Template classes and abstraction of type

[Raistlin]

I modified the example a bit to make it a bit more clear. I use a single value to type T here for simplicity, but of course this not what happens in reality. But it is just to illustrate the idea.

/*Use of the classes in the main program*/
 
//Check for type of files and allocate based on type
VolumeGeneral::type imageType;
 
//Determine the type based on input files
...
 
//Allocate with righ type 
VolumeGeneral* volume = 0;
 
if (imageType == typeU16)
	volume = new VolumeSpecified<unsigned short>();
 
else if (imageType == typeU8)
	volume = new VolumeSpecified<unsigned char>();
 
else
	break;
 
//Perform general operations
volume->init();
volume->doSomething();
 
//Perform type specific operations
if (volume->getType() == VolumeGeneral::typeU16)
{
	VolumeSpecified<unsigned short>* volumeS = dynamic_cast<VolumeSpecified<unsigned short>*>(volume);
	unsigned short testVariable = volumeS->getData();
}
 
delete volume;

/*Use of the classes in the main program*/

//Check for type of files and allocate based on type
VolumeGeneral::type imageType;

//Determine the type based on input files
...

//Allocate with righ type 
VolumeGeneral* volume = 0;

if (imageType == typeU16)
	volume = new VolumeSpecified<unsigned short>();

else if (imageType == typeU8)
	volume = new VolumeSpecified<unsigned char>();

else
	break;

//Perform general operations
volume->init();
volume->doSomething();

//Perform type specific operations
if (volume->getType() == VolumeGeneral::typeU16)
{
	VolumeSpecified<unsigned short>* volumeS = dynamic_cast<VolumeSpecified<unsigned short>*>(volume);
	unsigned short testVariable = volumeS->getData();
}

delete volume;

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I don't see how I can do this without the extra class. If I only have my template class, I always need to specify the type, right? I can not use an object of type template class T without specifying T. So I need a base class which does not have any type to provide an interface.

What do you mean by using a 'base class pointer' when you say I don't need a wrapper? It seems like I need to list all methods in my abstract base class, since I need it as interface for my methods. If you talk about casting some base class pointer to the right template type, it seems this is something I always need to do, regardless whether I am exporting or importing data, or I am doing something where the type does not need to be visible.

Maybe I just need some example code on how to apply what you mean in my case, since it seems I am missing out on something ...

[wysota]

class Image {
public:
  virtual void init() = 0;
  virtual void doSomething() = 0;
  virtual ~Image(){}
};
 
class Image16 : public Image {
public:
  Image16(unsigned short *data){
    //...
  }
  void init(){ ... }
  void doSomething(){ ... }
};
 
class Image8 : public Image {
 //...
};
 
Image *img;
if(type==16)
  img = new Image16(data);
else img = new Image8(data);
img->init();
img->doSomething();

class Image {
public:
  virtual void init() = 0;
  virtual void doSomething() = 0;
  virtual ~Image(){}
};

class Image16 : public Image {
public:
  Image16(unsigned short *data){
    //...
  }
  void init(){ ... }
  void doSomething(){ ... }
};

class Image8 : public Image {
 //...
};

Image *img;
if(type==16)
  img = new Image16(data);
else img = new Image8(data);
img->init();
img->doSomething();

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Isn't that what you want? You don't reference the actual class type anywhere apart from calling the constructor, so there is no problem of not knowing the type.

[Raistlin]

Thank you for that code wysota. However, it seems like I am basically doing the same thing. The only difference I see is that your solution has less code because you do not keep track of the type in your base class. However, I sometimes do need to do some type-specific operations (like in the example) to interface with the main program, so I have to keep track of it to get out the right type of data.

In conclusion, this is already a lot cleaner than what I started from. The only thing I need to do is to add all non-type specific methods as pure virtual methods to the base class.

[wysota]

However, I sometimes do need to do some type-specific operations (like in the example) to interface with the main program, so I have to keep track of it to get out the right type of data.

if((Image16 *image16 = dynamic_cast<Image16*>(img))!=0){
  image16->specificMethod16();
} else if((Image8 *image8 = dynamic_cast<Image8*>(img))!=0){
  image8->specificMethod8();
} else {
  std::cerr << "Unknown Image subclass encountered" << std::endl;
}

if((Image16 *image16 = dynamic_cast<Image16*>(img))!=0){
  image16->specificMethod16();
} else if((Image8 *image8 = dynamic_cast<Image8*>(img))!=0){
  image8->specificMethod8();
} else {
  std::cerr << "Unknown Image subclass encountered" << std::endl;
}

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[Raistlin]

Oh, thanks for that addition. I did not know casting to the wrong type returned 0 instead of a junk value.

[wysota]

That's the idea of dynamic_cast. If your Image class inherited QObject, you could also use qobject_cast.