Pascal - Variants

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Category: Chapter 4

Published: Monday, 15 April 2013 13:58

Written by Sternas Stefanos

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Pascal supports a unique type of storage named variants. You can assign any simple type of values in a variant variable. The type of a value stored in a variant is only determined at runtime. Almost any simple type can be assigned to variants: ordinal types, string types, int64 types.

Structured types such as sets, records, arrays, files, objects and classes are not assignment-compatible with a variant. You can also assign a pointer to a variant.

Free Pascal supports variants.

 

1. Declaring a Variant

You can declare variant type like any other types using the var keyword. The syntax for declaring a variant type is:

var
   v: variant;
 

Now this variant variable v can be assigned to almost all simple types including the enumerated types and vice versa.

type  
   color =(red, black, white);  
var  
   v : variant;  
   i : integer;  
   b :byte;  
   w : word;  
   q : int64;  
   e : extended;  
   d :double;  
   en : color;  
   as: ansistring;  
   ws : widestring;  
begin  
   v := i;  
   v := b;  
   v := w;  
   v := q;  
   v := e;  
   v := en;  
   v := d:  
   v :=as;  
   v := ws;  
end;
 

 

2. Example:

The following example would illustrate the concept:

Program exVariant;
uses variants;
type
   color =(red, black, white);
var
   v : variant;
   i : integer;
   r: real;
   c : color;
   as: ansistring;
begin
   i :=100;
   v:= i;
   writeln('Variant as Integer: ', v);
   r:=234.345;
   v:= r;
   writeln('Variant as real: ', v);
   c := red;
   v := c;
   writeln('Variant as Enumerated data: ', v);
   as:=' I am an AnsiString';
   v:=as;
   writeln('Variant as AnsiString: ', v);
 end.
 

When the above code is compiled and executed, it produces following result:

Variant as Integer: 100
Variant as real: 234.345
Variant as Enumerated data: 0
Variant as AnsiString: I am an AnsiString