wiiki - CallByValueResult

A ParameterPassing mode, used in AdaLanguage to handle "IN OUT" parameters. In CallByValueResult, the actual parameter supplied by the caller is copied into the callee's formal parameter; the function is run; and the (possibly modified) formal parameter is then copied back to the caller. This allows a function to modify the state of its caller, similar to what you get with CallByReference. The (semantic) differences between CallByReference and CallByValueResult are:

No alias is created between the formal and actual parameters. If lexical scoping is used, the difference can be apparent. Consider the following snippets in a mythical C/Ada-ish hybrid (LexicalScoping is present):

void outer (void) 
{
     int a = 5, b = 7;

     void inner (IN OUT int c; REF int d)
     {
          printf ("a: %d b: %d c: %d d: %d\n", a,b,c,d);
          a = 0; b = 9; c = 4; d = 6; 
          printf ("a: %d b: %d c: %d d: %d\n", a,b,c,d);
     },

     inner(a,b);
     printf ("a: %d b: %d\n",a,b);
},

The results of this should be

a: 5 b: 7 c: 5 d: 7
a: 0 b: 6 c: 4 d: 6  // d and b are aliases, so b=9 got clobbered
a: 4 b: 6            // a replaced with value of c on function exit.

Some languages allow CallByReference as an optimization of CallByValueResult if it can be shown that the two (due to lack of aliasing, etc.) are semantically equivalent.

CategoryLanguageFeature