[aspectc-user] Around advice and return values

[Olaf Spinczyk]

Hello,

Loesch Felix (CR/AEF4) wrote:
> Hello,
> 
> I tried to use around advice with return values with AspectC++, but I
> was unable to use return values inside
> advice code.
> 
> I tried the following example code but always got the following error of
> the AspectC++ compiler:
> 
> --- error begin ---
> 
> ../MinValue.ah: In member function `void MinValue::__a0_around(int, int,
> int)':
> ../MinValue.ah:9: error: return-statement with a value, in function
> declared 
> 
> --- error end ----
> 
> 
> ---- begin main.cpp ----
> #include <stdio.h>
> using namespace std;
> 
> // compute max value of i, j, k
> int REF(int i, int j, int k) {
>     int max = 0;
>     max = i;
>     if (j > max) { max = j; }
>     if (k > max) { max = k; }
>     return max;
> }
> 
> int main() {
>    printf("Reference value: %d", REF(3,5,4));
> } 
> --- end main.cpp---
> 
> MinValue.ah
> 
> --- begin MinValue.ah ----
> #include <stdio.h>
> 
> #include <stdio.h>
> 
> aspect MinValue {
> 	advice call("int REF(...)") && args(i,j,k) : int around(int
> i,int j,int k) {
> 		int min = 0;
> 		min = i;
> 		if (j < min) { min = j; }
> 		if (k < min) { min = k; }
> 		return min;
> 	}
> };
> 
> --- end MinValue.ah --- 
> 
> - Why does AspectC++ declare the inlined function as "void"? 
> - Why does AspectC++ not support return types of functions that are
> replaced by "around" advice?
> - Is there a workaround to support return statements in advice code?
> 
> 
> --
> Felix Loesch

AspectC++ supports the manipulation of the result value like this:

aspect MinValue {
	advice call("int REF(...)") && args(i,j,k) :
	around(int i,int j,int k) { // don't write *int* around here!
		int min = 0;
		min = i;
		if (j < min) { min = j; }
		if (k < min) { min = k; }
		*tjp->result () = min;
		// no return statement!
	}
};

Two additional remarks:

* an aspect that transforms a max function into a min function is
   critical, because it dramatically changes the semantics of the
   component code. It is not an example for best practices ;-).

* the advice code is normally inlined. Therefore, larger advice bodies
   should better be moved into a separate function, which is not inlined
   and called be the advice:

   aspect MinValue {
	int min (int i, int j, int k); // defined somewhere else
	advice call("int REF(...)") && args(i,j,k) :
	around(int i,int j,int k) { // don't write *int* around here!
		*tjp->result () = min (i, j, k);
		// no return statement!
	}
   };

Best regards,

Olaf