Add OFFSETOF keyword for BASIC

Author: totalspectrum

Changelog.txt

@@ -1,3 +1,6 @@
+Version 7.6.8
+- Add OFFSETOF to BASIC
+
 Version 7.6.7
 - Add OFFSETOF implementation
 - Fix a Windows crash

doc/basic.md

@@ -301,6 +301,7 @@ next
 new
 nil
 not
+offsetof
 open
 option
 or
@@ -2900,6 +2901,24 @@ Convert `x` into a string with `d` digits in base `base`. If `x` is too big to f
 ```
 Returns a string representing the unsigned integer `x` in base 8. Only the lowest `n` digits of the representation are included. If `n` is omitted or is 0 then the returned string is the minimum length needed to represent the unsigned value.
 
+### OFFSETOF
+
+```
+n = offsetof(typename, fieldname)
+```
+Returns the byte offset of the field `fieldname` within the class, struct, or union type `typename`. `typename` must name a type (not a variable), and `fieldname` must be a plain identifier naming a member of that type. The result is a compile-time integer constant suitable for use anywhere an integer is expected.
+
+For example:
+```
+class point
+  dim as integer x
+  dim as integer y
+end class
+
+print offsetof(point, y)
+```
+will print 4.
+
 ### ON X GOTO
 
 For compatibility only, FlexBASIC accepts statements like:

frontends/basic/basic.y

@@ -476,6 +476,7 @@ AdjustParamForByVal(AST *param)
 %token BAS_NEXT       "next"
 %token BAS_NIL        "nil"
 %token BAS_NOT        "not"
+%token BAS_OFFSETOF   "offsetof"
 %token BAS_ON         "on"
 %token BAS_OPEN       "open"
 %token BAS_OPTION     "option"
@@ -1681,6 +1682,10 @@ pseudofunc_expr:
         AST *ident = $5;
         $$ = NewAST(AST_HASMETHOD, typnam, ident);
     }
+  | BAS_OFFSETOF '(' typename ',' BAS_IDENTIFIER ')'
+    {
+        $$ = NewAST(AST_OFFSETOF, $3, $5);
+    }
   | BAS_SAMETYPES '(' typename ',' typename ')'
     {
         AST *expr = $3;

frontends/lexer.c

@@ -2665,6 +2665,7 @@ struct reservedword basic_keywords[] = {
     { "next", BAS_NEXT },
     { "not", BAS_NOT },
     { "nil", BAS_NIL },
+    { "offsetof", BAS_OFFSETOF },
     { "on", BAS_ON },
     { "open", BAS_OPEN },
     { "option", BAS_OPTION },