Tcl Source Code

Tcl_Obj System
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Discussion

If extensions need TclObjBeingDeleted then it should be public. Otherwise it should be file static.

TclFreeIntRep should be public. There should be no direct calling of another object type's procs.

There's code duplication in TclHashObjKey cribbed from Hash Tables.

Critical defect here: much of the API uses a signed int for object lengths (with negative numbers, typically -1, indicating “use strlen() to get the real length”). This is fine on 32-bit platforms, but massively wrong on 64-bit (except for ILP64 architectures, but that's a rare configuration).

There's a single avenue for transforming from one arbitrary object type to another -- through the string rep. This is effective, but limiting. Fully internal transformations cheat to avoid the problems, notably list to dict and back. This suggests alternative exchange forms could be useful. The two that come to mind are containers -- effectively lists -- and bytearrays.

There's only a single alternative intrep recalled at any time. This is sometimes unwelcome, and a fair bit of trickery is employed in places to avoid loss of a particularly precious intrep. Some consideration of a larger number of alternative representations able to co-exist might be taken. (Pros and cons)

Each object type fully controls the creation, duplication, and release of the internal rep. However it controls only the creation of the string rep. The overall value system defines that when it is time to release the string rep, it will be done with ckfree. This implies that every string rep must be a separately ckalloc'd chunk of memory and that implies a fair bit of memory management and string copying. The expense of this is mitigated by the Copy On Write scheme that keeps down the number of values that are nothing more than copies of others. Offering the object type control over the release of string reps would allow for other possibilities, where a set of values drawn from a small set of static strings might be done without memory management. Or string reps could be substrings of other strings kept alive via whatever means is required. This would imply relaxing the current requirement that objPtr->bytes values are NUL-terminated.

The reference counting scheme used to manage copy on write as well as memory management brings with it the constraint that these values are tied to a single thread at a time, and except where special care is taken that means a Tcl value is born, lives, and dies all in one thread. Values that must pass from one thread to another take a route through a string representation.