false and true

│ English (en) │

The constants false and true are used to define the false and true conditions of a boolean variable. They are manifest constants that are defined as part of the standard data types the compiler initially knows about.

These constant values must be predefined by the compiler as there is no way to define them in terms of anything else. They are defined via compiler/psystem.pas as part of the system unit.

As of FPC 3.0.0 false and true are no longer reserved words. Thus the following program is valid, compiles and is “usable”:

program falseAndTrue(input, output, stderr);

const
	true = 42;

begin
	writeLn(true);                 // prints 42
	//writeLn(true and false);     // does not compile
	writeLn(system.true and false) // prints FALSE
end.

Internal value

program falseDemo(input, output, stderr);

uses
	typInfo;

begin
	writeLn(false);                            // prints FALSE
	
	// enumerative actions ------------------------------------------
	writeLn(ord(false));                       // prints 0
	writeLn(succ(false));                      // prints TRUE
	// next two statements generate out-of-range compile-time warnings
	writeLn(pred(false));                      // prints TRUE
	writeLn(succ(succ(false)));                // prints TRUE
	
	// data type ----------------------------------------------------
	writeLn(sizeOf(false));                    // prints 1
	writeLn(bitSizeOf(false));                 // prints 8
	writeLn(PTypeInfo(typeInfo(false))^.kind); // prints tkBool
	writeLn(PTypeInfo(typeInfo(false))^.name); // prints Boolean
end.

When typecasting or interpreting any numeric value as a boolean value, it is important to know, that any non-zero value means true whilst only 0 (zero) is false.

Confer ISO 7185 § “Required simple-types”.

See also

• Boolean