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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.

Implementation-defined strict total order over pointers

A specialization of std::less for any pointer type yields the implementation-defined strict total order, even if the built-in < operator does not.

The implementation-defined strict total order is consistent with the partial order imposed by built-in comparison operators (<=>, (since C++20)<, >, <=, and >=), and consistent among following standard function objects:

std::less, std::greater, std::less_equal, and std::greater_equal, when the template argument is a pointer type or void (since C++14)

std::ranges::equal_to, std::ranges::not_equal_to, std::ranges::less, std::ranges::greater, std::ranges::less_equal, std::ranges::greater_equal, and std::compare_three_way

(since C++20)

Specializations

less<void>

(C++14)

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

Member types

Type	Definition
`result_type` (deprecated in C++17)(removed in C++20)	`bool`
`first_argument_type` (deprecated in C++17)(removed in C++20)	`T`
`second_argument_type` (deprecated in C++17)(removed in C++20)	`T`

These member types are obtained via publicly inheriting std::binary_function<T, T, bool>.

(until C++11)

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

For T which is not a pointer type, true if lhs < rhs, false otherwise.

For T which is a pointer type, true if lhs precedes rhs in the implementation-defined strict total order, false otherwise.

Exceptions

May throw implementation-defined exceptions.

Possible implementation

constexpr bool operator()(const T &lhs, const T &rhs) const 
{
    return lhs < rhs; // assumes that the implementation uses a flat address space
}

Example

#include <functional>
#include <iostream>
 
template<typename A, typename B, typename C = std::less<>>
bool fun(A a, B b, C cmp = C{})
{
    return cmp(a, b);
}
 
int main()
{
    std::cout
        << std::boolalpha
        << fun(1, 2)   << ' ' // true
        << fun(1.0, 1) << ' ' // false
        << fun(1, 2.0) << ' ' // true
        << std::less<int>{}(5, 5.6)    << ' ' // false: 5 < 5 (warn: implicit conversion)
        << std::less<double>{}(5, 5.6) << ' ' // true: 5.0 < 5.6
        << std::less<int>{}(5.6, 5.7)  << ' ' // false: 5 < 5 (warn: implicit conversion)
        << std::less{}(5, 5.6)         << ' ' // true: less<void>: 5.0 < 5.6
        << '\n';
}

Output:

true false true false true false true

equal_to	function object implementing `x == y` (class template)
greater	function object implementing `x > y` (class template)
ranges::less (C++20)	function object implementing `x < y` (class)