无涯教程-C++ Map - operator=函数映射中有以下三种使用operator =()的方式: Operat

映射中有以下三种使用operator =()的方式:

Operator =()用于通过替换旧内容(或复制内容)将新内容分配给映射集合，并在必要时修改大小。
Operator =()用于将一个映射集合的内容移动到另一个映射集合中，并在必要时修改其大小。
Operator = 用于将初始化器列表中的元素复制到映射集合。

operator= - 语法

copy(1) map& operator= (const map& x); //until C++ 11
copy (1) map& operator= (const map& x); //since C++ 11
move (2) map& operator= (map&& x); //since C++ 11
initializer list (3) map& operator= (initializer_list<value_type> il); //since C++ 11

copy(1) :- 将x中的所有元素复制到映射集合中。

copy(2) :- 将x的内容移动到映射集合中。

initializer_list(3) :- 将il的元素复制到映射集合中。

operator= - 参数

x : 具有相同类型的映射对象。

il : 初始化列表对象。

operator= - 返回值

这个*this指针。

operator= - 例子1

让我们看一个简单的示例，将一个映射的内容复制到另一个映射。

#include <iostream>
#include <map>
using namespace std;
int main(void) {
map<char, int> m1 = {
{a, 10},
{b, 20},
{c, 30} };
cout << "Map m1 contains following elements" << endl;
for (auto it = m1.begin(); it != m1.end(); ++it)
cout << it->first << " = " << it->second << endl;
map</span><span class="pun">&lt;</span><span class="kwd">char</span><span class="pun">,</span><span class="pln"> </span><span class="kwd">int</span><span class="pun">&gt;</span><span class="pln"> m2 </span><span class="pun">=</span><span class="pln"> m1</span><span class="pun">;</span><span class="pln">  
cout</span><span class="pun">&lt;&lt;</span><span class="str">"\nAfter Copying the elements from m1 to m2... \n"</span><span class="pun">;</span><span class="pln">  

cout </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">"\nMap m2 contains following elements"</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="kwd">auto</span><span class="pln"> it </span><span class="pun">=</span><span class="pln"> m2</span><span class="pun">.</span><span class="kwd">begin</span><span class="pun">();</span><span class="pln"> it </span><span class="pun">!=</span><span class="pln"> m2</span><span class="pun">.</span><span class="kwd">end</span><span class="pun">();</span><span class="pln"> </span><span class="pun">++</span><span class="pln">it</span><span class="pun">)</span><span class="pln">
  cout </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">first </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">" = "</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">second </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">

return 0;
}

输出:

Map m1 contains following elements
a = 10
b = 20
c = 30
After copying the elements from m1 to m2...
Map m2 contains following elements
a = 10
b = 20
c = 30

在上面的示例中，使用operator =()函数将一个映射图m1的内容复制到另一个映射图m2。

operator= - 例子2

让我们看一个简单的示例，将一个映射的元素移动到另一个。

#include <iostream>
#include <map>
using namespace std;
int main(void) {
map<char, int> m1 = {
{a, 1},
{b, 2},
{c, 3} };
  cout </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">"Map m1 contains following elements"</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="kwd">auto</span><span class="pln"> it </span><span class="pun">=</span><span class="pln"> m1</span><span class="pun">.</span><span class="kwd">begin</span><span class="pun">();</span><span class="pln"> it </span><span class="pun">!=</span><span class="pln"> m1</span><span class="pun">.</span><span class="kwd">end</span><span class="pun">();</span><span class="pln"> </span><span class="pun">++</span><span class="pln">it</span><span class="pun">)</span><span class="pln">
  cout </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">first </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">" = "</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">second </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">
  
map</span><span class="pun">&lt;</span><span class="kwd">char</span><span class="pun">,</span><span class="pln"> </span><span class="kwd">int</span><span class="pun">&gt;</span><span class="pln"> m2 </span><span class="pun">=</span><span class="pln"> move</span><span class="pun">(</span><span class="pln">m1</span><span class="pun">);</span><span class="pln"> 
cout</span><span class="pun">&lt;&lt;</span><span class="str">"\nAfter moving the elements from m1 to m2... \n"</span><span class="pun">;</span><span class="pln">  

cout </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">"\nMap m2 contains following elements"</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(</span><span class="kwd">auto</span><span class="pln"> it </span><span class="pun">=</span><span class="pln"> m2</span><span class="pun">.</span><span class="kwd">begin</span><span class="pun">();</span><span class="pln"> it </span><span class="pun">!=</span><span class="pln"> m2</span><span class="pun">.</span><span class="kwd">end</span><span class="pun">();</span><span class="pln"> </span><span class="pun">++</span><span class="pln">it</span><span class="pun">)</span><span class="pln">
  cout </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">first </span><span class="pun">&lt;&lt;</span><span class="pln"> </span><span class="str">" = "</span><span class="pln"> </span><span class="pun">&lt;&lt;</span><span class="pln"> it</span><span class="pun">-&gt;</span><span class="pln">second </span><span class="pun">&lt;&lt;</span><span class="pln"> endl</span><span class="pun">;</span><span class="pln">

return 0;
}

输出:

Map m1 contains following elements
a = 1
b = 2
c = 3
After moving the elements from m1 to m2...
Map m2 contains following elements
a = 1
b = 2
c = 3

在上面的示例中，使用operator =()函数将一个映射m1的内容移动到另一映射m2。

operator= - 例子3

让我们看一个简单的示例，将初始化列表中的内容复制到map。

#include <iostream>
#include <map>
using namespace std;
int main(void) {
map<char, int> m;
m =  {
{a, 100},
{b, 200},
{c, 300},
{d, 400}  };
cout << "Map contains the following elements" << endl;
for (auto it = m.begin(); it != m.end(); ++it)
cout << it->first << " = " << it->second << endl;
return 0;
}

输出:

Map contains the following elements
a = 100
b = 200
c = 300
d = 400

在上面的示例中，operator =()用于将内容从初始化列表复制到映射m。

operator= - 例子4

让我们看一个简单的例子。

#include <iostream>
#include <map>
using namespace std;
int main ()
{
map<char,int> first;
map<char,int> second;
first[x]=8;
first[y]=16;
first[z]=32;
second=first;               //第二个现在包含 3 个整数
first=map<char,int>();      //第一个现在是空的
cout << "Size of first: " << first.size() << 
;
cout << "Size of second: " << second.size() << 
;
return 0;
}

输出:

Size of first: 0
Size of second: 3

在上面的示例中，首先它将计算空图的大小，然后将一些元素添加到第一张图并复制到第二张图。

无涯教程-C++ Map - operator=函数