C++ pointer

[Hanks]

I am reading C++ design pattern and derivative pricing. Some code is like this

Wrapper& operator=(const Wrapper<T>& original)
{
      if(this != &original)
          {
             if(DataPtr != 0)
             delete DataPtr;
             DataPtr = (original.DataPtr != 0)? original.DataPtr->clone() : 0;
           }
return *this;
}

(DataPtr is a pointer data number of that class)
Why we have to check the following code and delete DataPtr first before assign it to new address?
if(DataPtr != 0) delete DataPtr;

 

[msj]

if DataPtr points to a dynamically allocated object this is your last chance to destroy that object and free memory

 

[Polter]

"if(DataPtr != 0) delete DataPtr;"
Probably a code taking into account this -- http://www.parashift.com/c -faq/double-delete-disaster.html -- but not this -- http://www.parashift.com/c -faq/delete-handles-null.html.

The rest is just the check-for-self-assignment-for-raw-ptrs'-wrappers idiom:
http://en.wikibooks.org/wiki/C++_Programming/Operators/Operator_Overloading#Assignment_operator
http://en.wikibooks.org/wiki/C++_Pr...ing_self-assignment_in_an_assignment_operator

 

[Hanks]

if DataPtr points to a dynamically allocated object this is your last chance to destroy that object and free memory

Thanks. At first I thought it just delete the pointer, the pointed object will not be deleted. Now know that is wrong.

 

[Hanks]

"if(DataPtr != 0) delete DataPtr;"
Probably a code taking into account this -- http://www.parashift.com/c -faq/double-delete-disaster.html -- but not this -- http://www.parashift.com/c -faq/delete-handles-null.html.

The rest is just the check-for-self-assignment-for-raw-ptrs'-wrappers idiom:
http://en.wikibooks.org/wiki/C++_Programming/Operators/Operator_Overloading#Assignment_operator
http://en.wikibooks.org/wiki/C++_Programming/Idioms#Handling_self-assignment_in_an_assignment_operator

Good material. I learned a lot. Thanks

 

[Daniel Duffy]

In some cases (e.g Singleton pattern) the assignment = is private as well as copy constructor.

DataPtr = (original.DataPtr != 0)? original.DataPtr->clone() : 0;

Q: Since original is a refernce why do we have to check if its data pointer is null?

 

[ExSan]

who said pointers are easy ???
10 lines of source code that deserve more than 10 hours studying them
I wonder how many people in this forum do really understand the previous posts? //DD.Poler.MJ
I have been working with pointers for years and still keep learning and discovering new things, I still can not say pointers are easy but yes they are amazing

line number 5 I rather use

if(!DataPtr)  {.... }                //if DataPtr is NULL

 

[Daniel Duffy]

In general, it is an essential design problem for deep copy of complex objects. What is the contract between objects?

A good copy ctor is essential, at least.

NULL is a bit scary; from now see nullptr

http://www.cprogramming.com/c++11/c++11-nullptr-strongly-typed-enum-class.html