Re: Replacement for MS STL?

"P.J. Plauger" <>
Mon, 2 Oct 2006 08:51:35 -0400
"Garry W" <> wrote in message

Here's another STL bug I found -

#2: Instance of illegal "pointer" usage (or non-usage).

I am doing something unusual: I'm providing my own definitions for
allocator::pointer and allocator::const_pointer. (I have a persistent -
memory-mapped - database to store that "huge" amount of data I have. The
persistent database requires the unusual pointers.) Doing something
always provides excellent opportunities for interesting bugs.

Specifically -

In the file "xstring" near line 2045 occurs this definition of std::string
character storage:

union _Bxty
{ // storage for small buffer or pointer to larger one
_Elem _Buf[_BUF_SIZE];
_Elem * _Ptr;
} _Bx;

First error is that the intrinsic-pointer usage "_Elem *" is obviously
- it has to be rebind<_Elem>::pointer or something. There shouldn't be any
naked pointers to elements in stored STL data.

No, it doesn't. Template class basic_string can be instantiated only
on PODs -- see Clause 21 of the C++ Standard. That precludes any
fancy tricks with redefining pointers or addressing. Note that template
class vector does not use the small-string optimization, because it
lacks this restriction.

More subtle error is that even with that change the program would still be
wrong: the arbitrary type "pointer" =may= have constructors (or
or assignment ops.) Mine do. And types that have any of those things are
illegal within C++ unions.

Then your program is ill formed. Not a bug in the implementation.

To achieve the equivalent effect of constructor-in-union, you might
do a bit of fancy footwork with alignment guarantees, manual
construction/destruction at the proper moments, and accessors that do
reference casts. At least, that's what I did in my quick work-around,
for putting in the alignment guarantee (my own pointers didn't need it.)

Hope this helps.

It does.

P.J. Plauger
Dinkumware, Ltd.

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