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Showing content from https://cplusplus.com/reference/functional/less/ below:

class template

<functional>

std::less 
template <class T> struct less;

Function object class for less-than inequality comparison

Binary function object class whose call returns whether the its first argument compares less than the second (as returned by operator <).

Generically, function objects are instances of a class with member function operator() defined. This member function allows the object to be used with the same syntax as a function call.

It is defined with the same behavior as:

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template <class T> struct less : binary_function <T,T,bool> {
  bool operator() (const T& x, const T& y) const {return x<y;}
};
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template <class T> struct less {
  bool operator() (const T& x, const T& y) const {return x<y;}
  typedef T first_argument_type;
  typedef T second_argument_type;
  typedef bool result_type;
};


Objects of this class can be used on standard algorithms such as sort, merge or lower_bound.

Template parameters
T
Type of the arguments to compare by the functional call.
The type shall support the operation (operator<).

Member types member type definition notes first_argument_type T Type of the first argument in member operator() second_argument_type T Type of the second argument in member operator() result_type bool Type returned by member operator()
Member functions
bool operator() (const T& x, const T& y)
Member function returning whether the first argument compares less than the second (x<y).

Example 
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// less example
#include <iostream>     // std::cout
#include <functional>   // std::less
#include <algorithm>    // std::sort, std::includes

int main () {
  int foo[]={10,20,5,15,25};
  int bar[]={15,10,20};
  std::sort (foo, foo+5, std::less<int>());  // 5 10 15 20 25
  std::sort (bar, bar+3, std::less<int>());  //   10 15 20
  if (std::includes (foo, foo+5, bar, bar+3, std::less<int>()))
    std::cout << "foo includes bar.\n";
  return 0;
}

Output:


See also
equal_to
Function object class for equality comparison (class template)
not_equal_to
Function object class for non-equality comparison (class template)
greater
Function object class for greater-than inequality comparison (class template)
greater_equal
Function object class for greater-than-or-equal-to comparison (class template)
less_equal
Function object class for less-than-or-equal-to comparison (class template)

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