C++ unordered_multimap::emplace_hint() 函数

C++ 的unordered_multimap::emplace_hint()函数用于使用提示或位置在unordered_multimap中插入新元素。该位置仅作为提示；它不决定插入的位置，并且通过插入新元素将容器大小增加一。此函数类似于emplace()函数。

语法

以下是unordered_map::emplace_hint()函数的语法。

iterator emplace_hint ( const_iterator position, Args&&... args );

参数

position − 插入元素的位置提示。
args − 它指定转发到构造新元素的参数。

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返回值

此函数返回指向新插入元素的迭代器。

示例 1

在下面的示例中，让我们看看emplace_hint()函数的使用。

Open Compiler

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm = {
      {'b', 2},
      {'c', 3},
      {'d', 4},
   };
   umm.emplace_hint(umm.begin(), 'b', 2);
   umm.emplace_hint(umm.end(), 'e', 3);
   cout << "Unordered multimap contains following elements" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

输出

如果我们运行上面的代码，它将生成以下输出：

Unordered multimap contains following elements
e = 3
b = 2
b = 2
c = 3
d = 4

示例 2

考虑下面的示例，我们将在此示例中在容器的起始位置和结束位置添加键值对。

Open Compiler

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<string, int> umm = {{"Aman", 490},{"Vivek", 485},{"Akash", 500},{"Sonam", 450}};
   cout << "multimap contains following elements before" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   cout<<"after use of the emplace_hint() function \n";
   umm.emplace_hint(umm.begin(), "Vivek", 440);
   umm.emplace_hint(umm.end(), "Aman", 460);
   umm.emplace_hint(umm.end(), "Akash", 460);
   cout << "multimap contains following elements" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

输出

以下是上述代码的输出：

multimap contains following elements before
Akash = 500
Vivek = 485
Sonam = 450
Aman = 490
after use of the emplace_hint() function 
multimap contains following elements
Aman = 460
Aman = 490
Sonam = 450
Vivek = 440
Vivek = 485
Akash = 460
Akash = 500

示例 3

让我们看下面的例子，我们将使用emplace_hint()并将键值对存储在随机顺序中。

Open Compiler

#include <iostream>
#include <unordered_map>
using namespace std;
int main() { 
   unordered_multimap<int, string> Ummap;
   auto it = Ummap.emplace_hint(Ummap.begin(), 1, "Jun");
   it = Ummap.emplace_hint(it, 1, "January");
   it = Ummap.emplace_hint(it, 2, "February");
   it = Ummap.emplace_hint(it, 3, "March");
   Ummap.emplace_hint(it, 3, "April");
   Ummap.emplace_hint(it, 2, "May");
 
   cout << "The unordered_multimap contains following element : \n";
   cout << "KEY\tELEMENT"<<endl;
   for (auto itr = Ummap.begin(); itr != Ummap.end(); itr++)
      cout << itr->first << "\t"<< itr->second << endl;
   return 0;
}

输出

上述代码的输出如下：

The unordered_multimap contains following element : 
KEY	ELEMENT
3	March
3	April
2	May
2	February
1	Jun
1	January

打印页面