C++ unordered_map::emplace() 函数

C++ 函数unordered_map::emplace()用于向容器插入新元素，并使容器大小增加一。只有当键不存在于容器中时，插入才会发生，这使得键是唯一的。

如果多次放置相同的键，地图只存储第一个元素，因为地图是一个不存储多个相同键值的容器。

语法

以下是 unordered_map::emplace() 函数的语法：

pair<iterator, bool>  emplace( Args&&... args);

参数

args − 表示要转发到元素构造函数的参数。

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返回值

返回一个pair，包含一个bool值（指示是否发生了插入）和一个指向新插入元素的迭代器。

示例 1

让我们看看 unordered_map::emplace() 函数的基本用法。

Open Compiler

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<char, int> um;
   um.emplace('a', 1);
   um.emplace('b', 2);
   um.emplace('c', 3);
   um.emplace('d', 4);
   um.emplace('e', 5);
   cout << "Unordered map contains following elements" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

输出

让我们编译并运行上面的程序，这将产生以下结果：

Unordered map contains following elements
e = 5
d = 4
c = 3
b = 2
a = 1

示例 2

在下面的示例中，我们创建了一个 unordered_map，然后我们将使用 emplace() 函数添加另外两个键值对。

Open Compiler

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

示例 3

考虑下面的示例，我们尝试使用 emplace() 函数更改现有键的值，如下所示：

Open Compiler

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

输出

以下是上述代码的输出：

Unordered map contains following elements before
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490
Unordered map contains same elements after the usages of emplace() function
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490

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