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.

Constant declaration

--                --       -   -         -          -  ----------
  constant declaration -identifier--=---expression--hintdirectives--;-|

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:

  One = 1;  

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


Only constants of the following types can be declared:

The following are all valid constant declarations:

  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.