dynamic ptr types

Hi,

I am just wondering if it is at all possible to redefine the type of a pointer at runtime? For example, say I have char *pBuffer, but instead of it pointing to a type char, I now want it to point to a type of short signed *pBuffer. However, I still want to be able to use the same indentifier i.e. pBuffer. I know you can use the reinterpret_cast operator, but that is not permenant, and I don't want to be using that constantly throughout my for loop when subscripting.

Any Ideas?

Thanks, Dene.

The closest match to what you want is a pointer to void. That's the generic pointer type in C++ and you can assign any pointer to it. You might have some issues with dereferencing the pointer if you use it in a hardcoded loop though. What exactly are you trying to do? It sounds like your design is off.

I am analysing a sine wave which is dependant upon the bits per sample, (which could either be 8 or 16). I was going to do this:

Code:

---

f(void *pSampleBuffer)
{
  bool bPtrType = false;
  int iSample1, iSample2;

  char *cptrData = (char*)pSampleBuffer;
  short signed *iptrData = (short signed *)pSampleBuffer;
       
  if((m_wfWaveFormat.wBitsPerSample / 8 == 2) bPtrType = true;

  for(int i = 0; i < bla bla - 1; i++)
  {
    if(bPtrType)
    {
      iSample1 = iptrData[i];
      iSample2 = iptrData[i + 1];
    }
    else
    {
      iSample1 = cptrData[i]
      iSample2 = cptrData[i + 1]
    }

    //then the rest of the code would use the iSample1 and iSample2

  }
}

---

You could tuck all of the nasty stuff safely in an object to avoid switching on arbitrary attributes. That would clean up your function a bit:

Code:

---

class sample_list {
public:
  sample_list(void *buffer, int size, bool is_byte);
  int next();
  bool done();
private:
  unsigned char *pbuffer;
  int            buf_size;
  int            current;
  bool          use_byte;
};

sample_list::sample_list(void *buffer, int size, bool is_byte)
  : pbuffer(static_cast<unsigned char *>(buffer))
  , buf_size(size)
  , current(0)
  , use_byte(is_byte)
{}

int sample_list::next()
{
  int ret;

  if (use_byte) {
    ret = pbuffer[current];
  }
  else {
    ret = (reinterpret_cast<short int *>(pbuffer))[current];
  }
  ++current;

  return ret;
}

bool sample_list::done()
{
  return current == buf_size;
}

void f(void *sample_buffer, int size)
{
  bool        is_byte_sample = m_wfWaveFormat.wBitsPerSample / 8 == 1;
  sample_list samples(sample_buffer, size, is_byte_sample);
  int        sample;

  while (!samples.done()) {
    sample = samples.next();
    // Use sample
  }
}

---

There are several ways to solve this problem, but none of them are really good and elegant at the same time. :(

Robc : Thanks very much for that solution, it is very good and encapsulates alot of the lower level stuff.

Cheers.