Ordinary constants

2.1 Ordinary constants

Ordinary constants declarations are constructed using an identifier name followed by an ”=” token, and followed by an optional expression consisting of legal combinations of numbers, characters, boolean values or enumerated values as appropriate. The following syntax diagram shows how to construct a legal declaration of an ordinary constant.

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Constant declaration

-- -- - - - - ----------
constant declaration -identifier--=---expression--hintdirectives--;-|
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The compiler must be able to evaluate the expression in a constant declaration at compile time. This means that most of the functions in the Run-Time library cannot be used in a constant declaration. Operators such as +, -, *, /, not, and, or, div, mod, ord, chr, sizeof, pi, int, trunc, round, frac, odd can be used, however. For more information on expressions, see chapter 12, page 534.

When a previously declared ordinary constant is used in the code, the compiler will insert the actual value of the constant instead of the constant name. That is, the following 2 pieces of code are entirely equivalent:

Const
One = 1;

begin
Writeln(One);
end.

The above will produce the same code as if one had written:

begin
Writeln(1);
end.

Only constants of the following types can be declared:

Ordinal types
Set types
Pointer types (but the only allowed value is Nil).
Real types
Char,
String

The following are all valid constant declarations:

Const
  e = 2.7182818;  { Real type constant. }
  a = 2;          { Ordinal (Integer) type constant. }
  c = ’4’;        { Character type constant. }
  s = ’This is a constant string’; {String type constant.}
  sc = chr(32)
  ls = SizeOf(Longint);
  P = Nil;
  Ss = [1,2];

Assigning a value to an ordinary constant is not permitted. Thus, given the previous declaration, the following will result in a compiler error:

s := ’some other string’;

For string constants, the type of the string is dependent on some compiler switches. If a specific type is desired, a typed constant should be used, as explained in the following section.

Prior to version 1.9, Free Pascal did not correctly support 64-bit constants. As of version 1.9, 64-bit constants can be specified.

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