##### cppreference.com

##### Search

Create account

##### Namespaces

- Page
- Discussion

##### Variants

##### Views

- View
- Edit
- History

##### Actions

# std::less

From cppreference.com
< cpp‎ | utility‎ | functional

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`.

### [edit] Specializations

A specialization of `std::less` for any pointer type yields a strict total order, even if the built-in operator< does not.The strict total order is consistent among specializations of std::less, std::greater, std::less_equal, and std::greater_equal for that pointer type, and is also consistent with the partial order imposed by the corresponding built-in operators (`<`, `>`, `<=` and `>=`).

If the function call operator of the specialization `std::less<void>` calls a built-in operator comparing pointers, it yields a strict total order even if the built-in operator< does not.This strict total order is consistent among the specializations `std::less<void>`, `std::greater<void>`, `std::less_equal<void>`, and `std::greater_equal<void>`, and is also consistent with the partial order imposed by the corresponding built-in operators. | (since C++14)

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) [edit]
| (since C++14)

### [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`

### [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

Run this code

    #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

owner_less (C++11) | provides mixed-type owner-based ordering of shared and weak pointers (class template) [edit]
greater | function object implementing x > y (class template) [edit]

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/less&oldid=84267"
Hidden category:

- Pages with unreviewed LWG DR marker