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

From cppreference.com
< cpp‎ | utility‎ | functional
(Mark that c++14 allows independent argument types)
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{{cpp/utility/functional/navbar}}
 
{{cpp/utility/functional/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl header | functional }}
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{{dcl header|functional}}
 
{{dcl rev multi
 
{{dcl rev multi
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| dcl1=
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|dcl1=
 
template< class T >
 
template< class T >
 
struct less;
 
struct less;
−
| since2=c++14 | dcl2=
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|since2=c++14|dcl2=
 
template< class T = void >
 
template< class T = void >
 
struct less;
 
struct less;
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{{dcl end}}
 
{{dcl end}}
  
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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}}.
−
 
+
−
{{cpp/utility/functional/pointer_order|<}}
+
  
 
===Specializations===
 
===Specializations===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/utility/functional/dsc less_void}}
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{{dsc inc|cpp/utility/functional/dsc less_void}}
 
{{dsc end}}
 
{{dsc end}}
  
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===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
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{{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=
+
{{member|{{small|std::less::}}operator()|2=
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl rev multi
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{{dcla|constexpr=c++14|1=
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| dcl1=
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bool operator()( const T& lhs, const T& rhs ) const;
 
bool operator()( const T& lhs, const T& rhs ) const;
−
| since2=c++14 | dcl2=
 
−
constexpr bool operator()( const T1& lhs, const T2& rhs ) const;
 
 
}}
 
}}
 
{{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}}
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{{par | lhs, rhs | values to compare}}
+
{{par|lhs, rhs|values to compare}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
−
For {{tt|T}} which is not a pointer type, {{c|true}} if {{c|1=lhs < rhs}}, {{c|false}} otherwise.
+
{{c|lhs < rhs}}.
  
−
For {{tt|T}} which is a pointer type, {{c|true}} if {{tt|lhs}} precedes {{tt|rhs}} in the implementation-defined strict total order, {{c|false}} otherwise.
+
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 T1 &lhs, const T2 &rhs) const  
+
constexpr bool operator()(const T& lhs, const T& rhs) const  
 
{
 
{
−
     return lhs < rhs; // assumes that the implementation uses a flat address space
+
     return lhs < rhs; // assumes that the implementation handles pointer total order
 
}
 
}
 
}}
 
}}
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===Example===
 
===Example===
 
{{example
 
{{example
−
| code=
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|code=
 
#include <functional>
 
#include <functional>
−
#include <iostream>
 
  
−
template <typename A, typename B, typename C = std::less<>>
+
template<typename A, typename B, typename C = std::less<>>
−
bool fun(A a, B b, C cmp = C{})
+
constexpr bool fun(A a, B b, C cmp = C{})
 
{
 
{
 
     return cmp(a, b);
 
     return cmp(a, b);
 
}
 
}
  
−
int main()
+
static_assert(fun(1, 2) == true);
−
{
+
static_assert(fun(1.0, 1) == false);
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    std::cout
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static_assert(fun(1, 2.0) == true);
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      << std::boolalpha
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static_assert(std::less<int>{}(5, 5.6) == false);  // 5 < 5 (warn: implicit conversion)
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      << fun(1, 2)   << ' ' // true
+
static_assert(std::less<double>{}(5, 5.6) == true); // 5.0 < 5.6
−
      << fun(1.0, 1) << ' ' // false
+
static_assert(std::less<int>{}(5.6, 5.7) == false); // 5 < 5 (warn: implicit conversion)
−
      << fun(1, 2.0) << ' ' // true
+
static_assert(std::less{}(5, 5.6) == true);         // less<void>: 5.0 < 5.6
−
      << std::less<int>{}(5, 5.6)   << ' ' // false: 5 < 5 (warn: implicit conversion)
+
 
−
      << std::less<double>{}(5, 5.6) << ' ' // true: 5.0 < 5.6
+
int main() {}
−
      << std::less<int>{}(5.6, 5.7) << ' ' // false: 5 < 5 (warn: implicit conversion)
+
−
      << std::less{}(5, 5.6)        << ' ' // true: less<void>: 5.0 < 5.6
+
−
      << '\n';
+
−
}
+
−
| output=true false true false true false true
+
 
}}
 
}}
 +
 +
===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 equal_to}}
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{{dsc inc|cpp/utility/functional/dsc equal_to}}
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{{dsc inc | cpp/utility/functional/dsc greater}}
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{{dsc inc|cpp/utility/functional/dsc greater}}
−
{{dsc inc | cpp/utility/functional/ranges/dsc {{SUBPAGENAMEE}}}}
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{{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

 
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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