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std::less

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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] Specializations

If the built-in operator< does not provide total order for pointers, a partial specialization of std::less for pointer types is provided, which guarantees total order.

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.

function object implementing x < y deducing argument and return types
(class template specialization) [edit]
(since C++14)

[edit] Member types

type definition
result_type bool
first_argument_type T
second_argument_type T

[edit] Member functions

operator()
checks if 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

true if lhs < rhs, false otherwise.

Exceptions

(none)

Possible implementation

constexpr bool operator()(const T &lhs, const T &rhs) const 
{
    return lhs < rhs;
}

[edit] 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

[edit] See also

provides mixed-type owner-based ordering of shared and weak pointers
(class template) [edit]