写时复制Copy-On-Write_综合

Copy-on-Write简称COW，不是奶牛，好处就是能保证数据的完整性，掉电的话容易恢复。

在执行复制和赋值的时候，不会真正的Copy，只有真正对内容进行修改时，才会进行复制，复制完之后再去修改-->懒复制

?Copy-On-Write一定使用了“引用计数”,必然有一个变量类似于RefCnt

?当第一个string对象str1构造时，string的构造函数会根据传入的参数从堆上分配内存

?当有其它string对象复制str1时，这个RefCnt会自动加1

?当有对象析构时，这个计数会减1；直到最后一个对象析构时，RefCnt为0，此时，程序才会真正的释放这块从堆上分配的内存

?在共享同一块内存的类发生内容改变时，才会发生Copy-On-Write

?比如string类的 []、=、+=、+、操作符赋值，还有一些string类中诸如insert、replace、append等成员函数

#include <stdio.h>
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;//std::string并不是线程安全的int main(void)
{string s1 = "hello,world";string s2 = s1;string s3 = s2;const char * p1 = s1.c_str();const char * p2 = s2.c_str();const char * p3 = s3.c_str();cout << "s1 = " << s1 << endl;cout << "s2 = " << s2 << endl;cout << "s3 = " << s3 << endl;printf("p1 = %p\n", p1);printf("p2 = %p\n", p2);printf("p3 = %p\n", p3);s3[0] = 'X';cout << "发生修改之后:" << endl;p1 = s1.c_str();p2 = s2.c_str();p3 = s3.c_str();cout << "s1 = " << s1 << endl;cout << "s2 = " << s2 << endl;cout << "s3 = " << s3 << endl;printf("p1 = %p\n", p1);printf("p2 = %p\n", p2);printf("p3 = %p\n", p3);return 0；
}

执行后的结果为：

s1 = hello,world
s2 = hello,world
s3 = hello,world
p1 = 0x204f028
p2 = 0x204f028
p3 = 0x204f028
发生修改之后:
s1 = hello,world
s2 = hello,world
s3 = Xello,world
p1 = 0x204f028
p2 = 0x204f028
p3 = 0x204f058

显而易见，当修改其内容时，发生Copy-On-Write

#include <stdio.h>
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;//std::string并不是线程安全的int main(void)
{string s1 = "hello,world";string s2 = s1;string s3 = s2;const char * p1 = s1.c_str();const char * p2 = s2.c_str();const char * p3 = s3.c_str();cout << "s1 = " << s1 << endl;cout << "s2 = " << s2 << endl;cout << "s3 = " << s3 << endl;printf("p1 = %p\n", p1);printf("p2 = %p\n", p2);printf("p3 = %p\n", p3);s3[0] = 'X';cout << "发生修改之后:" << endl;p1 = s1.c_str();p2 = s2.c_str();p3 = s3.c_str();cout << "s1 = " << s1 << endl;cout << "s2 = " << s2 << endl;cout << "s3 = " << s3 << endl;printf("p1 = %p\n", p1);printf("p2 = %p\n", p2);printf("p3 = %p\n", p3);cout << endl;cout << "读取s2[0]之后" << endl;cout << s2[0] << endl;p1 = s1.c_str();p2 = s2.c_str();p3 = s3.c_str();cout << "s1 = " << s1 << endl;cout << "s2 = " << s2 << endl;cout << "s3 = " << s3 << endl;printf("p1 = %p\n", p1);printf("p2 = %p\n", p2);printf("p3 = %p\n", p3);return 0;
}

执行结果为：

s1 = hello,world
s2 = hello,world
s3 = hello,world
p1 = 0x211e028
p2 = 0x211e028
p3 = 0x211e028
发生修改之后:
s1 = hello,world
s2 = hello,world
s3 = Xello,world
p1 = 0x211e028
p2 = 0x211e028
p3 = 0x211e058读取s2[0]之后
h
s1 = hello,world
s2 = hello,world
s3 = Xello,world
p1 = 0x211e028
p2 = 0x211e088
p3 = 0x211e058