std::less

Defined in header `<functional>`
template< class T > struct less;		(until C++14)
template< class T = void > struct less;		(since C++14)

Function object for performing comparisons. Unless specialized, invokes operator< on type T.

Specializations

A specialization of std::less for any pointer type yields a strict total order, even if the built-in operator< does not. The strict total order is consistent among specializations of std::less, std::greater, std::less_equal, and std::greater_equal for that pointer type, and is also consistent with the partial order imposed by the corresponding built-in operators (<, >, <= and >=).

If the function call operator of the specialization std::less<void> calls a built-in operator comparing pointers, it yields a strict total order even if the built-in operator< does not. This strict total order is consistent among the specializations std::less<void>, std::greater<void>, std::less_equal<void>, and std::greater_equal<void>, and is also consistent with the partial order imposed by the corresponding built-in operators.

(since C++14)

The standard library provides a specialization of std::less when T is not specified, which leaves the parameter types and return type to be deduced.

less<void>

function object implementing x < y deducing argument and return types
(class template specialization)

(since C++14)

Member types

Type	Definition
`result_type`(deprecated in C++17)	`bool`
`first_argument_type`(deprecated in C++17)	`T`
`second_argument_type`(deprecated in C++17)	`T`

(until C++20)

Member functions

operator()

checks whether the first argument is less than the second
(public member function)

std::less::operator()

bool operator()( const T& lhs, const T& rhs ) const;		(until C++14)
constexpr bool operator()( const T& lhs, const T& rhs ) const;		(since C++14)

Checks whether lhs is less than rhs.

Parameters

lhs, rhs

-

values to compare

Return value

true if lhs < rhs, false otherwise.

Exceptions

(none)

Possible implementation

constexpr bool operator()(const T &lhs, const T &rhs) const 
{
    return lhs < rhs;
}

Example

Run this code

#include <functional>
#include <iostream>
 
template <typename A, typename B, typename U = std::less<>>
bool f(A a, B b, U u = U())
{
    return u(a, b);
}
 
int main() 
{
    std::cout << std::boolalpha;   
    std::cout << f(5, 20) << '\n';
    std::cout << f(100, 10) << '\n';
}

Output:

true
false

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