GPC MANUAL: Schemata, Pointer arithmetics

-- Peter Gerwinski, Essen, Germany, http://home.pages.de/~Peter.Gerwinski/ Maintainer GNU Pascal - http://home.pages.de/~GNU-Pascal/ - gpc-980511 PGP key on request - 6C 94 45 BE 28 A4 96 - 0E CC E9 12 47 25 82 75 Fight the SPAM and UBE! - http://spam.abuse.net/ - http://maps.vix.com/ @c ============================================================================ @node Schemata @section Schema Types @cindex schemata @cindex types, schema Schemata are types that depend on a variable, called ``discriminant''. They are an ISO-10206 Extended Pascal extension. @smallexample Type RealArray ( n: Integer ) = array [ 1..n ] of Real; @end smallexample The type @samp{RealArray} in this example is called a Schema with the discriminant @samp{n}. To declare a variable of such a type, write @cindex Foo @smallexample Var Foo: RealArray ( 42 ); @end smallexample Schema-typed variables ``know'' about their discriminants. They can be accessed just like record fields: @smallexample writeln ( Foo.n ); (* yields 42 *) @end smallexample While types of variables must always have specified discriminants (which may be other variables), @emph{pointers} to schema variables may be without a discriminant: @cindex Bar @smallexample Type RealArrayPtr = ^RealArray; Var Bar: RealArrayPtr; @end smallexample When applying @samp{New} to such a pointer, you must specify the intended value of the discriminant as a parameter: @smallexample New ( Bar, 137 ); @end smallexample As a GNU extension, the above can also be written as @smallexample Bar:= New ( RealArrayPtr, 137 ); @end smallexample Schemata are not limited to arrays. They can be of any type that normally requires constant values in its definition, for instance subrange types, or records containing arrays etc. (Sets do not yet work.) To finish this section, here is a somewhat exotic example: @smallexample Type ColorType = ( red, green, blue ); ColoredInteger ( Color: ColorType ) = Integer; @end smallexample A @samp{ColoredInteger} behaves just like an ordinary integer, but it has an additional property @samp{Color} which can be accessed like a record field. @smallexample Var Foo: ColoredInteger ( green ); [...] Foo:= 7; if Foo.Color = red then inc ( Foo, 2 ) else Foo:= Foo div 3; @end smallexample @c ============================================================================ @node Pointer arithmetics @section Pointer Arithmetics @cindex pointer arithmetics GPC allows to increment, decrement, compare, and subtract pointers or to use them in @samp{for} loops just like the C language. @smallexample Var A: array [ 1..7 ] of Char; p, q: ^Char; i: Integer; [...] for p:= @@A [ 1 ] to @@A [ 7 ] do p^:= 'x'; q:= @@A [ 3 ]; while p > q do begin p^:= 'y'; dec ( p ); end (* while *); i:= q - p; @end smallexample Incrementing a pointer by one means to increment the address it contains by the size of the variable it is pointing to. For typeless pointers (@samp{Pointer}), the address is incremented by one instead. Similar things hold when decrementing a pointer. Subtracting two pointers yields the number of variables pointed to between both pointers, i.e.@: the difference of the addresses divided by the size of the variables pointed to. The pointers must be of the same type. @c ============================================================================ @node Type casts @section Type Casts @cindex type casts In some cases, especially when interfacing with other languages, Pascal's strong typing can be an obstacle. To temporarily circumvent this, GPC (and other Pascal compilers) defines explicit ``type casts''. To convert a value of one data type to another type, use the target type as a ``function'': @cindex Foo @cindex Bar @smallexample Type CharPtr = ^Char; CharArray = array [ 0..99 ] of Char; CharArrayPtr = ^CharArray; Var Foo: CharPtr; Bar: CharArrayPtr; [...] Foo:= CharPtr ( Bar ); @end smallexample Beware: Explicit type casts might have unexpected effects on different platforms since you cannot rely on a specific way the data is stored. If it is possible to avoid type casts (which holds for most cases), you should do that. For instance in the example above, you can make @samp{Foo} point to the first char in the array @samp{Bar^}: @smallexample Foo:= @@Bar^ [ 0 ]; @end smallexample When dealing with objects (see @ref{OOP}), it is often necessary---and safe---to cast a pointer to an object to a pointer of a ``bigger'' (derivated) object. In future releases, GPC will provide an operator @samp{as} for a safer approach to this problem. See also: @ref{absolute}. @c ============================================================================