__FB_EVAL__

Intrinsic define (macro) performed by the compiler.

Syntax
   __FB_EVAL__( arg )

Parameters
   arg
      argument

Description
   Evaluates the argument (constant-expression) at compile time.

   When the argument evaluation produces a string, __FB_EVAL__ returns a 
   string formatted with a preprocessor operator:
      - a Non-Escaped String Literal (of form: $"text"),
      - or an Escaped String Literal (of form: !"text") if needed.
   For other datatypes produced, simple Literals (of Integers, Floating 
   Points, Booleans) without prefix/suffix are returned.

   __FB_EVAL__ macro is useful where there is any of the following:
      - non functional expression (i.e. side-effects),
      - needs to be evaluated (i.e. simplified, respecting the operator 
      precedence) before passing it on,
      - used in a place where fbc would not allow an expression.

Example
   #print 1 + 2
   #print __FB_EVAL__( 1 + 2 )
   #print 4 * Atn(1)
   #print __FB_EVAL__( 4 * Atn(1) )

   /' Compiler output:
   1 + 2
   3
   4 * Atn(1)
   3.141592653589793
   '/
      

   '   In this example, the three '__FB_EVAL__' are absolutely mandatory in this 'assign()' macro.
   '   Even for '__FB_QUOTE__( __FB_EVAL__( expr ) )', because for the case of expr = cos(1/x),
   '   'cos(1/x)' must be properly evaluated before be quoted (after the previous 'assign("x", x+1)'),
   '   otherwise in that case 'cos(1/x+1)' is taken into account (giving 'cos(2)') instead of 'cos(1/(x+1))' (giving 'cos (1/2)')
   '   because the operator precedence is not applied by '__FB_QUOTE__'.

   #macro assign( sym, expr )
      __FB_UNQUOTE__( __FB_EVAL__( "#undef " + sym ) )
      __FB_UNQUOTE__( __FB_EVAL__( "#define " + sym + " " + __FB_QUOTE__( __FB_EVAL__( expr ) ) ) )
   #endmacro

   #define x

   assign( "x", 1 )
   Print x

   assign( "x", x+1 )
   Print x

   assign( "x", Cos(1/x) )
   Print x

   assign( "x", "hello" )
   Print x

   assign( "x", x+x )
   Print x

   /' Output:
    1
    2
    0.877582...
   hello
   hellohello
   '/
      

   See also __FB_ARG_EXTRACT__ example.

Version
   * Since fbc 1.08.0

Differences from QB
   * New to FreeBASIC

See also
   * __FB_QUOTE__
   * __FB_UNQUOTE__

