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std::less

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< cpp‎ | utility‎ | functional
 
 
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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.

Contents

[edit] 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 :

(since C++20)

[edit] Specializations

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

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)

[edit] 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.

[edit] 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
}

[edit] 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

[edit] See also

function object implementing x == y
(class template) [edit]
function object implementing x > y
(class template) [edit]