std::flat_set<Key,Compare,KeyContainer>:: emplace_hint
From zh.cppreference.net
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template
<
class
...
Args
>
iterator emplace_hint ( const_iterator hint, Args && ... args ) ; |
(自 C++23 起)
(自 C++26 起为 constexpr) |
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在容器中插入一个新元素，尽可能接近并位于 hint 指示位置之前。
键与映射值的构造函数将以与传递给函数时完全相同的参数调用，这些参数通过 std:: forward < Args > ( args ) ... 进行完美转发。
| 关于迭代器失效的信息复制自 |
内容参数
返回值指向被插入元素的迭代器，或指向阻止插入的元素的迭代器。 异常若因任何原因抛出异常，此函数不产生任何效果（ 强异常安全保证 ）。 复杂度
示例运行此代码 #include <chrono>
#include <cstddef>
#include <functional>
#include <iomanip>
#include <iostream>
#include <flat_set>
const int n_operations = 100'500'0;
std::size_t set_emplace()
{
std::flat_set<int> set;
for (int i = 0; i < n_operations; ++i)
set.emplace(i);
return set.size();
}
std::size_t set_emplace_hint()
{
std::flat_set<int> set;
auto it = set.begin();
for (int i = 0; i < n_operations; ++i)
{
set.emplace_hint(it, i);
it = set.end();
}
return set.size();
}
std::size_t set_emplace_hint_wrong()
{
std::flat_set<int> set;
auto it = set.begin();
for (int i = n_operations; i > 0; --i)
{
set.emplace_hint(it, i);
it = set.end();
}
return set.size();
}
std::size_t set_emplace_hint_corrected()
{
std::flat_set<int> set;
auto it = set.begin();
for (int i = n_operations; i > 0; --i)
{
set.emplace_hint(it, i);
it = set.begin();
}
return set.size();
}
std::size_t set_emplace_hint_closest()
{
std::flat_set<int> set;
auto it = set.begin();
for (int i = 0; i < n_operations; ++i)
it = set.emplace_hint(it, i);
return set.size();
}
double time_it(std::function<std::size_t()> set_test,
const char* what = nullptr,
double ratio = 0.0)
{
const auto start = std::chrono::system_clock::now();
const std::size_t setsize = set_test();
const auto stop = std::chrono::system_clock::now();
const std::chrono::duration<double, std::milli> time = stop - start;
if (what != nullptr && setsize > 0)
std::cout << std::setw(8) << time << " for " << what << " (ratio: "
<< (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n";
return time.count();
}
int main()
{
std::cout << std::fixed << std::setprecision(2);
time_it(set_emplace); // cache warmup
const auto x = time_it(set_emplace, "plain emplace");
time_it(set_emplace_hint, "emplace with correct hint", x);
time_it(set_emplace_hint_wrong, "emplace with wrong hint", x);
time_it(set_emplace_hint_corrected, "corrected emplace", x);
time_it(set_emplace_hint_closest, "emplace using returned iterator", x);
}
可能的输出： 22.91ms for plain emplace (ratio: 1.00)
40.73ms for emplace with correct hint (ratio: 0.56)
3713.24ms for emplace with wrong hint (ratio: 0.01)
2824.25ms for corrected emplace (ratio: 0.01)
5.96ms for emplace using returned iterator (ratio: 3.86)
参见
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