1

Here is my code below:

template<class t>
class smart_ptr{
    t *ptr;
public: 
    smart_ptr(t *p):ptr(p){cout<<"smart pointer copy constructor is called"<<endl;}    
    smart_ptr(const smart_ptr &sm){cout<<"copy constructor is called"
                                   <<endl;ptr=sm->ptr;}   
    ~smart_ptr(){cout<<"smart pointer destructor is called"<<endl;delete(ptr);}
    t& operator *(){cout<<"returning the * of pointer"<<endl;return(*ptr);}
    t* operator ->(){cout<<"returning the -> of pointer"<<endl;return(ptr);}
    t& operator=(const t &lhs){ptr=lhs->ptr;cout<<"assignement operator called"
                                 <<endl;return *this;}

    };
    class xxx{
            int x;
    public:
            xxx(int y=0):x(y){cout<<"xxx constructor called"<<endl;}
            ~xxx(){cout<<"xxx destructor is called"<<endl;}
            void show(){cout<<"the value of x="<<x<<endl;}
    };
int main(int argc, char *argv[])
{
    xxx *x1=new xxx(50);
    smart_ptr<xxx> p2(new xxx(60));
    smart_ptr<xxx> p1(x1);
    p1->show();
    smart_ptr<xxx> p3(p2);     //calling copy construcotr is giving error
    p3=p1;                     //calling assignment operator is giving error
    p2->smart_ptr<class xxx>::~smart_ptr<class xxx>(); //calling smart pointer 
                                                        destructor gives error
    return 0;
}

I am getting compilation errors while compiling this file because of wrong copy constructor, assignment operator and destructor codes.

The errors are:

  smart_pointer_impl.cpp: In function ‘int main(int, char**)’:

  smart_pointer_impl.cpp:33: error: ‘smart_ptr<xxx>’ is not a base of ‘xxx’

  smart_pointer_impl.cpp: In copy constructor ‘smart_ptr<t>::smart_ptr(const 
  smart_ptr<t>&) [with t = xxx]’:

   smart_pointer_impl.cpp:28:   instantiated from here

   smart_pointer_impl.cpp:8: error: passing ‘const smart_ptr<xxx>’ as ‘this’ argument  
   of ‘t* 

   smart_ptr<t>::operator->() [with t = xxx]’ discards qualifiers

   smart_pointer_impl.cpp:8: error: ‘class xxx’ has no member named ‘ptr’

Please find where I am wrong in the above functions. Any help is welcome.

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6
  • 4
    Could you post the errors you are getting? Commented Nov 7, 2013 at 15:24
  • 3
    sm.ptr instead of sm->ptr. operator= should be taking smart_ptr and not t. Commented Nov 7, 2013 at 15:26
  • 2
    Besides the compilation errors, calling the copy constructor will cause a double free of the pointer on destruction. Commented Nov 7, 2013 at 15:28
  • 1
    @Angew Regarding operator= I'd say that he might provide one taking t and one taking a smart_ptr<t>. They should both return this as a smart_ptr<t>& though. Commented Nov 7, 2013 at 15:43
  • 1
    @user1158692 Given the body of the assignment op the OP posted, they were obviously after the copy assignment op, though. And I think an overload taking t would be weird. Taking a t*, maybe, but just t doesn't feel right. Commented Nov 7, 2013 at 18:43

2 Answers 2

Reset to default
3

One obvious error is that your assignment operator should take and return references to smart_ptr, not t:

  smart_ptr& operator = (const smart_ptr& lhs)
//^^^^^^^^^                    ^^^^^^^^^

Next, your copy constructor calls sm->ptr when it should be calling sm.ptr, since -> has been overloaded to return a t anyway:

smart_ptr(const smart_ptr& sm) : ptr(sm.ptr) { .... }
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2

Compilation errors could be fixed with the following changes:

    // copy ctor
    smart_ptr(const smart_ptr &sm)
      : ptr(sm.ptr)
    {
       cout<<"copy constructor is called" << endl;
    }

    // destructor's invocation
    p2.~smart_ptr();

However there's a logic error in the copy constructor as the underlying object will be deleted twice (or more times).

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