map as an alias of its third template parameter (Compare).

Example

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// map::key_comp
#include <iostream>
#include <map>

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

  std::map<char,int>::key_compare mycomp = mymap.key_comp();

  mymap['a']=100;
  mymap['b']=200;
  mymap['c']=300;

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

  char highest = mymap.rbegin()->first;     // key value of last element

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

  std::cout << '\n';

  return 0;
}


Output:
mymap contains:
a => 100
b => 200
c => 300

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 safety

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

See also

  • 997351548 2018-02-20
  • 903691547 2018-02-20
  • 9127171546 2018-02-20
  • 2234751545 2018-02-20
  • 1685381544 2018-02-20
  • 911091543 2018-02-20
  • 9281021542 2018-02-20
  • 632231541 2018-02-20
  • 9751491540 2018-02-20
  • 8333621539 2018-02-20
  • 995001538 2018-02-20
  • 6477441537 2018-02-20
  • 511981536 2018-02-19
  • 2072881535 2018-02-19
  • 6024461534 2018-02-19
  • 7594061533 2018-02-19
  • 9311341532 2018-02-19
  • 4928331531 2018-02-19
  • 38561530 2018-02-19
  • 7776611529 2018-02-19