Difference between revisions of "cpp/iterator/reverse iterator/operator cmp"
From cppreference.com
< cpp | iterator | reverse iterator
(Detached this page from the {{cpp/iterator/adaptor/operator cmp}} template, and updated the description.) |
m (→Example: revived.) |
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@6@ {{cpp/enable if|{{c|1=lhs.base() <= rhs.base()}} is well-formed and convertible to {{c/core|bool}}}}. | @6@ {{cpp/enable if|{{c|1=lhs.base() <= rhs.base()}} is well-formed and convertible to {{c/core|bool}}}}. | ||
}} | }} | ||
| + | |||
===Parameters=== | ===Parameters=== | ||
| Line 79: | Line 80: | ||
{{example | {{example | ||
|code= | |code= | ||
| + | #include <cassert> | ||
#include <iterator> | #include <iterator> | ||
| Line 86: | Line 88: | ||
// ↑ └───── x, y | // ↑ └───── x, y | ||
// └──────── z | // └──────── z | ||
| − | |||
// “x” and “y” are equal, but “x” is less than “z” (reversely) | // “x” and “y” are equal, but “x” is less than “z” (reversely) | ||
std::reverse_iterator<int*> | std::reverse_iterator<int*> | ||
| Line 92: | Line 93: | ||
y{std::rend(a) - std::size(a)}, | y{std::rend(a) - std::size(a)}, | ||
z{std::rbegin(a) + 1}; | z{std::rbegin(a) + 1}; | ||
| − | + | ||
// two-way comparisons | // two-way comparisons | ||
| − | + | assert( x == y ); | |
| − | + | assert(!(x != y)); | |
| − | + | assert(!(x < y)); | |
| − | + | assert( x <= y ); | |
| − | + | assert(!(x == z)); | |
| − | + | assert( x != z ); | |
| − | + | assert( x < z ); | |
| − | + | assert( x <= z ); | |
| − | + | ||
// three-way comparisons | // three-way comparisons | ||
| − | + | assert( x <=> y == 0 ); | |
| − | + | assert(!(x <=> y < 0)); | |
| − | + | assert(!(x <=> y > 0)); | |
| − | + | assert(!(x <=> z == 0)); | |
| − | + | assert( x <=> z < 0 ); | |
| − | + | assert(!(x <=> z > 0)); | |
} | } | ||
}} | }} | ||
Revision as of 16:06, 2 November 2024
| Defined in header <iterator>
|
||
template< class Iter1, class Iter2 > bool operator==( const std::reverse_iterator<Iter1>& lhs, |
(1) | (constexpr since C++17) |
template< class Iter1, class Iter2 > bool operator!=( const std::reverse_iterator<Iter1>& lhs, |
(2) | (constexpr since C++17) |
template< class Iter1, class Iter2 > bool operator< ( const std::reverse_iterator<Iter1>& lhs, |
(3) | (constexpr since C++17) |
template< class Iter1, class Iter2 > bool operator<=( const std::reverse_iterator<Iter1>& lhs, |
(4) | (constexpr since C++17) |
template< class Iter1, class Iter2 > bool operator> ( const std::reverse_iterator<Iter1>& lhs, |
(5) | (constexpr since C++17) |
template< class Iter1, class Iter2 > bool operator>=( const std::reverse_iterator<Iter1>& lhs, |
(6) | (constexpr since C++17) |
| template< class Iter1, std::three_way_comparable_with<Iter1> Iter2 > constexpr std::compare_three_way_result_t<Iter1, Iter2> |
(7) | (since C++20) |
Compares the underlying iterators of lhs and rhs.
- The result of equality comparisons are preserved (i.e. equal underlying iterators imply equal reverse iterators).
- The result of relational comparisons are reversed (i.e. a greater underlying iterator implies a lesser reverse iterator).
|
1) This overload participates in overload resolution only if lhs.base() == rhs.base() is well-formed and convertible to bool.
2) This overload participates in overload resolution only if lhs.base() != rhs.base() is well-formed and convertible to bool.
3) This overload participates in overload resolution only if lhs.base() > rhs.base() is well-formed and convertible to bool.
4) This overload participates in overload resolution only if lhs.base() >= rhs.base() is well-formed and convertible to bool.
5) This overload participates in overload resolution only if lhs.base() < rhs.base() is well-formed and convertible to bool.
6) This overload participates in overload resolution only if lhs.base() <= rhs.base() is well-formed and convertible to bool.
|
(since C++20) |
Contents |
Parameters
| lhs, rhs | - | iterator adaptors to compare |
Return value
1) lhs.base() == rhs.base()
2) lhs.base() != rhs.base()
3) lhs.base() > rhs.base()
4) lhs.base() >= rhs.base()
5) lhs.base() < rhs.base()
6) lhs.base() <= rhs.base()
7) rhs.base() <=> lhs.base()
Notes
operator<=> returns rhs.base() <=> lhs.base() rather than lhs.base() <=> rhs.base() because this is a reverse iterator.
Example
Run this code
#include <cassert> #include <iterator> int main() { int a[]{0, 1, 2, 3}; // ↑ └───── x, y // └──────── z // “x” and “y” are equal, but “x” is less than “z” (reversely) std::reverse_iterator<int*> x{std::rend(a) - std::size(a)}, y{std::rend(a) - std::size(a)}, z{std::rbegin(a) + 1}; // two-way comparisons assert( x == y ); assert(!(x != y)); assert(!(x < y)); assert( x <= y ); assert(!(x == z)); assert( x != z ); assert( x < z ); assert( x <= z ); // three-way comparisons assert( x <=> y == 0 ); assert(!(x <=> y < 0)); assert(!(x <=> y > 0)); assert(!(x <=> z == 0)); assert( x <=> z < 0 ); assert(!(x <=> z > 0)); }
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 280 | C++98 | heterogeneous assignment was not allowed | allowed |
