how can I avoid the use of #if in a polymorphic pr

Question

Let's try to run the following code:

#include <stdio.h>
#define MY_MACRO1(isArray,y) do { \
                      if(isArray) \
                        printf("%d", y[0]); \
                      else \
                        printf("%d", y); \
                     }while(0)

int main()
{
    int a = 38;
    int b[]={42};

    MY_MACRO1(0,a);

    return 0;
}

it returns the error:

main.c: In function ‘main’:
main.c:12:39: error: subscripted value is neither array nor pointer nor vector
                         printf("%d", y[0]); \

Ok, so we would need a #if statement to run y[0] only if the variable is an array:

#define MY_MACRO2(isArray,y) do { \
                      #if isArray \
                      printf("%d", y[0]); \
                      #else \
                      printf("%d", y); \
                      #endif \
                     }while(0)

int main()
{
    int a = 38;
    int b[]={42};

    MY_MACRO2(0,a);

    return 0;
}

But it returns :

main.c:11:28: error: '#' is not followed by a macro parameter
 #define MY_MACRO2(isArray,y) do { \

Is there anyway to call a #if statement inside a macro? if not, how can I do such a thing?

note: I'm using IAR 8.20.2

(this link does not help)

I you want to know why I would not like to use 2 different macros is because I need to to something like this (pseudo-code):

myFunction(int or array):
   doSomethingWhereIntAndArrayBehavesDifferentlyLikePrintf();
   doSomethingelse();
   doSomethingelse();
   doSomethingWhereIntAndArrayBehavesDifferentlyLikePrintf();
   doSomethingelse();

• It is pretty handy : you can factorize code.
• It's a way of implementing polymorphism.
• It mimics the C++ template feature.

Answer 1

Is there anyway to call a #if statement inside a macro?

Not possible.

if not, how can I do such a thing?

You could use C11 _Generic:

#include <stdio.h>

void int_func (int obj)
{
  printf("%d\n", obj);
}

void int_arr_func (const int* obj)
{
  printf("%d\n", obj[0]);
}

void float_func (float obj)
{
  printf("%f\n", obj);
}

#define MY_MACRO2(y) _Generic((y), \
  int:   int_func,                 \
  int*:  int_arr_func,             \
  float: float_func ) (y)


int main (void)
{
    int a = 38;
    int b[]={42};
    float pi = 3.14f;

    MY_MACRO2(a);
    MY_MACRO2(b);
    MY_MACRO2(pi);
    return 0;
}

Answer 2

Sum up and another works around using VLA & macros.

Using _Generic

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

static inline void printNonArray_u32 (uint32_t obj)
{
    printf("interger >> uint32_t : %u\n",obj);
}
static inline void printNonArray_u16 (uint16_t obj)
{
    printf("interger >> uint16_t : %u\n",obj);
}
static inline void printNonArray_u8 (uint8_t obj)
{
    printf("interger >> uint8_t : %d\n",obj);
}
static inline void printArray_u32 (const uint32_t* obj)
{
    printf("array >> uint32_t : "); 
    for(uint32_t i = 0 ; i<sizeof(obj)/sizeof(uint32_t) ; i++ ) 
      printf("%d - ",(uint32_t)obj[i]); 
    printf("\n"); 
}
static inline void printArray_u16 (const uint16_t* obj)
{
    printf("array >> uint16_t : "); 
    for(uint32_t i = 0 ; i<sizeof(obj)/sizeof(uint16_t) ; i++ ) 
      printf("%d - ",(uint16_t)obj[i]); 
    printf("\n"); 
}
static inline void printArray_u8(const uint8_t* obj)
{
    printf("array >> uint8_t : "); 
    for(uint32_t i = 0 ; i<sizeof(obj)/sizeof(uint8_t) ; i++ ) 
      printf("%d - ",(uint8_t)obj[i]); 
    printf("\n"); 
}

#define MY_POLYMORPHIC_PRINT2(y)  _Generic((y), \
                                  uint32_t:   printNonArray_u32, \
                                  uint32_t*:  printArray_u32, \
                                  uint16_t:   printNonArray_u16, \
                                  uint16_t*:  printArray_u16, \
                                  uint8_t:   printNonArray_u8, \
                                  uint8_t*:  printArray_u8 ) (y)

int main()
{
    uint32_t i1 = 257;
    uint16_t i2 = 257;
    uint8_t  i3 = 25;
    uint32_t a1[]={42,43,44,257};
    uint16_t a2[]={22,23,24,257};
    uint8_t  a3[]={12,13,14,25};


    printf("MY_POLYMORPHIC_PRINT2 - _Generic\n");      
    MY_POLYMORPHIC_PRINT2(i1);           //interger >> uint32_t : 257                                             >> OK                                                                                                                                                                                                
    MY_POLYMORPHIC_PRINT2(i2);           //interger >> uint16_t : 257                                             >> OK                                                                                                                                                                              
    MY_POLYMORPHIC_PRINT2(i3);           //interger >> uint8_t : 25                                               >> OK                                                                                                                                                                                                            
    MY_POLYMORPHIC_PRINT2(a1);           //array >> uint32_t : 42 - 43 -                                          >> FAILS                                                                                                                                                                                                 
    MY_POLYMORPHIC_PRINT2(a2);           //array >> uint16_t : 22 - 23 - 24 - 257 -                               >> OK                                                                                                                                                                                    
    MY_POLYMORPHIC_PRINT2(a3);           //array >> uint8_t : 12 - 13 - 14 - 25 - 253 - 127 - 0 - 0 -             >> FAILS                             

    return 0;
}

pros:

• simple macro + simple functions
• you don't need C++ library
• you don't need "isArray" anymore
• you don't need #if
• type-safe

cons:

• how to get the array size? --> gives some bad results without
• limited to C11

Using VLA (no ##)

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define MY_POLYMORPHIC_PRINT1(y, type) do { \
                      if((sizeof(y)/sizeof(type) != 1) && (sizeof(y)/sizeof(type) != sizeof(y))) \
                      { \
                          type arrayBuffer[sizeof(y)/sizeof(type)]; \
                          memcpy(&arrayBuffer,&y,sizeof(y)); \
                          printf("array >> "#type" : "); \
                          for(uint32_t i = 0 ; i<sizeof(y)/sizeof(type) ; i++ ) \
                          printf("%d - ",arrayBuffer[i]); \
                          printf("\n"); \
                      } \
                      else \
                      { \
                          type intBuffer = (type)*((type*)(&y)); \
                          printf("integer >> "#type" : %d\n",intBuffer); \
                      } \
                     }while(0)

int main()
{
    uint32_t i1 = 257;
    uint16_t i2 = 257;
    uint8_t  i3 = 25;
    uint32_t a1[]={42,43,44,257};
    uint16_t a2[]={22,23,24,257};
    uint8_t  a3[]={12,13,14,25};

    printf("MY_POLYMORPHIC_PRINT1 - VLA\n");
    MY_POLYMORPHIC_PRINT1(i1,uint32_t);// integer >> uint32_t : 257                                               >> OK                                                                                                     
    MY_POLYMORPHIC_PRINT1(i2,uint16_t);// integer >> uint16_t : 257                                               >> OK  
    MY_POLYMORPHIC_PRINT1(i3,uint8_t); // integer >> uint8_t : 25                                                 >> OK  
    MY_POLYMORPHIC_PRINT1(i1,uint8_t); // integer >> uint8_t : 1                                                  >> POK  wrong type, casting is working                                                                                        
    MY_POLYMORPHIC_PRINT1(a1,uint32_t);// array >> uint32_t : 42 - 43 - 44 - 257 -                                >> OK
    MY_POLYMORPHIC_PRINT1(a2,uint16_t);// array >> uint16_t : 22 - 23 - 24 - 257 -                                >> OK
    MY_POLYMORPHIC_PRINT1(a3,uint8_t); // integer >> uint8_t : 12                                                 >> FAILS
    MY_POLYMORPHIC_PRINT1(a1,uint16_t); // integer >> uint16_t : 42 - 0 - 43 - 0 - 44 - 0 - 257 - 0 -             >> POK  wrong type, casting is somewhat working

    return 0;
}

pros:

• flexible
• compatibility : C99, C11 (VLA option), ...
• just one (ugly) macro
• you don't need C++ library
• you don't need "isArray" anymore
• you don't need array size as an argument
• you don't need #if

cons:

• not type-safe --> gives some bad results
• not compatible before C99
• need type info

Using VLA + ## (string concatenation macro)

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define MY_VLA_PRINT_ARRAY(y, type) do{\
                          type arrayBuffer[sizeof(y)/sizeof(type)]; \
                          memcpy(&arrayBuffer,&y,sizeof(y)); \
                          printf("array >> "#type" : "); \
                          for(uint32_t i = 0 ; i<sizeof(y)/sizeof(type) ; i++ ) \
                          printf("%d - ",arrayBuffer[i]); \
                          printf("\n"); \
                          }while(0)

#define MY_VLA_PRINT_NOT_ARRAY(y, type) do{\
                          type intBuffer = (type)*((type*)(&y)); \
                          printf("integer >> "#type" : %d\n",intBuffer); \
                          }while(0)

#define MY_POLYMORPHIC_PRINT3( y, isArray, type) do { \
                      MY_VLA_PRINT_ ## isArray (y, type); \
                     }while(0)

int main()
{
    uint32_t i1 = 257;
    uint16_t i2 = 257;
    uint8_t  i3 = 25;
    uint32_t a1[]={42,43,44,257};
    uint16_t a2[]={22,23,24,257};
    uint8_t  a3[]={12,13,14,25};


    printf("MY_POLYMORPHIC_PRINT3 -  ## + VLA\n");
    MY_POLYMORPHIC_PRINT3(i1,NOT_ARRAY,uint32_t); // integer >> uint32_t : 257                                    >> OK                                                                                                     
    MY_POLYMORPHIC_PRINT3(i2,NOT_ARRAY,uint16_t); // integer >> uint16_t : 257                                    >> OK  
    MY_POLYMORPHIC_PRINT3(i3,NOT_ARRAY,uint8_t);  // integer >> uint8_t : 25                                      >> OK  
    MY_POLYMORPHIC_PRINT3(i1,NOT_ARRAY,uint8_t);  // integer >> uint8_t : 1                                       >> POK  wrong type, casting is working                                                                                        
    MY_POLYMORPHIC_PRINT3(i1,ARRAY,uint8_t);      // array >> uint8_t : 1 - 1 - 0 - 0 -                           >> POK (does not crash)                                                                                       
    MY_POLYMORPHIC_PRINT3(a1,ARRAY,uint32_t);     // array >> uint32_t : 42 - 43 - 44 - 257 -                     >> OK
    MY_POLYMORPHIC_PRINT3(a2,ARRAY,uint16_t);     // array >> uint16_t : 22 - 23 - 24 - 257 -                     >> OK
    MY_POLYMORPHIC_PRINT3(a3,ARRAY,uint8_t);      // array >> uint8_t : 12 - 13 - 14 - 25 -                       >> OK
    MY_POLYMORPHIC_PRINT3(a1,ARRAY,uint16_t);     // array >> uint16_t : 42 - 0 - 43 - 0 - 44 - 0 - 257 - 0 -     >> POK  wrong type, casting is somewhat working
    MY_POLYMORPHIC_PRINT3(a1,NOT_ARRAY,uint16_t); // integer >> uint16_t : 42                                     >> POK (does not crash)

    return 0;
}

pros:

• only solution that works for all test cases
• quite simple macros
• you don't need C++ library
• you don't need #if

cons:

• not compatible before C99
• need isArray
• need type info
• is not type-safe --> but did not give some too bad results

macro ## without VLA:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define MY_NON_VLA_PRINT_ARRAY_U32(y) do{\
                          printf("array >> uint32_t : "); \
                          for(uint32_t i = 0 ; i< sizeof(y) / sizeof(uint32_t)  ; i++ ) \
                            printf("%d - ",(uint32_t)*((uint32_t*)&y+(uint32_t*)i)); \
                          printf("\n"); \
                          }while(0)
#define MY_NON_VLA_PRINT_ARRAY_U16(y) do{\
                          printf("array >> uint16_t : "); \
                          for(uint32_t i = 0 ; i< sizeof(y) / sizeof(uint16_t)  ; i++ ) \
                            printf("%d - ",(uint16_t)*((uint16_t*)&y+(uint16_t*)i)); \
                          printf("\n"); \
                          }while(0)
#define MY_NON_VLA_PRINT_ARRAY_U8(y) do{\
                          printf("array >> uint8_t : "); \
                          for(uint32_t i = 0 ; i< sizeof(y) / sizeof(uint8_t)  ; i++ ) \
                            printf("%d - ",(uint8_t)*((uint8_t*)&y+(uint8_t*)i)); \
                          printf("\n"); \
                          }while(0)
#define MY_NON_VLA_PRINT_NOT_ARRAY_U32(y) do{\
                          printf("integer >> uint32_t : %d\n",(uint32_t)y); \
                          }while(0)
#define MY_NON_VLA_PRINT_NOT_ARRAY_U16(y) do{\
                          printf("integer >> uint16_t : %d\n",(uint16_t)y); \
                          }while(0)
#define MY_NON_VLA_PRINT_NOT_ARRAY_U8(y) do{\
                          printf("integer >> uint8_t : %d\n",(uint8_t)y); \
                          }while(0)


#define MY_POLYMORPHIC_PRINT4( y, isArray, type) do { \
                      MY_NON_VLA_PRINT_ ## isArray ## _ ## type (y); \
                     }while(0)


int main()
{
    uint32_t i1 = 257;
    uint16_t i2 = 257;
    uint8_t  i3 = 25;
    uint32_t a1[]={42,43,44,257};
    uint16_t a2[]={22,23,24,257};
    uint8_t  a3[]={12,13,14,25};


    printf("MY_POLYMORPHIC_PRINT4 -  ## + no VLA\n");
    MY_POLYMORPHIC_PRINT4(i1,NOT_ARRAY,U32);                                                                                         
    MY_POLYMORPHIC_PRINT4(i2,NOT_ARRAY,U16);      
    MY_POLYMORPHIC_PRINT4(i3,NOT_ARRAY,U8);       
    MY_POLYMORPHIC_PRINT4(i1,NOT_ARRAY,U8);                                                                                      
    MY_POLYMORPHIC_PRINT4(i1,ARRAY,U8);                                                                          
    MY_POLYMORPHIC_PRINT4(a1,ARRAY,U32);          
    MY_POLYMORPHIC_PRINT4(a2,ARRAY,U16);          
    MY_POLYMORPHIC_PRINT4(a3,ARRAY,U8);           
    MY_POLYMORPHIC_PRINT4(a1,ARRAY,U16);          
    MY_POLYMORPHIC_PRINT4(a1,NOT_ARRAY,U16);      

    //does not compile:
    // error: invalid operands to binary + (have ‘uint32_t * {aka unsigned int *}’ and ‘uint32_t * {aka unsigned int *}’)
    //                        printf("%d - ",(uint32_t)*((uint32_t*)&y+(uint32_t*)i)); \
    // error: invalid operands to binary + (have ‘uint16_t * {aka  short unsigned int *}’ and ‘uint16_t * {aka short unsigned int *}’)
    //                        printf("%d - ",(uint16_t)*((uint16_t*)&y+(uint16_t*)i)); \
    // error: invalid operands to binary + (have ‘uint8_t * {aka unsigned char *}’ and ‘uint8_t * {aka unsigned char *}’)
    //                        printf("%d - ",(uint8_t)*((uint8_t*)&y+(uint8_t*)i)); \

    return 0;
}

pros:

• would be compatible before C99
• you don't need C++ library
• you don't need #if

cons:

• does not compile!
• a lot of macros
• need isArray
• need type info
• is not type-safe

Answer 3

Let me first say that I don't think you should use macro for this. You should have 2 separate functions instead, with possibly additional _Generic expression as shown in Lundins answer.

However, it is possible to do with multiple macro defines:

#define MY_MACRO(isArray,y) MY_MACRO_ ## isArray (y)
#define MY_MACRO_0(y) printf("%d", y)
#define MY_MACRO_1(y) printf("%d", y[0])

Answer 4

You can use BOOST_PP_IF:

#include <boost/preprocessor/if.hpp>

#define MY_MACRO1(isArray,y) printf("%d", BOOST_PP_IF(isArray, (y)[0], (y)))

See it live on Coliru