Difference between revisions of "cpp/utility/functional/less"
From cppreference.com
< cpp | utility | functional
Andreas Krug (Talk | contribs) m (const T & -> const T&) |
(Wording update for total pointer order.) |
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{{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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===Return value=== | ===Return value=== | ||
| − | + | {{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}} | ||
| Line 57: | Line 55: | ||
constexpr bool operator()(const T& lhs, const T& rhs) const | constexpr bool operator()(const T& lhs, const T& rhs) const | ||
{ | { | ||
| − | return lhs < rhs; // assumes that the implementation | + | return lhs < rhs; // assumes that the implementation handles pointer total order |
} | } | ||
}} | }} | ||
| Line 90: | Line 88: | ||
true false true false true false true | 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=== | ||
Revision as of 01:03, 26 December 2023
| 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 |
Specializations
| (C++14) |
function object implementing x < y deducing parameter and return types (class template specialization) |
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) |
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; |
(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
lhs < rhs.
If T is a pointer type, the result is consistent with the implementation-defined strict total order over pointers.
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 } |
Example
Run this code
#include <functional> #include <iostream> template<typename A, typename B, typename C = std::less<>> bool fun(A a, B b, C cmp = C{}) { return cmp(a, b); } int main() { std::cout << std::boolalpha << fun(1, 2) << ' ' // true << fun(1.0, 1) << ' ' // false << fun(1, 2.0) << ' ' // true << std::less<int>{}(5, 5.6) << ' ' // false: 5 < 5 (warn: implicit conversion) << std::less<double>{}(5, 5.6) << ' ' // true: 5.0 < 5.6 << 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
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 |
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) |
