问题
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.
回答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;
}
回答2:
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
回答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])
回答4:
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
来源：https://stackoverflow.com/questions/60636573/how-can-i-avoid-the-use-of-if-in-a-polymorphic-print-macro