为什么C ++复制构造函数会失败？

Question

#include <iostream>
#include <string>

using namespace std;

class A
{
public:
    A() { i=1; j=2;};
    A (A &obj) { i= obj.i+100; j= obj.j+100;};
     int i;
     int j;
};

class B:public A
{
public:
    B():A() {i=10; j=20; k=30;};

    B(A &obj) {  A::A(obj); k=10000; };//

    int k;
};

int main()
{ 
    A dog;
    B mouse(dog);
    cout<<mouse.i<<endl;
    cout<<mouse.k<<endl;

    return 0;
}

I try to write a copy constructor for the derived class that takes advantage of the copy constructor for the base class. I expect that mouse.i should be 101, but in fact the compiling result is 1. The value for mouse.k is 10000, which is expected. I was wondering what's wrong with my code.

Answer 1

You must use the initialization list to call the parent's constructor (and you should do it for all other members also):

B(A const& obj) : A(obj), k(10000) {}

Additionally, when copying you do not modify the original object, so you should take a const reference to it. That will allow you to copy from constant objects (or through constant references), improving const-correctness.

Answer 2

In this constructor:

B(A &obj) {  A::A(obj); k=10000; };

A::A(obj); does not initialise the base sub-object; instead, it creates a local object also called obj . It's equivalent to A::A obj; , which is equivalant to A obj; . [UPDATE: or possibly it does something else, or possibly it's ill-formed - in any event, it's wrong.]

You want to use an initialiser list:

B(A & obj) : A(obj), k(10000) {}

Also, you almost certainly want the constructor parameters to be A const & , to allow construction from constant objects or temporaries.

Answer 3

You should initialize the base class like this:

B(A &obj):A(obj) {  k=10000; }

(more on this at What are the rules for calling the superclass constructor? ). And a side-note: use const for copy constructor arguments:

A (const A &obj) {...}

EDIT :

The preferred way to initialize instance members is through an initialization list, so your ctor will look like

B(A &obj):A(obj), k(10000) { }

Answer 4

#include <iostream>
#include <string>

using namespace std;

class A
{
public:
A()
{
    i=1;
    j=2;
}

A (A &obj)
{
    i= obj.i+100;
    j= obj.j+100;
}

 int i;
 int j;
};

class B:public A
{
public:
B():A() {i=10; j=20; k=30;}

B(A &obj)
:A(obj)
 {
  //A::A(obj);
  k=10000;
  }//

int k;
};

int main()
{
    A dog;
    B mouse(dog);
    cout<<mouse.i<<endl;
    cout<<mouse.k<<endl;

    return 0;
}

This work for me

B(A &obj)
{
    A::A(obj)
}

Is illegal on gcc compiler