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

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< cpp‎ | utility‎ | functional
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{{cpp/title|less}}
 
{{cpp/title|less}}
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{{cpp/utility/functional/sidebar}}
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{{cpp/utility/functional/navbar}}
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{{ddcl list begin}}
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{{dcl begin}}
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{{ddcl list header | functional }}
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{{dcl header|functional}}
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{{ddcl list item |
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{{dcl rev multi
 +
|dcl1=
 
template< class T >
 
template< class T >
 +
struct less;
 +
|since2=c++14|dcl2=
 +
template< class T = void >
 
struct less;
 
struct less;
 
}}
 
}}
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{{ddcl list end}}
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{{dcl end}}
  
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Function object for performing comparisons. Implements {{c|operator<}} on type {{tt|T}}.
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Function object for performing comparisons. The main template invokes {{c|operator<}} on type {{tt|T}}.
  
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===Notes===
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===Specializations===
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{{cpp/utility/functional/pointer_order|operator<|std::less}}
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{{dsc begin}}
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{{dsc inc|cpp/utility/functional/dsc less_void}}
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{{dsc end}}
  
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===Member types===
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{{cpp/utility/functional/member types|result=bool|arg1=T|arg2=T}}
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{{tdcl list begin}}
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{{tdcl list hitem | type | definition }}
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{{tdcl list item | {{tt|result_type}} | {{tt|bool}}}}
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{{tdcl list item | {{tt|first_argument_type}} | {{tt|T}} }}
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{{tdcl list item | {{tt|second_argument_type}} | {{tt|T}} }}
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{{tdcl list end}}
+
  
 
===Member functions===
 
===Member functions===
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{{dcl list begin}}
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{{dsc begin}}
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{{dcl list mem fun | operator() | nolink=true | checks if 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}}
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{{dcl list end}}
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{{dsc end}}
  
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{{member | {{small|std::less::}}operator() | 2=
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{{member|{{small|std::less::}}operator()|2=
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{{ddcl | 1=
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{{dcl begin}}
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{{dcla|constexpr=c++14|1=
 
bool operator()( const T& lhs, const T& rhs ) const;
 
bool operator()( const T& lhs, const T& rhs ) const;
 
}}
 
}}
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{{dcl end}}
  
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Checks whether {{tt|lhs}} is ''less'' than {{tt|rhs}}.
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Checks whether {{c|lhs}} is ''less'' than {{c|rhs}}.
  
 
===Parameters===
 
===Parameters===
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{{param list begin}}
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{{par begin}}
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{{param list item | lhs, rhs | values to compare}}
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{{par|lhs, rhs|values to compare}}
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{{param list end}}
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{{par end}}
  
 
===Return value===
 
===Return value===
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{{c|true}} if {{c|1=lhs < rhs}}, {{c|false}} otherwise.
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{{c|lhs < rhs}}.
  
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===Exceptions===
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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]].
−
(none)
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 +
{{cpp/impldef exception}}
  
 
===Possible implementation===
 
===Possible implementation===
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{{eq fun cpp | 1=
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{{eq fun|1=
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bool operator()(const T &lhs, const T &rhs) const  
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constexpr bool operator()(const T& lhs, const T& rhs) const  
 
{
 
{
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     return lhs < rhs;
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     return lhs < rhs; // assumes that the implementation handles pointer total order
 
}
 
}
 
}}
 
}}
 
}}
 
}}
 +
 +
===Example===
 +
{{example
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|code=
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#include <functional>
 +
 +
template<typename A, typename B, typename C = std::less<>>
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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() {}
 +
}}
 +
 +
===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===
−
{{dcl list begin}}
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{{dsc begin}}
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{{dcl list template | cpp/memory/dcl list owner_less}}
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{{dsc inc|cpp/utility/functional/dsc equal_to}}
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{{dcl list end}}
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{{dsc inc|cpp/utility/functional/dsc greater}}
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{{dsc inc|cpp/utility/functional/ranges/dsc {{SUBPAGENAMEE}}}}
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{{dsc end}}
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 21:14, 13 July 2024

 
C++
 
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)
(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++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

(C++14)
 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

Run this code
#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]
(C++20)
 constrained function object implementing x < y
(class) [edit]
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