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pointers

Started by BloodLust666 Sep 7, 2006 at 12:23 PM 19 replies 1.6k views

Original Post

82

Sep 7, 2006 at 12:23 PM

I have an instance where i need to take a pointer, create a new pointer using "new" and copy the values from the passed pointer into the new one so the old one can be deleted and the new one will retain the values from the old one.

200

Sep 7, 2006 at 12:32 PM

Assuming C++. Implement a copy constructor for your class then just do:

MyClass *newPointer = new MyClass( *oldPointer );//since the values have been copied//you can happily delete the old instance without data lossdelete oldPointer;

here's a little tut on Copy Constructors: http://www.fredosaurus.com/notes-cpp/oop-condestructors/copyconstructors.html

-me

1,022

Sep 7, 2006 at 12:33 PM

What you described is usually referred to as 'cloning.' It's a common idiom in C++.

You have to write a clone() member function yourself. Yuo can sometime leverage the copy constructor.

200

Sep 7, 2006 at 12:37 PM

A _very_ important thing to remember is that if your class has member variables which are pointers, those variables will also have to be copy-constructed.

-me

Author

82

Sep 7, 2006 at 12:46 PM

so i would have to create my own copy constructor and copy constructor those pointers as well?

200

Sep 7, 2006 at 12:54 PM

Quote:
Original post by EvilKnuckles666
so i would have to create my own copy constructor and copy constructor those pointers as well?

correct. if you just assign the values of the member variable pointers then you're just copying a memory address, not the data itself. So you'll have to implement copy-constructors for all the member variable objects also. If they're dynamically allocated arrays, you'll have to new [] and then memcpy the data, etc.

-me

Author

82

Sep 7, 2006 at 1:15 PM

does this look correct?

class myClass
{
myClass(myClass &Copy)
{
m_pOne = *Copy.m_pOne;
m_pTwo = *Copy.m_pTwo;
m_Three = Copy.m_Three;
};

// ...

};

344

Sep 7, 2006 at 1:17 PM

The "deep copy" issue is one of the most problamatic in all of programming, because there is no such thing as a "correct" one-size-fits-all solution.

But for your case I think you want to implement either the "deepest copy reasonable" / "copy everything I can create" version or the "copy everything I own" version. I would start with the "copy everything I own" version.

Which simply means this, every single thing you destroy in your destructur, you should create a new copy instance of in your assignment operator and copy constructor.

So if you destructor does this:

delete x;
// or something like this:
x.Dispose();
x = null;

in your copy functions you should use:

x = new XsType(*(rhs.x));

not:

x = rhs.x;

Author

82

Sep 7, 2006 at 1:28 PM

ok, i got that much, but how do i copy a value from one pointer to the value of another?

i thought this: but it doesn't work

*newpointer = *oldpointer

Author

82

Sep 7, 2006 at 2:09 PM

what's wrong with this code? it's giving me an error

myClass(myClass &Copy)
{
m_pOne = NULL;
m_pTwo = NULL;

memcpy((void*)m_pOne , (void*)Copy.m_pOne, sizeof(myClass));
memcpy((void*)m_pTwo , (void*)Copy.m_pTwo, sizeof(myClass));
m_Three= Copy.m_Three;
};

238

Sep 7, 2006 at 2:14 PM

You are writing to two NULL pointers, which is a Bad Thing^TM. Have these pointers point to something beforehand.

Also, you seem to be memcpy'ing classes. This is a Worse Thing^TM, since classes often have no definite memory layout and/or contain pointers. Classes should be copied using their copy constructor.

182

Sep 7, 2006 at 2:21 PM

Never use memcpy to copy classes!

You're telling it to write to a NULL memory address, twice. You will be getting a NULL pointer exception.

Your solution will look like the following.

newpointer = new myInternalClass(*oldpointer);

Author

82

Sep 7, 2006 at 2:25 PM

only bad thing is when i call that "new" i'd have to delete it too, and i wouldn't know which instances i copied from and which one's i created and didn't use "new" so i woudln't know which ones to call "delete" on

238

Sep 7, 2006 at 2:27 PM

Well, if you don't use new, you have no memory to write to, so it crashes.

Author

82

Sep 7, 2006 at 2:33 PM

well, how will i know which ones i called new to and which ones not?

25

Sep 7, 2006 at 2:38 PM

Remember that you don't have to call new or delete for every pointer variable you use. You have to call new and delete for every dynamicly allocated value you use. You could have multiple pointers point to the same object, but you only need to use new and delete once.

I could be wrong, but I get the impression that you're lacking a basic understanding of pointers and freestore memory. I'd recommend looking up tutorials on that.

189

Sep 7, 2006 at 2:39 PM

The point is you're not supposed to assign the created objects to another class, you should create new ones.

class A { public:  A() {   member0 = 0;  }    A(const A &src) {   member0 = src.member0;  }  private:  int member0;}class B { public:  B() {   member0 = new A;      member1 = new int[10];   for(int i=0;i<10;i++)    member1 = 0;      member2 = 0;  }    B(const B &src) {   member0 = new A(*src.member0);      member1 = new int[10];   for(int i=0;i<10;i++)    member1 = src.member1;      member2 = src.member2;  }    ~B() {   delete member0;   delete[] member1;  }  private:  A *member0;  int *member1  int member2;}

This way you can just create a copy of your class with
B *newB = new B(*oldB);

Additionaly it's better to use initalization lists, if you know what those are.

If you don't want to create new copies of the objects inside the class you should not make a copy, but rather a swap.
Create a new empty class object, then swap the internal variables with the old one, and then delete the old one.

Author

82

Sep 7, 2006 at 3:18 PM

ok... there's no way i'm going to be able to do this because i can't get a hold of the class at all.... it's an API struct.

here's what i'm REALLY trying to do.. maybe someone can help.

ok, i have a template that has an Add() function, internally it adds to a map<> template with a std::string and Type (the template part). the Add() created a new item using "new" and puts it into the map<>. BUT now what i'm trying to do is make an Insert() that lets you put a pointer to an already created object and add that into the map<> but i noticed when i get that object back, it doesn't have the values i put in it. here's my code.

void Insert(std::string str, Type* Item){	Type *newType = new Type(*Item); // this is where i want to have a point                                         // get created and have the same values                                         // as the old one	m_MapList.insert( std::pair<std::string, Type*>(str, newType));};

this would work and all, but because of the struct i'm using that apart of an API, i can't get a hold of the copy structure. sooo... is there another way i could do it with the map<> maybe?

thanks

edit: sry, i had other stuff that wasn't right in there

344

Sep 7, 2006 at 3:28 PM

So are you saying the class you are using does not have a copy-constructor / assignment operator?

If the API class you are using IS copyable - then you could use either a std::map or std::map depending on the semantics you want for ownership, and the efficiency you need to achieve.

std::map would always use value / copy semantics

std::map would use reference semantics by default, but if you implemented it as you showed above (using new Type(...)) then you are getting value semantics with a pointer.

If the API class you are using is not copyable, then you cannot copy it. Therefore your only choice is std::map AND you cannot make it have value semantics (because you cannot call new) ... you can ONLY use reference semantics ... so you would need to deal with ownership management issues rather than copy-creation issuse.

For instance you could use a smarter pointer .... many to choose from, but boost seems to be the way to go ...

then when you Add() the element, you do whatever management required of your smart poitner (often nothing but strait assignment).

Author

82

Sep 7, 2006 at 4:10 PM

my template is defined as map, i'm using all pointers. doing so there's no way of taking a value from one, creating a new one and putting those values into my map?

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