A binary predicate that performs the less-than operation ( operator<) on its arguments.

template <class Type = void>
struct less : public binary_function <Type, Type, bool>  
{
    bool operator()(const Type& Left, const Type& Right) const;
 };

// specialized transparent functor for operator<
template <>
struct less<void>  
{
  template <class T, class U>
  auto operator()(T&& Left, U&& Right) const`
    -> decltype(std::forward<T>(Left) <std::forward<U>(Right));
 };

Type, T, U
Any type that supports an operator< that takes operands of the specified or inferred types.

Left
The left operand of the less-than operation. The unspecialized template takes an lvalue reference argument of type Type. The specialized template does perfect forwarding of lvalue and rvalue reference arguments of inferred type T.

Right
The right operand of the less-than operation. The unspecialized template takes an lvalue reference argument of type Type. The specialized template does perfect forwarding of lvalue and rvalue reference arguments of inferred type U.

The result of Left < Right. The specialized template does perfect forwarding of the result, which has the type that's returned by operator<.

The binary predicate less< Type> provides a strict weak ordering of a set of element values of type Type into equivalence classes, if and only if this type satisfies the standard mathematical requirements for being so ordered. The specializations for any pointer type yield a total ordering of elements, in that all elements of distinct values are ordered with respect to each other.

// functional_less.cpp  
// compile with: /EHsc  
#include <vector>  
#include <algorithm>  
#include <functional>  
#include <iostream>  
  
struct MyStruct {  
   MyStruct(int i) : m_i(i){}  
  
   bool operator < (const MyStruct & rhs) const {  
      return m_i < rhs.m_i;  
   }     
  
   int m_i;  
};  
  
int main() {  
   using namespace std;  
   vector <MyStruct> v1;  
   vector <MyStruct>::iterator Iter1;  
   vector <MyStruct>::reverse_iterator rIter1;  
  
   int i;  
   for ( i = 0 ; i < 7 ; i++ )       
       v1.push_back( MyStruct(rand()));  
  
   cout << "Original vector v1 = ( " ;  
   for ( Iter1 = v1.begin() ; Iter1 != v1.end() ; Iter1++ )   
cout << Iter1->m_i << " ";  
   cout << ")" << endl;  
  
   // To sort in ascending order,  
   sort( v1.begin( ), v1.end( ), less<MyStruct>());  
  
   cout << "Sorted vector v1 = ( " ;  
   for ( Iter1 = v1.begin() ; Iter1 != v1.end() ; Iter1++ )   
cout << Iter1->m_i << " ";  
   cout << ")" << endl;  
 }  

Original vector v1 = (41 18467 6334 26500 19169 15724 11478)
Sorted vector v1 = (41 6334 11478 15724 18467 19169 26500)

Header: <functional>

Namespace: std

C++ Standard Library Reference