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# Struct copy inside itself

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10 replies to this topic

### #1ak09  Members

Posted 01 May 2013 - 06:38 PM

struct wheel
{
wheel *copy;
wheel()
{
// variables assignment
copy = (wheel *)malloc(sizeof(wheel));
*copy = *this;
}
// variables...
};


Is this a reasonable code? Will it behavior nice for my purpose of having a struct's copy that way?

### #2Khatharr  Members

Posted 01 May 2013 - 06:56 PM

No.

What are you trying to do, please?

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

### #3Brother Bob  Moderators

Posted 01 May 2013 - 06:57 PM

This depends greatly on how you handle the copying, and what a "copy" actually means in this case. You need to differentiate between the original and the copy in order to handle the copy of the original object as a copy, and not an original in itself that has yet another copy of itself.

Explain why you need this and what you intend to do with this copy instead and we may able to provide an actual answer to the specific question, or even provide a better solution.

### #4alnite  Members

Posted 01 May 2013 - 07:19 PM

Also, be very wary of recursive copying.

### #5zacaj  Members

Posted 01 May 2013 - 07:36 PM

Your code *is* valid.  copy will be an exact copy of the struct, including containing its own copy that points to itself.

### #6ak09  Members

Posted 01 May 2013 - 07:40 PM

I'm trying to have a copy of my dozens of variables struct inside it, because it'll only be used in that header file. There will be no reason why my main file should know about this copy's existance.
Using the above struct as an example, should be a funcion named spin(wheel w), which is called millions of times. All I need, for a faster performance, is to copy the struct once, right after it's creation, and not on every spin call.

### #7ak09  Members

Posted 01 May 2013 - 07:42 PM

Your code *is* valid.  copy will be an exact copy of the struct, including containing its own copy that points to itself.

Yeah, it's working fine so far.

### #8Khatharr  Members

Posted 01 May 2013 - 08:21 PM

The ctor does not set any variables (other than copy), so copy will simply point to block of memory identified as being of type 'wheel' with undefined values.

?

Edited by Khatharr, 01 May 2013 - 08:23 PM.

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

### #9Servant of the Lord  Members

Posted 01 May 2013 - 08:21 PM

Is this C or C++? You're using malloc and structs, which implies C-style programming, but you have a constructor, implying it's actually C++.

If C++, why not do something like this:

class Wheel
{
public:
Wheel()
{
//Do the thing that you only want to do once here.
}

}

Or this:

class Wheel
{
public:
Wheel()
{
//Do the thing that you only want to do once here.
}

void Spin()
{
this->doThatThingYouOnlyDoOnce();
}

private:

void doThatThingYouOnlyDoOnce()
{
}
}

The first option is much better though, because if a function says "Spin()", I expect the function to do what it says. Coding is hard enough without function names lying to you.

Maybe you want the constructor to spin, and you actually want a function called "GetResultOfSpin()"?

It's perfectly fine to abbreviate my username to 'Servant' or 'SotL' rather than copy+pasting it all the time.
All glory be to the Man at the right hand... On David's throne the King will reign, and the Government will rest upon His shoulders. All the earth will see the salvation of God.
Of Stranger Flames -

### #10rip-off  Moderators

Posted 02 May 2013 - 01:46 AM

The code can become invalid if you add a non-trivial copy constructor to the wheel class, which you would have to do to avoid leaking memory.

I cannot understand exactly what you want. Have you considered passing the wheel by (const) reference instead? If copies are expensive, what about making your wheel class noncopyable? Are you trying to use something like the pimpl idiom or the flyweight pattern?

### #11King Mir  Members

Posted 02 May 2013 - 12:33 PM

It looks like you need to think about your problem more, because I cannot see a useful application of what you're trying to do.

That said, the way to create a copy on the heap is this:
struct wheel
{
wheel *copy;
wheel()
{
copy = new wheel(*this);
}
//also needs copy constructor, assignment, and destructor, to handel freeing correctly.
};

Even better you can use this:

#include<memory>

struct wheel
{
std::shared_ptr<wheel> copy;
wheel()
{
copy.reset(new wheel(*this));
}
//copy constructor, assignment, and destructor not needed; free does not need to be called.
};

Old topic!

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