How to declare a C function with an undetermined return type? - objective-c

Question

Can I declare a C function with an undetermined return type (without C compiler warning)? The return type could be int, float, double, void *, etc.
undetermined_return_type miscellaneousFunction(undetermined_return_type inputValue);
And you can use this function in other functions to return a value (although that could be a run time error):
BOOL isHappy(int feel){
return miscellaneousFunction(feel);
};
float percentage(float sales){
return miscellaneousFunction(sales);
};
What I'm looking for:
To declare and to implement a C function (or Obj-C method) with an undefined-return-type could be useful for aspect-oriented programming.
If I could intercept Obj-C messages in another function in run time, I might return the value of that message to the original receiver or not with doing something else action. For example:
- (unknown_return_type) interceptMessage:(unknown_return_type retValOfMessage){
// I may print the value here
// No idea how to print the retValOfMessage (I mark the code with %???)
print ("The message has been intercepted, and the return value of the message is %???", retValOfMessage);
// Or do something you want (e.g. lock/unlock, database open/close, and so on).
// And you might modify the retValOfMessage before returning.
return retValOfMessage;
}
So I can intercept the original message with a little addition:
// Original Method
- (int) isHappy{
return [self calculateHowHappyNow];
}
// With Interception
- (int) isHappy{
// This would print the information on the console.
return [self interceptMessage:[self calculateHowHappyNow]];
}

Answer 1

You can use a void * type.
Then for example:
float percentage(float sales){
return *(float *) miscellaneousFunction(sales);
}
Be sure not to return a pointer to a object with automatic storage duration.

Answer 2

You may use the preprocessor.
#include <stdio.h>
#define FUNC(return_type, name, arg) \
return_type name(return_type arg) \
{ \
return miscellaneousFunction(arg); \
}
FUNC(float, undefined_return_func, arg)
int main(int argc, char *argv[])
{
printf("\n %f \n", undefined_return_func(3.14159));
return 0;
}

Answer 3

May be a union as suggested by thejh
typedef struct
{
enum {
INT,
FLOAT,
DOUBLE
} ret_type;
union
{
double d;
float f;
int i;
} ret_val;
} any_type;
any_type miscellaneousFunction(any_type inputValue) {/*return inputValue;*/}
any_type isHappy(any_type feel){
return miscellaneousFunction(feel);
}
any_type percentage(any_type sales){
return miscellaneousFunction(sales);
}
Here with ret_type you can know data type of return value and ret_type. i,f,d can give you corresponding value.
All elements will use same memory space and only one should be accessed.

Answer 4

Straight C doesn't support dynamically-typed variables (variants) since it is statically typed, but there might be some libraries that do what you want.