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Difference between revisions of "cpp/utility/functional/less"

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< cpp‎ | utility‎ | functional
m (Text replace - "(===\s*Exceptions?\s*===\s+)\(none\)" to "{{cpp/impldef exception}}")
m (~)
 
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{{cpp/utility/functional/navbar}}
 
{{cpp/utility/functional/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
{{dcl header | functional }}
+
{{dcl header|functional}}
{{dcl rev begin}}
+
{{dcl rev multi
{{dcl | until=c++14 |
+
|dcl1=
 
template< class T >
 
template< class T >
 
struct less;
 
struct less;
}}
+
|since2=c++14|dcl2=
{{dcl | since=c++14 |
+
template< class T = void >
template< class T {{=}} void >
+
 
struct less;
 
struct less;
 
}}
 
}}
{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
Function object for performing comparisons. Unless specialized, invokes {{c|operator<}} on type {{tt|T}}.
+
Function object for performing comparisons. The main template invokes {{c|operator<}} on type {{tt|T}}.
  
 
===Specializations===
 
===Specializations===
{{cpp/utility/functional/pointer_order|operator<|std::less}}
 
 
{{rev begin}}
 
{{rev | since=c++14 |
 
The standard library provides a specialization of {{tt|std::less}} when {{tt|T}} is not specified, which leaves the parameter types and return type to be deduced.
 
 
{{dsc begin}}
 
{{dsc begin}}
{{dsc inc | cpp/utility/functional/dsc less_void}}
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{{dsc inc|cpp/utility/functional/dsc less_void}}
 
{{dsc end}}
 
{{dsc end}}
}}
 
{{rev end}}
 
  
 
{{cpp/utility/functional/member types|result=bool|arg1=T|arg2=T}}
 
{{cpp/utility/functional/member types|result=bool|arg1=T|arg2=T}}
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===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
{{dsc mem fun | operator() | nolink=true | checks whether the first argument is ''less'' than the second}}
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{{dsc mem fun|operator()|nolink=true|checks whether the first argument is ''less'' than the second}}
 
{{dsc end}}
 
{{dsc end}}
  
{{member | {{small|std::less::}}operator() | 2=
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{{member|{{small|std::less::}}operator()|2=
 
{{dcl begin}}
 
{{dcl begin}}
{{dcl rev begin}}
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{{dcla|constexpr=c++14|1=
{{dcl |until=c++14|
+
 
bool operator()( const T& lhs, const T& rhs ) const;
 
bool operator()( const T& lhs, const T& rhs ) const;
 
}}
 
}}
{{dcl |since=c++14|
 
constexpr bool operator()( const T& lhs, const T& rhs ) const;
 
}}
 
{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
Checks whether {{tt|lhs}} is ''less'' than {{tt|rhs}}.
+
Checks whether {{c|lhs}} is ''less'' than {{c|rhs}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
{{par | lhs, rhs | values to compare}}
+
{{par|lhs, rhs|values to compare}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
{{c|true}} if {{c|1=lhs < rhs}}, {{c|false}} otherwise.
+
{{c|lhs < rhs}}.
 +
 
 +
If {{tt|T}} is a pointer type, the result is consistent with the [[cpp/language/operator comparison#Pointer total order|implementation-defined strict total order over pointers]].
  
 
{{cpp/impldef exception}}
 
{{cpp/impldef exception}}
  
 
===Possible implementation===
 
===Possible implementation===
{{eq fun | 1=
+
{{eq fun|1=
constexpr bool operator()(const T &lhs, const T &rhs) const  
+
constexpr bool operator()(const T& lhs, const T& rhs) const  
 
{
 
{
     return lhs < rhs;
+
     return lhs < rhs; // assumes that the implementation handles pointer total order
 
}
 
}
 
}}
 
}}
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===Example===
 
===Example===
 
{{example
 
{{example
| code=
+
|code=
 
#include <functional>
 
#include <functional>
#include <iostream>
 
  
template <typename A, typename B, typename U = std::less<>>
+
template<typename A, typename B, typename C = std::less<>>
bool f(A a, B b, U u = U())
+
constexpr bool fun(A a, B b, C cmp = C{})
 
{
 
{
     return u(a, b);
+
     return cmp(a, b);
 
}
 
}
  
int main()  
+
static_assert(fun(1, 2) == true);
{
+
static_assert(fun(1.0, 1) == false);
    std::cout << std::boolalpha;   
+
static_assert(fun(1, 2.0) == true);
    std::cout << f(5, 20) << '\n';
+
static_assert(std::less<int>{}(5, 5.6) == false)// 5 < 5 (warn: implicit conversion)
    std::cout << f(100, 10) << '\n';
+
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)
| output=
+
static_assert(std::less{}(5, 5.6) == true);        // less<void>: 5.0 < 5.6
true
+
 
false
+
int main() {}
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=2562|std=C++98|before=the pointer total order might be inconsistent|after=guaranteed to be consistent}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
{{dsc inc | cpp/utility/functional/dsc greater}}
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{{dsc inc|cpp/utility/functional/dsc equal_to}}
 +
{{dsc inc|cpp/utility/functional/dsc greater}}
 +
{{dsc inc|cpp/utility/functional/ranges/dsc {{SUBPAGENAMEE}}}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 21:14, 13 July 2024

 
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
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.

Contents

[edit] Specializations

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

[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)

[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;
(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.

[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 handles pointer total order
}

[edit] 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() {}

[edit] 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

[edit] See also

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