std::less

 

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Operator wrappers plus minus negate multiplies    divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
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unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*) mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

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

Run this code

#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

See also

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)