C + +: Default copy constructor performs shallow copy

C + +, a copy constructor is generated by default, and when a pointer appears in a class, the copy performs a shallow copy, that is, only the address of the pointer is copied, and the data is not copied;

So in class, when using pointers, you need to be aware of; If you want to use a deep copy, you can add a copy constructor.

The following code, if you do not add a copy constructor, will run an error, but you can compile,

At runtime, the program cannot execute because deletion (delete[]) is two times the same address space as Str refers to.

Code:

* * * main.cpp * * Created on:2014.4.15 * author:spike//*vs2012*/#include <iostream  
      
> #include <cstring> #include <vector> #include <memory> using namespace std;  
    Class CDemo {Public:cdemo (): Str (NULL) {};  
        ~cdemo () {static int i=0; if (str) {std::cout << "&demo" << i++ << "=" << (int*) this <<, str =  
            "<< (int*) str << Std::endl;  
        Delete[] STR;  
        }///copy constructor CDemo (const cdemo& CD) {this->str = new Char[strlen (CD.STR) + 1];  
    strcpy (This->str, CD.STR);  
} char* str;  
      
};  
    int main () {CDemo D1;  
    D1.STR = new CHAR[32];  
    strcpy (D1.str, "Caroline");  
    std::vector<cdemo>* A1 = new std::vector<cdemo> (); A1->push_back (D1); Perform a copy constructor std::cout << "D1.str =" << d1.str << Std::endl;  
    Std::cout << "(*a1) [0].STR =" << (*A1) [0].str << Std::endl;  
    strcpy (D1.str, "Wendy");  
    Std::cout << "d1.str =" << d1.str << Std::endl;  
    Std::cout << "(*a1) [0].STR =" << (*A1) [0].str << Std::endl;  
    Delete A1;  
return 0; }

Output:

D1.str = Caroline  
(*a1) [0].str  = Caroline d1.str  
= Wendy  
(*A1) [0].str  = Wendy  
&demo0 = 0x312570, str = 0x312548  
&demo1 = 0x22fec8, str = 0x312548

Author: csdn Blog spike_king

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