pointer in a class

Hello,
I'm wondering when is necessary/better declare a pointer to an object in a class or a pointer to a vector in a class

Code:

---

class A {
  public:
   vector<flaoat> fvec;
   //or
  vector<float>* fpvec;
  A() {  
      fvec.resize(1000);
      //or
      fpvec = new vector<float>(1000);
  }
};

---

What's the thing that lead me to opt one or other?
THe same question if I had a Person p[10]; in the class. Should be better take a pointer to an array of person? How to choice?

thanks

Quote:

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Originally Posted by mickey

I'm wondering when is necessary/better declare a pointer to an object in a class or a pointer to a vector in a class

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The only reasons to declare a pointer instead of an object I see are:
1. The object pointed by the pointer is not owned by the containing object or the instance is shared among more than one containing object.
2. One wants to only forward declare a class in the header file to speed up compilation of dependent objects and slow down initialization of objects.
3. The platform the application is running on has limitations on the stack size and as much data as possible is pushed onto the heap.

Apart from those three situations I see no reason to use pointers instead of objects.

Although if you do decide not to use a pointer, it might be a good idea to give the vector its initial size in the initialization list:

Code:

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A() : fvec(1000) {}

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Oh, I just thought of another reason to use a pointer instead of a full object - when the use of the object is optional and the full object is allocated only when needed - then you'll need a pointer. On the other hand it doesn't make much sense to do that with a vector because it already lazy allocates it data on the heap, so both conditions are fulfilled with using a vector object instead of vector pointer. The difference in memory footprint will be minimal (unless you resize the vector when constructing it - then there is no lazy allocation there).

Is there any change to use forward declaration with vector, like this:

Code:

---

class vector; // ?
 
class A {
    vector<float>* fvec;
.......................
};

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Your code that depends on class A will compile faster as it will not have to parse vector's header file each time A's header is included. The application will run a bit slower though, as it'll have to allocate the vector at some point.

Quote:

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Originally Posted by wysota

Your code that depends on class A will compile faster as it will not have to parse vector's header file each time A's header is included.

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but I asked how write it (this above doens't compile)

Quote:

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Originally Posted by wysota

The application will run a bit slower though, as it'll have to allocate the vector at some point.

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I guess the cause is around the allocation on the heap...

I think the forward declaration has to look something like:

Code:

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template <class T> class vector;

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I'd say it should be "class vector<float>;". I don't see a reason to use a pointer to a vector in this situation, though... It's just asking for trouble and a necessity to write more code.

Quote:

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Originally Posted by wysota

I'd say it should be "class vector<float>;". I don't see a reason to use a pointer to a vector in this situation, though... It's just asking for trouble and a necessity to write more code.

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Code:

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class vector<float>;
 
class Shape {
	const std::vector<float>* _sides;

---

It doesn't compile; compile gets many error on the first line (like as I missed the ";").
I know It's unuseful, but I'd like compile it for study....
thanks.

Probably because you didn't put it in a namespace. Try:

Code:

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namespace std { class vector<float>; }

---

or:

Code:

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namespace std { template <class T> vector; }

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nothing to do

IMHO the pointer shouldn't be const.

Quote:

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Originally Posted by wysota

IMHO the pointer shouldn't be const.

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nothing to do again....

What do you mean by "nothing to do"?

Quote:

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Originally Posted by wysota

What do you mean by "nothing to do"?

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Sorry, my english is not correct.....It means that compiler gets the same errors (a lot) on the line "class vector<float>;" (I took off "const" as you suggested) . Of course, if I include vector.h, there aren't problems...I don't understand how to do this forward declaration:crying:

Sorry, I made a mistake in my template declaration. Try:

Code:

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namespace std { template <class T> class vector; }

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That compiles fine on my computer.

Quote:

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Originally Posted by Shadowfiend

Sorry, I made a mistake in my template declaration. Try:

Code:

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namespace std { template <class T> class vector; }

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That compiles fine on my computer.

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Maybe it can be the solution; but then, my problem is in a other place; try to compile this ,please (It doens't compile on .net2005)

Code:

---

//main.cpp
#include <iostream>
#include "forward.h"
#include <vector>
using namespace std;
 
int main (int argc, char argv[]) {
 
	Forward f;	
	vector<float>* fvec = new vector<float>; //EDIT <----------------------------
 
	return EXIT_SUCCESS;
}
//forward.h
#ifndef FORWARD_H
#define FORWARD_H
 
namespace std { template <class T> class vector; } 
// #include <vector> // with only this, all compiles
 
class Forward {
	std::vector<float>* _vec;
public:
	Forward();
	~Forward();
};
 
#endif FORWARD_H
 
//forward.cpp
#include "forward.h"
#include <vector>
using namespace std;
 
Forward::Forward() {
 
	_vec = new vector<float>;
}
Forward::~Forward() {}

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Ah, that's right. std::vector takes an std::allocator as its second parameter. Well that just gets nasty...

Here's a place where this same issue is discussed: http://objectmix.com/c/251244-forwar...compliant.html

Looks like it's possible, but the advantage is negligible and there is not necessarily a guarantee that it will work.

Sorry, I did a mistake; what I want to show is that the code doens't still work; see the code I correct it....what I say is that, with your solution, if I put #define <vector> in such file that include even "forward.h", the compiler gets errors; does anyone know how to correct the last code above ,please?

Is it really worth the trouble? Use precompiled headers and your problem will disappear (along side a 100MB of your disk space).

Quote:

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Originally Posted by wysota

Is it really worth the trouble? Use precompiled headers and your problem will disappear (along side a 100MB of your disk space).

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When I start to do this forward declaration, I didn't think it was "particularly". What I'd like understand to this point is:
1. Why these all problems
2. why 100MB !!!
3. When Does the forward declaration is recommended

Thanks in advance.

Quote:

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Originally Posted by mickey

1. Why these all problems

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You should know, it's your code :)

Quote:

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2. why 100MB !!!

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When you use precompiled headers this is more or less the amount of spaces those headers take after precompilation.

Quote:

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3. When Does the forward declaration is recommended

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We've been all over this question... It's not recommended in this particular case, just include the vector and skip to some more interesting work.

Concerning (1) -- because STL container classes use enough template magic that forward-declaring them is a pain.

As for (3) -- forward declaration is mostly to speed up compilation times. Precompiled headers take care of that problem to some extent by only compiling the header part once, eliminating most of the overhead (so that the vector header doesn't need to be parsed every time a file is compiled that includes your class header that uses it).

In this case, obviously the effort to forward-declare a vector isn't worth the small time savings during compilation.