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std::less - cppreference.com

From cppreference.com

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. The main template invokes operator< on type T.

Specializations

function object implementing x < y deducing parameter and return types
(class template specialization) [edit]

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

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;

 (constexpr since C++14)

Checks whether lhs is less than rhs.

Parameters

lhs, rhs - values to compare

Return value

lhs < rhs.

If T is a pointer type, the result is consistent with the implementation-defined strict total order over pointers.

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 handles pointer total order
}

Example

#include <functional>

template<typename A, typename B, typename C = std::less<>>
constexpr bool fun(A a, B b, C cmp = C{})
{
    return cmp(a, b);
}

static_assert(fun(1, 2) == true);
static_assert(fun(1.0, 1) == false);
static_assert(fun(1, 2.0) == true);
static_assert(std::less<int>{}(5, 5.6) == false);   // 5 < 5 (warn: implicit conversion)
static_assert(std::less<double>{}(5, 5.6) == true); // 5.0 < 5.6
static_assert(std::less<int>{}(5.6, 5.7) == false); // 5 < 5 (warn: implicit conversion)
static_assert(std::less{}(5, 5.6) == true);         // less<void>: 5.0 < 5.6

int main() {}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2562 C++98 the pointer total order might be inconsistent guaranteed to be consistent

See also