Copy-on-write and STL string

Copy-on-write(COW) 是计算机程序设计中的一种优化策略。The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, you can give them pointers to the same resource. This fiction can be maintained until a caller tries to modify its "copy" of the resource, at which point a true private copy is created to prevent the changes becoming visible to everyone else. All of this happens transparently to the callers. The primary advantage is that if a caller never makes any modifications, no private copy need ever be created. 

STL中的string的实现就用到了copy on write策略。String is a reference counted, copy on write string class

 

#include <string>
using namespace std;
main()
...{    
    string str1 = "hello world";    
    string str2 = str1;    
    
    
    printf ("Sharing the memory: ");    
    printf (" str1's address: %x ", str1.c_str() );    
    printf (" str2's address: %x ", str2.c_str() );    
        
    str1[1]='q';
    str2[1]='w';    
    
    printf ("After Copy-On-Write: ");    
    printf (" str1's address: %x ", str1.c_str() );    
    printf (" str2's address: %x ", str2.c_str() );
    return 0;
}