90 of 208
ZBasic Language Reference 82 ZX Microcontroller Family Dim i as Integer Sub Main() Call foo(5) ' this call is unambiguous Call foo(i) ' this call is ambiguous Call foo("abc") ' this call is unambiguous End Sub In the example above, for the first invocation of foo() in Main() the compiler determines that the first overload of foo() having the ByVal parameter is the only one whose signature is compatible with the invocation. This is not ambiguous because the constant value cannot be passed by reference so the overload of foo() with the ByRef parameter cannot possibly be used. In contrast, for the second invocation of foo(), the first two overloads are both compatible resulting in an ambiguous reference. 3.20 Aliases Occasionally, it is useful to be able to access a variable or parts of a variable as different types at different times. Although this can be accomplished by using the System Library routines BlockMove() or RamPeek()/RamPoke() it is simpler and more efficient to use the concept of an alias. Simply stated, defining an alias tells the compiler to generate code to access a variable or part of a variable as if it were a different type. To be clear, no new data space is allocated by defining an alias. It simply provides a different way of accessing previously defined space. The syntax for defining an alias is similar to that for defining a variable. For example, the syntax for defining an alias at the module level is shown below. {Public | Private | Dim} <name>[(<dim-list>)] As <type> Alias <var-ref> As with normal variables, Dim has exactly the same effect as Private. Within a subroutine, a function or any block structure, a local alias may be defined using the syntax shown below. Dim <name>[(<dim-list>)] As <type> Alias <var-ref> In both cases the <var-ref> element is the name of a RAM variable or the name of another alias optionally including a parenthesized set of one or more constant index expressions. The parenthesized index list is only allowed, of course, if the referent item is an array. Examples Dim ival as Integer, fval as Single Dim buf(1 to 20) as Byte Dim b As Byte Alias fval Dim c As Byte Alias buf(2) Dim c2(1 to 3) As Byte Alias buf(3) Dim bval(1 to 5) As Byte Alias ival The first alias definition allows you to read/write the least significant byte of the Single value fval. The second definition allows direct access to the second byte of the buf variable. The third example shows how to define a sub-array within an array. The fourth example shows an alias being defined that spans more than one variable. Although the compiler allows this form its use is discouraged because the effect depends on the order in which the compiler chooses to allocate data items. Recursive alias definitions are not allowed; an error message will be issued by the compiler when a recursive definition is detected. Note that is not allowed to define an alias that is a String type. You may, however, define an alias that overlays a String variable although this is not often useful.