Constant Expressions
Constant term defines an expression that can be evaluated at compile time.
Preamble:
A constant expression is an expression that contains only constants (this means that it does not contain any variables at the evaluation time) with some operators/functions allowed, and possibly parentheses to override the operator precedence order.
A constant expression can be evaluated during compilation rather than at run time, and can be used in any place that a constant can occur.
Constant expressions are mainly required:
- as arguments for the preprocessor directives and intrinsic defines macros,
- as initializers for the Enum/Type structures and global/static variables,
- as data constants for the Data Statements.
List of operators/functions allowed in constant expressions
fbc is not a scripting language and does not have a full feature macro processor on the front end.
Constant folding is an optimization in fbc that takes constant expressions, evaluates the constant expression at compile time and produces a single constant result.
For constant folding to work, a compile time implementation of the operators/functions must be written.
Only the following operators/functions with constant arguments can be used to constitute constant expressions:
`Bit, BitReset, BitSet, HiByte, HiWord, LoByte, LoWord
+, -, *, /, \, ^, Mod, Shl, Shr
=, <>, <=, >=, <, >
And, Eqv, Imp, Or, Not, Xor
Abs, Acos, Asin, Atan2, Atn, Cos, Exp, Fix, Frac, Int, Log, Sgn, Sin, Sqr, Tan
Asc, Chr, CVD, CVI, CVL, CVLongInt, CVS, CVShort, Len, SizeOf
Cast, CBool, CByte, CDbl, CInt, CLng, CLngInt, CShort, CSign, CSng, CUByte, CUInt, CULng, CULngInt, CUnsg, CUShort
more obviously all predefined symbols`
Basic example of using constant expressions
Example illustrating three different use-cases:
`
#define pi 4 * Atn(1)
Dim Shared As Double d = Sqr(2)
Type pt
Dim As Integer x = 300 * Cos(pi / 6)
Dim As Integer y = 300 * Sin(pi / 6)
End Type
Dim As pt p
Print pi '' 3.14159...
Print d '' 1.41421...
Print p.x, p.y '' 260 150
Sleep
`
See also

- Differences between Expressions and Statements
- Variable Initializers