how to define a type at run time

Hi,guys,

there are two data frames,the first gives the defining data of the second,so I thought it would define data type in the second frame according to the first frame at run time?
how can I resolve it? I have been puzzled long time...too hard to me, anyone could help me...thx!

Can't undestand what you want. Can you give a real example?

You cannot define a new type at runtime.

What you might want to do is dynamically create an existing type at runtime based on runtime information.

You'll also need to determine what you are going to do with the result when you have it.

You should, in this case, use an abstract class factory which will create the appropriate class and then you call the appropriate method on it (which must also be abstract).

You can create adapters for existing classes that implement your interfaces with the concrete classes that already exist.

thanks for your advice,could you give me some articles or sample codes about your idea? Are there any cool links or forums about design pattern?I just know something about design pattern...but I am a novice about using abstract class factory pattern...thx

Quote:

---

Originally Posted by NMTop40

You cannot define a new type at runtime.

What you might want to do is dynamically create an existing type at runtime based on runtime information.

You'll also need to determine what you are going to do with the result when you have it.

You should, in this case, use an abstract class factory which will create the appropriate class and then you call the appropriate method on it (which must also be abstract).

You can create adapters for existing classes that implement your interfaces with the concrete classes that already exist.

---

i think u r looking for something like this have a look for void pointer.

Code:

---

void *pv;                // Okay
int  *pint; int i;
int main()              // main has no return value
{
  pv = &i;
  // Cast optional in C required in C++
  pint = (int *)pv;
}

---

Code:

---

int *frame;
void *frameSecond;
int *IntSecFrameData;
char *CharSecFrameData;
float *FloatSecFrameData;


frame=getFrameFirst();

switch(*frame)
{

  case IntData:
          intSecFrameData=(int *)getFrameSecond();
          break;
  case CharData:
          charSecFrameData=(char *)getFrameSecond();
          break;
  case FloatData:
          FloatSecFrameData=(float *)getFrameSecond();
          break;
  default:
          frameSecond=(void *)getFrameSecond();
          break;

}

---

I think you looking ..for something like above stuff.........



-Anant

RTTI'll help you!
typeid( your_variable ).name()

one little problem - different compilers has different types of name strings
for example, int - "i", "int", "integer" etc. for different C++ compilers. Check your compiler's names before using.

..but void*... I don't know, how it'll work after casting to void and backwards. But it gives dynamic type of variable, but not for referenced type.

!and!
you must always remember!
if you want to use dynamic type identification, it means, that you make an error in design time. 90% of using RTTI - result of such errors.
For example, you can use virtual functions instead of RTTI.

thx...but how to define data struct at run time? The type of data wasnt defined utill at run time...it seemed that the type of data was defined at compile-time in your code

Quote:

---

Originally Posted by humptydumpty

i think u r looking for something like this have a look for void pointer.

Code:

---

void *pv;                // Okay
int  *pint; int i;
int main()              // main has no return value
{
  pv = &i;
  // Cast optional in C required in C++
  pint = (int *)pv;
}

---

---

thx for your advice,your advice is good,but my second frame is not fixed...maybe it includes two char,three int..maybe it includes other data type... data type in the second frame was defined at run time in my problem...maybe NMTop40's advice help me more...I wanna something like abract factory class pattern...but I am still puzzled...

Quote:

---

Originally Posted by anantwakode

Code:

---

int *frame;
void *frameSecond;
int *IntSecFrameData;
char *CharSecFrameData;
float *FloatSecFrameData;


frame=getFrameFirst();

switch(*frame)
{

  case IntData:
          intSecFrameData=(int *)getFrameSecond();
          break;
  case CharData:
          charSecFrameData=(char *)getFrameSecond();
          break;
  case FloatData:
          FloatSecFrameData=(float *)getFrameSecond();
          break;
  default:
          frameSecond=(void *)getFrameSecond();
          break;

}

---

I think you looking ..for something like above stuff.........



-Anant

---

Quote:

---

Originally Posted by NMTop40

You cannot define a new type at runtime.

What you might want to do is dynamically create an existing type at runtime based on runtime information.

You'll also need to determine what you are going to do with the result when you have it.

You should, in this case, use an abstract class factory which will create the appropriate class and then you call the appropriate method on it (which must also be abstract).

You can create adapters for existing classes that implement your interfaces with the concrete classes that already exist.

---

can u explain in detail about the abstract factory class with a code as an example or some reference material if possible

For some basic sample code, look at http://www.earlpurple.com/CodeBase/Factory.h