dynamic ptr types

This is a discussion on dynamic ptr types within the C++ Programming forums, part of the General Programming Boards category; Hi, I am just wondering if it is at all possible to redefine the type of a pointer at runtime? ...

  1. #1
    Registered User subdene's Avatar
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    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.
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  2. #2
    Compulsive Liar Robc's Avatar
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    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.

  3. #3
    Registered User subdene's Avatar
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    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
    
      }
    }
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  4. #4
    Compulsive Liar Robc's Avatar
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    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.

  5. #5
    Registered User subdene's Avatar
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    Robc : Thanks very much for that solution, it is very good and encapsulates alot of the lower level stuff.

    Cheers.
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