* doc/bison.texi (Type Generation): New section.
---
doc/bison.texi | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/doc/bison.texi b/doc/bison.texi
index aa49ce7..04cb773 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -211,6 +211,7 @@ Defining Language Semantics
* Value Type:: Specifying one data type for all semantic values.
* Multiple Types:: Specifying several alternative data types.
+* Type Generation:: Generating the semantic value type.
* Union Decl:: Declaring the set of all semantic value types.
* Structured Value Type:: Providing a structured semantic value type.
* Actions:: An action is the semantic definition of a grammar rule.
@@ -3639,6 +3640,7 @@ the numbers associated with @var{x} and @var{y}.
@menu
* Value Type:: Specifying one data type for all semantic values.
* Multiple Types:: Specifying several alternative data types.
+* Type Generation:: Generating the semantic value type.
* Union Decl:: Declaring the set of all semantic value types.
* Structured Value Type:: Providing a structured semantic value type.
* Actions:: An action is the semantic definition of a grammar rule.
@@ -3713,6 +3715,9 @@ Specify the entire collection of possible data types.
There are several
options:
@itemize @bullet
@item
+let Bison compute the union type from the tags you assign to symbols;
+
+@item
use the @code{%union} Bison declaration (@pxref{Union Decl, ,The Union
Declaration});
@@ -3734,6 +3739,56 @@ and for groupings with the @code{%type} Bison
declaration (@pxref{Type
Decl, ,Nonterminal Symbols}).
@end itemize
+@node Type Generation
+@subsection Generating the Semantic Value Type
+@cindex declaring value types
+@cindex value types, declaring
+@findex %define api.value.type union
+
+The special value @code{union} of the @code{%define} variable
+@code{api.value.type} instructs Bison that the tags used with the
+@code{%token} and @code{%type} directives are genuine types, not names of
+members of @code{YYSTYPE}.
+
+For example:
+
+@example
+%define api.value.type union
+%token <int> INT "integer"
+%token <int> 'n'
+%type <int> expr
+%token <char const *> ID "identifier"
+@end example
+
+@noindent
+generates an appropriate value of @code{YYSTYPE} to support each symbol
+type. The name of the member of @code{YYSTYPE} for tokens than have a
+declared identifier @var{id} (such as @code{INT} and @code{ID} above, but
+not @code{'n'}) is @code{yytype_@var{id}}. The other symbols have
+unspecified names on which you should not depend; instead, relying on C
+casts to access the semantic value with the appropriate type:
+
+@example
+/* For an "integer". */
+yylval.yytype_INT = 42;
+return INT;
+
+/* For an 'n', also declared as int. */
+*((int*)&yylval) = 42;
+return 'n';
+
+/* For an "identifier". */
+yylval.yytype_ID = "42";
+return IDENTIFIER;
+@end example
+
+
+This feature is new, and user feedback would be most welcome.
+
+A similar feature is provided for C++ that in addition overcomes C++
+limitations (that forbid non-trivial objects to be part of a @code{union}):
+@samp{%define api.value.type variant}, see @ref{C++ Variants}.
+
@node Union Decl
@subsection The Union Declaration
@cindex declaring value types
--
1.8.1.3
