std::less
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Defined in header
<functional>
template< class T >
struct less;
(until C++14)
struct less;
template< class T = void >
struct less;
(since C++14)
struct less;
Function object for performing comparisons. The main template invokes operator< on type T.
Contents
[edit] Specializations
(C++14)
(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()
(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 |