Passing+substructures+to+procedures

//by Richard Russell, August 2006//

As described in the [|main documentation] you can pass entire structures to a procedure or function as follows:

code format="bb4w" DIM object{name$,pos{x,y,z}} object.pos.x = 1.0 object.pos.y = 2.0 object.pos.z = 3.0 PRINT FN_distance(object{}) END

DEF FN_distance(s{}) = SQR(s.pos.x^2 + s.pos.y^2 + s.pos.z^2) code However you //cannot// pass an entire **substructure** to a procedure or function. The following code doesn't work (it will crash BBC BASIC):

code format="bb4w" REM This doesn't work - don't try it at home! DIM object{name$,pos{x,y,z}} object.pos.x = 1.0 object.pos.y = 2.0 object.pos.z = 3.0 PRINT FN_distance(object.pos{}) END

DEF FN_distance(p{}) = SQR(p.x^2 + p.y^2 + p.z^2) code In most cases passing the parent structure instead, as in the first example, isn't a major inconvenience, however it does mean that the substructure //must always have the same name// (in this case **pos{}**).

It would be desirable to be able to use a common **FN_distance** function (for example) even if the substructure doesn't always have the same name; in particular this would make it easier to incorporate the function in a library. There is a way of achieving this, which involves passing to the procedure or function //both// the parent structure //and// the substructure. Here's how you would do it for this particular example:

code format="bb4w" DIM object{name$,pos{x,y,z}} object.pos.x = 1.0 object.pos.y = 2.0 object.pos.z = 3.0 PRINT FN_distance(object{},object.pos{}) END DEF FN_distance(s{},p{}) !(^p{}+4) += !(^s{}+4) = SQR(p.x^2 + p.y^2 + p.z^2) code Note the manipulation of memory in **FN_distance** which makes this work.

Now you //can// use the same function with a differently-named substructure:

code format="bb4w" DIM newobj{type%,loc{x,y,z}} newobj.loc.x = 4.0 newobj.loc.y = 5.0 newobj.loc.z = 6.0 PRINT FN_distance(newobj{},newobj.loc{}) END code