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--- multiple_return_values [2017/03/17 15:45] – created korshun
+++ multiple_return_values [2019/04/21 21:32] (current) – korshun
@@ Line 1: / Line 1: @@
 ====== Multiple return values ======
-acsutils uses a simple convention for functions returning multiple values, like [[functions:RotateVector]]:
+Some ACSUtils functions return multiple values (e.g. [[math#vector_math|vector math functions]], but also some non-math functions). ACS doesn't support returning multiple values from a function.
-There are global variables r1 through r8. The functions write their return values to them.
+ACSUtils uses a convention to fake returning multiple values from a function. The convention is as follows: there are 8 global variables with names ''r1'' through ''r8''. If a function returns multiple values, it actually returns no value and writes the first "returned" value to ''r1'', the second one to ''r2'', and so on.
-    RotateVector(x, y, angle); // Call the function
+Here's an example, ''RotateVector'' returns X and Y:
-    int newX = r1; // Get the first returned value
-    int newY = r2; // And the second one
+<code>
+RotateVector(x, y, angle);
+int newX = r1;
+int newY = r2;
+</code>
+<note warning>
+You should retrieve the return values **as soon as possible**, or else they may get overwritten by other functions that return multiple values.
+Don't do this:
+<code>
+SomeFunctionWithMultipleReturnValues(...);
+SomeOtherFunction(r1);
+int b = r2;
+</code>
+Because ''SomeOtherFunction'' may call functions that **overwrite ''r2''**! Instead, strictly follow this style:
+<code>
+SomeFunctionWithMultipleReturnValues(...);
+int a = r1;
+int b = r2;
+SomeOtherFunction(a);
+</code>
+</note>
+<note important>Another shortcoming is that ''r1''..''r8'' are declared ''raw'' in BCC to be able to accomodate any type of return value, and thus bypass [[strict typing]], even if it's enabled.
+<code>
+ParseFixed("1.0", 10);
+int error = r1;
+int result = r2; // no compiler error, despite assigning a fixed to int
+</code>
+</note>
+But overwriting argument variables immediately after calling the function is valid:
+<code>
+RotateVector(x, y, angle);
+x = r1;
+y = r2;
+</code>
+An example of a function returning three values:
+<code>
+normalize3d(x, y, z);
+int newX = r1;
+int newY = r2;
+int newZ = r3;
+</code>
-<note warning>You should retrieve the return values **as soon as possible**, or else they may get overwritten by other functions that return multiple values.</note>

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