头文件： <algorithm>
(1)template< class BidirIt, class UnaryPredicate > BidirIt partition( BidirIt first, BidirIt last, UnaryPredicate p );
(C++11 前)template< class ForwardIt, class UnaryPredicate > ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
(C++11 起) (C++20 前)template< class ForwardIt, class UnaryPredicate > constexpr ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
(C++20 起)template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >ForwardIt partition( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, UnaryPredicate p );(2)(C++17 起)(1)：将[first,last]间的元素分为两个区间，第一个区间是符合规则p或者partition的元素集合，第二个区间则是不符合规则p或者partition的元素集合
(2)：同 (1) ，但按照 policy 执行。此重载仅若 std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (C++20 前)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (C++20 起) 为 true 才参与重载决议。
重点说一下返回值，返回值返回第二组元素的首元素的迭代器
先看一个简单的例子
// partition algorithm example #include <iostream> // std::cout #include <algorithm> // std::partition #include <vector> // std::vector bool IsOdd (int i) { return (i==5); } int main () { std::vector<int> myvector; // set some values: for (int i=1; i<10; ++i) myvector.push_back(i); // 1 2 3 4 5 6 7 8 9 std::vector<int>::iterator bound; bound = std::partition (myvector.begin(), myvector.end(), IsOdd); // print out content: std::cout << "odd elements:"; for (std::vector<int>::iterator it=myvector.begin(); it!=bound; ++it) std::cout << ' ' << *it; std::cout << '\n'; std::cout << "even elements:"; for (std::vector<int>::iterator it=bound; it!=myvector.end(); ++it) std::cout << ' ' << *it; std::cout << '\n'; return 0; }运行结果：
再来看一个相对复杂一些的例子
#include <algorithm> #include <iostream> #include <iterator> #include <vector> #include <forward_list> template <class ForwardIt> void quicksort(ForwardIt first, ForwardIt last) { if(first == last) return; auto pivot = *std::next(first, std::distance(first,last)/2); ForwardIt middle1 = std::partition(first, last, [pivot](const auto& em){ return em < pivot; }); ForwardIt middle2 = std::partition(middle1, last, [pivot](const auto& em){ return !(pivot < em); }); quicksort(first, middle1); quicksort(middle2, last); } int main() { std::vector<int> v = {0,1,2,3,4,5,6,7,8,9}; std::cout << "Original vector:\n "; for (int elem : v) std::cout << elem << ' '; auto it = std::partition(v.begin(), v.end(), [](int i){return i % 2 == 0;}); std::cout << "\nPartitioned vector:\n "; std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " ")); std::cout << " * "; std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " ")); std::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92}; std::cout << "\nUnsorted list:\n "; for (int n : fl) std::cout << n << ' '; std::cout << '\n'; quicksort(std::begin(fl), std::end(fl)); std::cout << "Sorted using quicksort:\n "; for(int fi : fl) std::cout << fi << ' '; std::cout << '\n'; }运行结果：
auto it = std::partition(v.begin(), v.end(), [](int i){return i % 2 == 0;}); //当前语句执行完成时，数据被分为两组，前一组为偶数，后一组为奇数,it是后一句首元素的迭代器。
std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " ")); //从数组v的首元素开始输出，直到碰见*it
