C++面向对象基础--动态内存（new的用法）

C++ 程序中的内存分为两个部分：

栈：在函数内部声明的所有变量都将占用栈内存。

堆：这是程序中未使用的内存，在程序运行时可用于动态分配内存。

在 C++ 中，我们可以使用new运算符为给定类型的变量在运行时分配堆内的内存，这会返回所分配的空间地址。如果不再需要动态分配的内存空间，可以使用 delete 运算符，删除之前由 new 运算符分配的内存。

1.new和delete的常规简要用法

new 运算符来为任意的数据类型动态分配内存的通用语法：

1 new data-type;
其中，data-type可以是包括数组在内的任意内置的数据类型，也可以是包括类或结构在内的用户自定义的任何数据类型。例如我们可以定义一个指向int类型数据的指针p，然后请求内存，该内存在执行时被分配。

1 int* p = NULL; // 初始化为null的指针
2 p = new int; // 为变量请求内存
如果自由存储区已被用完，可能无法成功分配内存。所以建议检查 new 运算符是否返回 NULL 指针，并采取以下适当的操作：

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1 int* p = NULL;
2 if (!(p = new int))
3 {
4 cout << “Error: out of memory.” << endl;
5 exit(1);
6
7 }
8 p = new int;
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如果某个已经动态分配内存的变量不再需要使用时，我们可以使用 delete 操作符释放它所占用的内存，如下所示：

1 delete p; // 释放p所指向的内存
new和delete完整用法示例：

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1 #include
2 using namespace std;
3
4 int main()
5 {
6 int* p = NULL; // 初始化为 null 的指针
7 p = new int; // 为变量请求内存
8 *p= 12345; // 在分配的地址存储值
9 cout << "Value of p : " << *p << endl;
10 delete p; // 释放内存
11 return 0;
12 }
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运行结果：

1 Value of p : 12345
2.数组的动态内存分配
（1）一维数组：

1 // 动态分配,数组长度为 n
2 int *array=new int [n]；
3
4 //释放内存
5 delete [] array;
（2）二维数组：

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1 int **array
2 // 假定数组第一维长度为 m， 第二维长度为 n
3 // 动态分配空间
4 array = new int *[m];
5 for( int i=0; i<m; i++ )
6 {
7 array[i] = new int [n] ;
8 }
9 //释放
10 for( int i=0; i<m; i++ )
11 {
12 delete [] arrary[i];
13 }
14 delete [] array;
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二维数组实例测试：

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1 #include
2 using namespace std;
3
4 int main()
5 {
6 int** p;
7 int i, j; //p[4][8]
8 //开始分配4行8列的二维数据
9 p = new int* [4];
10 for (i = 0; i < 4; i++)
11 {
12 p[i] = new int[8];
13 }
14
15 for (i = 0; i < 4; i++)
16 {
17 for (j = 0; j < 8; j++)
18 {
19 p[i][j] = j * i;
20 }
21 }
22 //打印数据
23 for (i = 0; i < 4; i++)
24 {
25 for (j = 0; j < 8; j++)
26 {
27 if (j == 0)
28 {
29 cout << endl;
30 }
31 cout << p[i][j] << “\t”;
32 }
33 }
34 //开始释放申请的堆
35 for (i = 0; i < 4; i++)
36 {
37 delete[] p[i];
38 }
39 delete[] p;
40 return 0;
41 }
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代码运行结果：

（3）三维数组：

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1 int*** array;
2 // 假定数组第一维为 m， 第二维为 n， 第三维为h
3 // 动态分配空间
4 array = new int** [m];
5 for (int i = 0; i < m; i++)
6 {
7 array[i] = new int* [n];
8 for (int j = 0; j < n; j++)
9 {
10 array[i][j] = new int[h];
11 }
12 }
13 //释放
14 for (int i = 0; i < m; i++)
15 {
16 for (int j = 0; j < n; j++)
17 {
18 delete[] array[i][j];
19 }
20 delete[] array[i];
21 }
22 delete[] array;
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三维数组测试实例：

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1 #include
2 using namespace std;
3
4 int main()
5 {
6 int i, j, k; // p[2][3][4]
7
8 int*** p;
9 p = new int** [2];
10 for (i = 0; i < 2; i++)
11 {
12 p[i] = new int* [3];
13 for (j = 0; j < 3; j++)
14 {
15 p[i][j] = new int[4];
16 }
17 }
18
19 //输出 p[i][j][k] 三维数据
20 for (i = 0; i < 2; i++)
21 {
22 for (j = 0; j < 3; j++)
23 {
24 for (k = 0; k < 4; k++)
25 {
26 p[i][j][k] = i + j + k;
27 cout << p[i][j][k] << " ";
28 }
29 cout << endl;
30 }
31 cout << endl;
32 }
33
34 // 释放内存
35 for (i = 0; i < 2; i++)
36 {
37 for (j = 0; j < 3; j++)
38 {
39 delete[] p[i][j];
40 }
41 }
42 for (i = 0; i < 2; i++)
43 {
44 delete[] p[i];
45 }
46 delete[] p;
47 return 0;
48 }
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代码运行结果：

3.对象的动态内存分配

示例代码：

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1 #include
2 using namespace std;
3
4 class test
5 {
6 public:
7 test() {
8 cout << “调用构造函数！” << endl;
9 }
10 ~test() {
11 cout << “调用析构函数！” << endl;
12 }
13 };
14
15 int main()
16 {
17 test* testArray = new test[4];
18 delete[] testArray; // 删除数组
19 return 0;
20 }
复制代码
代码运行结果：

如果要为一个包含四个test对象的数组分配内存，构造函数将被调用 4 次，同样地，当删除这些对象时，析构函数也将被调用相同的次数（4次）。
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