std::pop_heap
(on partitioned ranges)
<algorithm>
Swaps the value in the position first and the value in the position last - 1 and makes the subrange [
first,
last - 1)
into a heap. This has the effect of removing the first element from the heap [
first,
last)
.
[
first,
last)
is a heap with respect to comp.If any of the following conditions is satisfied, the behavior is undefined:
[
first,
last)
is empty.
[
first,
last)
is not a heap with respect to the corresponding comparator.
RandomIt
is not ValueSwappable.
The signature of the comparison function should be equivalent to the following:
bool cmp(const Type1& a, const Type2& b);
While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1
and Type2
regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1
a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them.
RandomIt
must meet the requirements of LegacyRandomAccessIterator.
Compare
must meet the requirements of Compare.
Given \(\scriptsize N\)N as std::distance (first, last):
#include <algorithm> #include <iostream> #include <string_view> #include <type_traits> #include <vector> void println(std::string_view rem, const auto& v) { std::cout << rem; if constexpr (std::is_scalar_v <std::decay_t <decltype(v)>>) std::cout << v; else for (int e : v) std::cout << e << ' '; std::cout << '\n'; } int main() { std::vector <int> v{3, 1, 4, 1, 5, 9}; std::make_heap (v.begin(), v.end()); println("after make_heap: ", v); std::pop_heap(v.begin(), v.end()); // moves the largest to the end println("after pop_heap: ", v); int largest = v.back(); println("largest element: ", largest); v.pop_back(); // actually removes the largest element println("after pop_back: ", v); }
Output:
after make_heap: 9 5 4 1 1 3 after pop_heap: 5 3 4 1 1 9 largest element: 9 after pop_back: 5 3 4 1 1
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 1205 | C++98 | the behavior was unclear if [ first, last) is empty
|
the behavior is undefined in this case |