C中具有空指针的动态类型变量（最终是动态类型转换）

Question

im just came into the question with dynamic type of variable (not really dynamic, but should be determind in the runtime), the situation is like this:

i have a function which accept a double array convert it into the integer and write to a file, the integer can have different Bitlength, like 8, 16 and 32. As it is a array, i want to use a pointer to access the final result (array). So i use the void pointer with malloc and switch case now, but it will be needed to add switch case every where when i trying to access or modify this array, my question is, is there a better way to do this?

current code is like:

void foo(double * arr, int len, int iBits, FILE *fh)
{
      void * newArr;
      int iBytePerElement, iBase,i;

      iBytePerElement = iBits / 8;
      iBase = (1 << (iBits - 1)) - 1;

      switch (iBytePerElement)
      {
             case 1:
                  {
                         newArr = (int8_t *) malloc(sizeof(int8_t)*len);
                         break;
                  }
             case 2:
                  {
                         newArr = (int16_t *) malloc(sizeof(int16_t)*len);
                         break;
                  }
             case 4:
                  {
                         newArr = (int32_t *) malloc(sizeof(int32_t)*len);
                         break;
                  }
      }

      for (i = 0; i < len; ++i)
      {
              switch (iBitPerElement)
              {
                     case 1:
                          {
                                ((int8_t *)newArr)[i] = (int8_t)(arr[i]*iBase);
                                 break;
                          }
                     case 2:
                          {
                                ((int16_t *)newArr)[i] = (int16_t)(arr[i]*iBase);
                                 break;
                          }
                     case 4:
                          {
                                ((int32_t *)newArr)[i] = (int32_t)(arr[i]*iBase);
                                 break;
                          }
              }
      }
      fwrite(newArr, iBytePerElement, iBytePerElement*len,fh);
}

Answer 1

Making a few assumptions about what you probably meant where your example code is undefined (commented), here's how I might do it:

#include <stdint.h>
#include <stdlib.h>
#include <limits.h>
void *foo(double * arr, int len, int iBits)  //should return the malloced array not void
{
      void * newArr;
      int iBytePerElement, iBase, i;

      iBytePerElement = iBits / 8;
      iBase = (1 << (iBits - 1)) - 1;

      if(NULL==(newArr = malloc(iBits/CHAR_BIT))) return NULL; //replace the first switch
      //which obviously meant to alloc sizeof(int16_t)*len in the case 2 branch (not (sizeof(int8_t)*len) etc

      for (i = 0; i < len; ++i) {
          switch(iBytePerElement){
          break; case 1: ((int8_t *)newArr)[i] = arr[i]*iBase; 
          break; case 2: ((int16_t *)newArr)[i] = arr[i]*iBase;
          break; case 4: ((int32_t *)newArr)[i] = arr[i]*iBase;
          }
      }
      return newArr;
}

Basically, you only need the second switch.

If you wanted to get rid of the 2nd switch, you could replace it with some function pointer play. For example:

#include <stdint.h>
#include <stdlib.h>
#include <limits.h>

#define MK_FN(Bits) \
void to##Bits(void *newArr, double const*arr, int len) \
{ \
    int i; for(i=0; i < len; i++) ((int##Bits##_t *)newArr)[i] = arr[i]*((1<<(Bits-1)-1));  \
} 
MK_FN(8)
MK_FN(16)
MK_FN(32)



void *foo(double * arr, int len, int iBits)  //should return the malloced array not void
{
      void * newArr;
      int iBytePerElement = iBits / 8;

      if(NULL==(newArr = malloc(iBits/CHAR_BIT))) return NULL;
      ((void (*[])(void *,double const*, int)){ [1]=to8, [2]=to16, [4]=to32, })[iBits](newArr,arr,len);
      return newArr;
}