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

The partial specialization of std::less for any pointer type yields a total order, even if the built-in operator< does not.

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`	`bool`
`first_argument_type`	`T`
`second_argument_type`	`T`

Member functions

operator()

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

std::less::operator()

1	`bool` `operator()(` `const` `T& lhs,` `const` `T& rhs )` `const;`

(until C++14)

1	`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

#include <functional>
#include <iostream>
  
template <typename A, typename B, typename U = std::less<int>>
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

std::less

Specializations

Member types

Member functions

std::less::operator()

Parameters

Return value

Exceptions

Possible implementation

Example

See also