何时需要自定义拷贝构造函数

包含动态内存分配的类需要自定义拷贝构造函数。

无定义默认为浅拷贝，此时参数对象和创建对象的指针成员指向同一块内存，调用二者的析构函数时第一对象调用释放内存成功，第二个对象会再次进行释放内存，此时运行时出错double free

故：应定义，且为深拷贝

浅拷贝实例代码：

 1 #include<iostream>
 2
 3 class A
 4 {
 5     public:
 6         A()
 7         {
 8             std::cout << "constructor is called" << std::endl;
 9             n = new int[10];
10             n[0] = 1;
11         }
12         // 默认拷贝构造函数
13         /*A(const A &t)
14         {
15             std::cout << "copy constructor is called" << std::endl;
16             n = t.n;
17         }
18         */
19
20         ~A()
21         {
22             std::cout << "destructor is called" << std::endl;
23             delete n;
24         }
25
26         void get() { std::cout << "n[0]: " << n[0] << std::endl; }
27     private:
28         int *n;
29 };
30
31 int main(void)
32 {
33     A a;
34     A b(a);
35     b.get();
36
37     return 0;
38 }

编译运行后结果：

深拷贝实例代码：

 1 #include<iostream>
 2 #include<string.h>
 3
 4 class A
 5 {
 6     public:
 7         A()
 8         {
 9             std::cout << "constructor is called" << std::endl;
10             n = new int[10];
11             n[0] = 1;
12         }
13         // 深拷贝
14         A(const A &t)
15         {
16             std::cout << "copy constructor is called" << std::endl;
17             n = new int[10];
18             memcpy(n, t.n, 10);
19         }
20
21         ~A()
22         {
23             std::cout << "destructor is called" << std::endl;
24             delete n;
25         }
26
27         void get() { std::cout << "n[0]: " << n[0] << std::endl; }
28     private:
29         int *n;
30 };
31
32 int main(void)
33 {
34     A a;
35     A b(a);
36     b.get();
37
38     return 0;
39 } 

编译运行结果：