BYVAL BYREF BYCOPY

[Mike Trader]

Im trying to make a function that can handle LONGs and SINGLEs as inputs.

I am confused about my declarations:
FUNCTION MaxList( Var1 AS SINGLE, Var2 AS SINGLE ) AS SINGLE
blah
Blah
blah
END FUNCTION

even tho I have declared Var1 and Var2 as SINGLEs I want to be able to call the function with LONGs ie

Global i AS LONG, J AS LONG, x AS SINGLE
x = Maxlist( i, j )

is this BYVAL or BYCOPY?


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Kind Regards
Mike

[Lance Edmonds]

When you pass a LONG value as parameter to sub/function that expects a BYREF SINGLE, you'll have to use BYCOPY - the compiler will then do an implicit type conversion and passes the result BYCOPY .

That is, it has to be evaluated as an expression, which BYCOPY will effectively do for you.

You can force this two ways, thus:

FUNCTION MaxList( Var1 AS SINGLE, Var2 AS SINGLE ) AS SINGLE
    FUNCTION = var1 + var2
END FUNCTION
 
FUNCTION PBMAIN
    DIM a AS LONG, b AS LONG, Result!
    a = 10 : b = 20
    Result! = MaxList(BYCOPY a,BYCOPY b)
    Result! = MaxList((a),(b))
END FUNCTION

If you want the value back from the function (because the function is expecting BYREF parameters), you'll have to pass a SINGLE so it gets passed BYREF instead of BYCOPY.

You can override the type checking by using BYVAL VARPTR(lngvar&) where lngvar& is a memory variable, but since the function expects the 32-bit value stored in the 32-bits occupied by lngvar& to actually be a SINGLE value, the results will likely be unpredictable, since the internal format of a LONG variable and the internal format of a SINGLE are quite different.

If you try to use BYVAL VARPTR(lngVal&), you'll get unpredictable results at best. If you use BYVAL lngval& you almost certainly get a GPF.

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Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Mike Trader]

Thx Lance,

That makes it clear.

How does a conversion for a SINGLE to a LONG take place internally? Is it truncation? or the same as INT()


------------------
Kind Regards
Mike

[Lance Edmonds]

It uses CSNG().

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Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Tom Hanlin]

Err, CLNG.

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Tom Hanlin
PowerBASIC Staff

[Lance Edmonds]

Oops, good catch! CSNG() to produce a Single, CLNG() to produce a Long.

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Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>