* doc/ObjectType.3: Updated documentation of the Tcl_ObjType

        struct to match expectations of Tcl 8.5 [Bug 1917650].

1 files changed, 74 insertions, 36 deletions

diff --git a/doc/ObjectType.3 b/doc/ObjectType.3
index f7bdb9f..a24f9c6 100644
--- a/doc/ObjectType.3
+++ b/doc/ObjectType.3
@@ -4,7 +4,7 @@
 '\" See the file "license.terms" for information on usage and redistribution
 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 '\" 
-'\" RCS: @(#) $Id: ObjectType.3,v 1.16 2007/12/13 15:22:31 dgp Exp $
+'\" RCS: @(#) $Id: ObjectType.3,v 1.17 2008/06/30 14:01:34 dgp Exp $
 '\" 
 .so man.macros
 .TH Tcl_ObjType 3 8.0 Tcl "Tcl Library Procedures"
@@ -49,7 +49,10 @@ They are used to register new object types, look up types,
 and force conversions from one type to another.
 .PP
 \fBTcl_RegisterObjType\fR registers a new Tcl object type
-in the table of all object types supported by Tcl.
+in the table of all object types that \fBTcl_GetObjType\fR
+can look up by name.  There are other object types supported by Tcl
+as well, which Tcl chooses not to register.  Extensions can likewise
+choose to register the object types they create or not.
 The argument \fItypePtr\fR points to a Tcl_ObjType structure that
 describes the new type by giving its name
 and by supplying pointers to four procedures
@@ -60,11 +63,11 @@ it is replaced with the new type.
 The Tcl_ObjType structure is described
 in the section \fBTHE TCL_OBJTYPE STRUCTURE\fR below.
 .PP
-\fBTcl_GetObjType\fR returns a pointer to the Tcl_ObjType
+\fBTcl_GetObjType\fR returns a pointer to the registered Tcl_ObjType
 with name \fItypeName\fR.
 It returns NULL if no type with that name is registered.
 .PP
-\fBTcl_AppendAllObjTypes\fR appends the name of each object type
+\fBTcl_AppendAllObjTypes\fR appends the name of each registered object type
 as a list element onto the Tcl object referenced by \fIobjPtr\fR.
 The return value is \fBTCL_OK\fR unless there was an error
 converting \fIobjPtr\fR to a list object;
@@ -74,7 +77,8 @@ in that case \fBTCL_ERROR\fR is returned.
 if possible.
 It creates a new internal representation for \fIobjPtr\fR
 appropriate for the target type \fItypePtr\fR
-and sets its \fItypePtr\fR member to that type.
+and sets its \fItypePtr\fR member as determined by calling the 
+\fItypePtr->setFromAnyProc\fR routine.  
 Any internal representation for \fIobjPtr\fR's old type is freed.
 If an error occurs during conversion, it returns \fBTCL_ERROR\fR
 and leaves an error message in the result object for \fIinterp\fR
@@ -82,12 +86,23 @@ unless \fIinterp\fR is NULL.
 Otherwise, it returns \fBTCL_OK\fR.
 Passing a NULL \fIinterp\fR allows this procedure to be used
 as a test whether the conversion can be done (and in fact was done).
+.VS 8.5
+.PP
+In many cases, the \fItypePtt->setFromAnyProc\fR routine will
+set \fIobjPtr->typePtr\fR to the argument value \fItypePtr\fR,
+but that is no longer guaranteed.  The \fIsetFromAnyProc\fR is
+free to set the internal representation for \fIobjPtr\fR to make
+use of another related Tcl_ObjType, if it sees fit.
+.VE 8.5
 .SH "THE TCL_OBJTYPE STRUCTURE"
 .PP
 Extension writers can define new object types by defining four
-procedures,
-initializing a Tcl_ObjType structure to describe the type,
-and calling \fBTcl_RegisterObjType\fR.
+procedures and
+initializing a Tcl_ObjType structure to describe the type.
+Extension writers may also pass a pointer to their Tcl_ObjType
+structire to \fBTcl_RegisterObjType\fR if they wish to permit
+other extensions to look up their Tcl_ObjType by name with
+the \fBTcl_GetObjType\fR routine.
 The \fBTcl_ObjType\fR structure is defined as follows:
 .PP
 .CS
@@ -102,8 +117,9 @@ typedef struct Tcl_ObjType {
 .SS "THE NAME FIELD"
 .PP
 The \fIname\fR member describes the name of the type, e.g. \fBint\fR.
-Extension writers can look up an object type using its name
-with the \fBTcl_GetObjType\fR procedure.
+When a type is registered, this is the name used by callers
+of \fBTcl_GetObjType\fR to lookup the type.  For unregistered
+types, the \fIname\fR field is primarily of value for debugging.
 The remaining four members are pointers to procedures
 called by the generic Tcl object code:
 .SS "THE SETFROMANYPROC FIELD"
@@ -124,22 +140,32 @@ unless \fIinterp\fR is NULL.
 If \fIsetFromAnyProc\fR is successful,
 it stores the new internal representation,
 sets \fIobjPtr\fR's \fItypePtr\fR member to point to
-\fIsetFromAnyProc\fR's \fBTcl_ObjType\fR, and returns \fBTCL_OK\fR.
+the \fBTcl_ObjType\fR struct corresponding to the new
+internal representation, and returns \fBTCL_OK\fR.
 Before setting the new internal representation,
 the \fIsetFromAnyProc\fR must free any internal representation
 of \fIobjPtr\fR's old type;
 it does this by calling the old type's \fIfreeIntRepProc\fR
 if it is not NULL.
-As an example, the \fIsetFromAnyProc\fR for the built-in Tcl integer type
+.PP
+As an example, the \fIsetFromAnyProc\fR for the built-in Tcl list type
 gets an up-to-date string representation for \fIobjPtr\fR
 by calling \fBTcl_GetStringFromObj\fR.
-It parses the string to obtain an integer and,
-if this succeeds,
-stores the integer in \fIobjPtr\fR's internal representation
-and sets \fIobjPtr\fR's \fItypePtr\fR member to point to the integer type's
+It parses the string to verify it is in a valid list format and
+to obtain each element value in the list, and, if this succeeds,
+stores the list elements in \fIobjPtr\fR's internal representation
+and sets \fIobjPtr\fR's \fItypePtr\fR member to point to the list type's
 Tcl_ObjType structure.
+.PP
 Do not release \fIobjPtr\fR's old internal representation unless you
 replace it with a new one or reset the \fItypePtr\fR member to NULL.
+.PP
+The \fIsetFromAnyProc\fR member may be set to NULL, if the routines
+making use of the internal representation have no need to derive that
+internal representation from an arbitrary string value.  However, in
+this case, passing a pointer to the type to Tcl_ConvertToType() will
+lead to a panic, so to avoid this possibility, the type
+should \fInot\fR be registered.
 .SS "THE UPDATESTRINGPROC FIELD"
 .PP
 The \fIupdateStringProc\fR member contains the address of a function
@@ -153,17 +179,27 @@ typedef void (Tcl_UpdateStringProc) (Tcl_Obj *\fIobjPtr\fR);
 \fIobjPtr\fR's \fIbytes\fR member is always NULL when it is called.
 It must always set \fIbytes\fR non-NULL before returning.
 We require the string representation's byte array
-to have a null after the last byte, at offset \fIlength\fR;
-this allows string representations that do not contain null bytes
+to have a null after the last byte, at offset \fIlength\fR,
+and to have no null bytes before that; this allows string representations 
 to be treated as conventional null character-terminated C strings.
+These restrictions are easily met by using Tcl's internal UTF encoding
+for the string representation, same as one would do for other
+Tcl routines accepting string values as arguments.
 Storage for the byte array must be allocated in the heap by \fBTcl_Alloc\fR
 or \fBckalloc\fR.  Note that \fIupdateStringProc\fRs must allocate
 enough storage for the string's bytes and the terminating null byte.
-The \fIupdateStringProc\fR for Tcl's built-in list type, for example,
-builds an array of strings for each element object
-and then calls \fBTcl_Merge\fR
-to construct a string with proper Tcl list structure.
-It stores this string as the list object's string representation.
+.PP
+The \fIupdateStringProc\fR for Tcl's built-in double type, for example,
+calls Tcl_PrintDouble to write to a buffer of size TCL_DOUBLE_SPACE,
+then allocates and copies the string representation to just enough
+space to hold it.  A pointer to the allocated space is stored in
+the \fIbytes\fR member.
+.PP
+The \fIupdateStringProc\fR member may be set to NULL, if the routines
+making use of the internal representation are written so that the
+string representation is never invalidated.  Failure to meet this
+obligation will lead to panics or crashes when \fBTcl_GetStringFromObj\fR
+or other similar routines ask for the string representation.
 .SS "THE DUPINTREPPROC FIELD"
 .PP
 The \fIdupIntRepProc\fR member contains the address of a function
@@ -180,12 +216,12 @@ Before the call,
 \fIsrcPtr\fR's internal representation is valid and \fIdupPtr\fR's is not.
 \fIsrcPtr\fR's object type determines what
 copying its internal representation means.
+.PP
 For example, the \fIdupIntRepProc\fR for the Tcl integer type
 simply copies an integer.
-The built-in list type's \fIdupIntRepProc\fR
-allocates a new array that points at the original element objects;
-the elements are shared between the two lists
-(and their reference counts are incremented to reflect the new references).
+The built-in list type's \fIdupIntRepProc\fR uses a far more
+sophisticated scheme to continue sharing storage as much as it
+reasonably can.
 .SS "THE FREEINTREPPROC FIELD"
 .PP
 The \fIfreeIntRepProc\fR member contains the address of a function
@@ -198,17 +234,19 @@ typedef void (Tcl_FreeInternalRepProc) (Tcl_Obj *\fIobjPtr\fR);
 The \fIfreeIntRepProc\fR function can deallocate the storage
 for the object's internal representation
 and do other type-specific processing necessary when an object is freed.
-For example, Tcl list objects have an \fIinternalRep.otherValuePtr\fR
-that points to an array of pointers to each element in the list.
-The list type's \fIfreeIntRepProc\fR decrements
-the reference count for each element object
-(since the list will no longer refer to those objects),
-then deallocates the storage for the array of pointers.
+.PP
+For example, the list type's \fIfreeIntRepProc\fR respects
+the storage sharing scheme established by the \fIdupIntRepProc\fR
+so that it only frees storage when the last object sharing it
+is being freed.
+.PP
 The \fIfreeIntRepProc\fR member can be set to NULL
 to indicate that the internal representation does not require freeing.
-The \fIfreeIntRepProc\fR implementation should not access the
-\fIbytes\fR member of the object, as this may potentially have already
-been deleted.
+The \fIfreeIntRepProc\fR implementation must not access the
+\fIbytes\fR member of the object, since Tcl makes its own internal
+uses of that field during object deletion.  The defined tasks for
+the \fIfreeIntRepProc\fR have no need to consult the \fIbytes\fR
+member.
 .SH "SEE ALSO"
 Tcl_NewObj, Tcl_DecrRefCount, Tcl_IncrRefCount
 .SH KEYWORDS