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}}") |
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{{cpp/utility/functional/navbar}} | {{cpp/utility/functional/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
− | {{dcl header | functional }} | + | {{dcl header|functional}} |
− | {{dcl rev | + | {{dcl rev multi |
− | + | |dcl1= | |
template< class T > | template< class T > | ||
struct less; | struct less; | ||
− | + | |since2=c++14|dcl2= | |
− | + | template< class T = void > | |
− | template< class T | + | |
struct less; | struct less; | ||
}} | }} | ||
− | |||
{{dcl end}} | {{dcl end}} | ||
− | Function object for performing comparisons. | + | Function object for performing comparisons. The main template invokes {{c|operator<}} on type {{tt|T}}. |
===Specializations=== | ===Specializations=== | ||
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− | |||
− | |||
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{{dsc begin}} | {{dsc begin}} | ||
− | {{dsc inc | cpp/utility/functional/dsc less_void}} | + | {{dsc inc|cpp/utility/functional/dsc less_void}} |
{{dsc end}} | {{dsc end}} | ||
− | |||
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{{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}} | + | {{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}} | ||
− | {{ | + | {{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}} | {{dcl end}} | ||
− | Checks whether {{ | + | 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| | + | {{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 |
} | } | ||
}} | }} | ||
Line 71: | Line 59: | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
− | + | |code= | |
#include <functional> | #include <functional> | ||
− | |||
− | template <typename A, typename B, typename | + | template<typename A, typename B, typename C = std::less<>> |
− | bool | + | constexpr bool fun(A a, B b, C cmp = C{}) |
{ | { | ||
− | return | + | 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 | |
− | true | + | |
− | + | 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}} | + | {{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
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] 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) | |
function object implementing x > y (class template) | |
(C++20) |
constrained function object implementing x < y (class) |