浅拷贝类型的危险构造函数

来源：互联网发布：海信电视mac是什么编辑：程序博客网时间：2024/05/16 05:53

（1）浅拷贝

比如定义一个Book类，

执行

Book bookA;

Book bookB(bookA)或Book bookB = bookA

如果没有实现拷贝构造函数，将会使用系统默认的拷贝构造函数，而系统默认生成的拷贝构造函数属于浅拷贝，当存在指针时，拷贝的仅仅是指针的地址，拷贝后，两个对象指向的内存地址相同，当其中一个对象销毁执行析构函数时，另外的一个对象的指针变成了迷途指针（野指针，指针悬挂），将会导致同一块内存被释放2次。

（2）而深拷贝，对于存在指针的情况时，拷贝的将是指针指向的内容，而不是单纯的赋值指针的地址，经深拷贝后的指针是指向两个不同地址的指针（各自有各自的堆内存）。

浅拷贝的例子，代码实例

#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string.h>static int a=0;class Book{public:Book(){m_pchBookname = NULL;}Book(const char *str){if(NULL != str){int len =  strlen(str) + 1;m_pchBookname = new char[len];if(NULL != m_pchBookname){memset(m_pchBookname , 0, len);strncpy(m_pchBookname, str, len -1);}}else{m_pchBookname = NULL;}}Book(const Book & otherBook)//浅拷贝{m_pchBookname = otherBook.m_pchBookname;}~Book(){a++;std::cout<<"~Book " <<std::endl;printf("\nm_pchBookname=%p  a = %d\n",m_pchBookname,a);if(NULL != m_pchBookname){delete m_pchBookname;}m_pchBookname = NULL;}void  print(){std::cout<<m_pchBookname<<std::endl;}private:char *m_pchBookname;};int main(){Book  bookA("BookA Name");Book  bookB(bookA);//bookB.~Book();    return 0;}

看见没有浅拷贝的后果，double free

改进后，即改成深拷贝

#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string.h>static int a=0;class Book{public:Book(){m_pchBookname = NULL;}Book(const char *str){if(NULL != str){int len =  strlen(str) + 1;m_pchBookname = new char[len];if(NULL != m_pchBookname){memset(m_pchBookname , 0, len);strncpy(m_pchBookname, str, len -1);}}else{m_pchBookname = NULL;}}Book(const Book & otherBook)//深拷贝{if(NULL != otherBook.m_pchBookname){int len =  strlen(otherBook.m_pchBookname) + 1;m_pchBookname = new char[len];if(NULL != m_pchBookname){memset(m_pchBookname , 0, len);strncpy(m_pchBookname,otherBook.m_pchBookname, len -1);}}else{m_pchBookname = NULL;}}~Book(){a++;std::cout<<"~Book " <<std::endl;printf("\nm_pchBookname=%p  a = %d\n",m_pchBookname,a);if(NULL != m_pchBookname){delete m_pchBookname;}m_pchBookname = NULL;}void  print(){std::cout<<m_pchBookname<<std::endl;}private:char *m_pchBookname;};int main(){Book  bookA("BookA Name");Book  bookB(bookA);//bookB.~Book();    return 0;}

通过声明拷贝构造函数和重载符=拷贝为私有的，禁止拷贝。

#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string.h>static int a=0;class Book{public:Book(){m_pchBookname = NULL;}Book(const char *str){if(NULL != str){int len =  strlen(str) + 1;m_pchBookname = new char[len];if(NULL != m_pchBookname){memset(m_pchBookname , 0, len);strncpy(m_pchBookname, str, len -1);}}else{m_pchBookname = NULL;}}~Book(){a++;std::cout<<"~Book " <<std::endl;printf("\nm_pchBookname=%p  a = %d\n",m_pchBookname,a);if(NULL != m_pchBookname){delete m_pchBookname;}m_pchBookname = NULL;}void  print(){std::cout<<m_pchBookname<<std::endl;}private:Book(const Book & otherBook){}Book & operator = (const Book & otherBook){return *this;}char *m_pchBookname;};int main(){Book  bookA("BookA Name");Book  bookB(bookA);    Book  bookC = bookA;//bookB.~Book();    return 0;}

阅读全文

0 0