Traps and pitfalls for VC6, ATL, C

1
Traps and pitfalls for VC6, ATL, C

• May be obvious ?
• May be not ?
• Mark Bartosik

2
Danagers of _bstr_t

• _bstr_t_bstr_t(const char narrow_str)
• Unhandled exception 0xC00000FDbecause it
  allocates from the stack 2 x strlen(narrow_str)
• operator const char () constoperator char ()
• both return a pointer to PRIVATE data
• const char p get_str()
• unsigned len strlen(p) // bang ???! error ??

3
Danagers of _bstr_t

• operator const wchar_t () constoperator wchar_t
  ()
• both return a pointer to PRIVATE data
• BSTR p get_str()
• unsigned len SysStringLen(p) // bang ???!
  err??

4
Alternatives with _bstr_t

• const wchar_t GetWString() const
• const char GetString() const
• But not realistic. Once the extraction operators
  exist, you cannot avoid them.
• pragma depracatedsee Visual Studio .NET

5
Dangers of CComBSTR

• What is wrong with the following member
  functions?operator BSTR() constBSTR
  operator()
• int aint p a

6
CComPtrltTgt CComQIPtrltTgt

• T operator()
• Gives access to internal datacan cause a leak
  (if not NULL)
• T operator()
• Does not access to IUnknownoperator-gt() returns
  proxy object with private IUnknown
• operator T()
• Gives implicit access to raw pointer

7
_com_ptr_tltTgt

• T operator()
• T operator-gt()Exposes Iunknown, unlike CComPtr
  CComQIPtr which return a proxy object
• Behaves like CComQIPtr but uses exceptions for
  error reporting.Auto declared by import

8

• variant_t public VARIANT

9
How to choose ?

• Consistency is best
• Consistently use _bstr_t or CComBSTR
• Consistently use CComPtr or _com_ptr_tIf using
  import prefer _com_ptr_t
• Does the rest of your code use exceptions or
  magic return values?
• Write your own? (based on CComPtr)

10
collections

• stdmapltunsigned, employee_tgtstdmapltunsigned
  , CComPtrltIEmployeegt gtstdmapltunsigned,
  CAdaptltCComPtrltIEmployeegt gt gt
• Also when erasing elements, dont saw off the
  branch that you are sitting on. (General STL
  rule)Store objects NOT dumb pointers (smart
  pointers are objects)Remember that
  my_collection.erase(iter) invalidates iter.use
  iter my_collection.erase(iter)Dont
  forget about the algorithms include ltalgorithmgt

11
import

• Can use instead of .h, even for
  implementationImplement the raw_ functions.
• Read the .tlh filesBe careful some functions
  will return IDispatchPtr others will return
  IDispatch . It depends on the IDL.Reading the
  .tlh is a thinking aid, keep it open in a
  window.

12
Debugging

• Prefer __stdcall over __thiscall
• Optimizer off
• Consider releasing with optimizer off (except
  allow inlines for likes of STL)
• Symbol files on (PDB not PDB for edit continue),
  Link with debug info,Do not separate types
• Debug with Visual Studio .NET

13
(more) lifetime management

• CObjfoo() CSLock __anonymous__(m_critsec)
  m_sub_object.Release() // Bang!
• CObjfoo() IUnknownPtr __anonymous__(this)
  CSLock __anonymous__(m_critsec)
  m_sub_object.Release()

14
(more) lifetime management

• Do not mix strong and weak pointers
• What if you need a C pointer to a CComObject
  derived object?stdpairltCOurClass,
  UnknownPtrgt orstruct
• UnknownPtr strong_ref COurClass
  weak_ref

15
Patterns

• Avoid Singleton anti-pattern
• Once per what?Per network? Per sub-net? Per
  machine? Per apartment ? Per security context ?
  Per process? Per thread? Per transaction? Per
  user ? Etc.The ATL singleton is once per
  process.
• Use a more natural language idiom
• C globals are naturally per process.
• C variables in a shared data seg are naturally
  per machine
• VB module variables are naturally per thread
  (apartment)
• But avoid global pointers to COM objects (issues
  with CRT and COM initialization order)

16
Cyclic references Connection points

• Wizard generated code is just bad (has some
  bugs)Also it is synchronoussee www.cuj.com
  (search for Bartosik)
• Lack of type safety (IDispatch)

17
Build consideration

• Prefer dynamic link CRT(but there are install
  issues)
• Avoid creating COM components within a group

18
Libraries

• STL
• WTL
• boost.org

19
Leak Browser

• Much of COM is about ownership / lifetime
  management, but poor C language binding, thus
  the bugs.
• Will find all your leaks
• Soon will find all your references to deleted
  objects
• Soon will find all use of uninitialized memory
• Contact Bartosik for updates / latest build.

20
casts

• reinterpret_castltTOgt(from)reinterpret the bit
  pattern (unsafe and forceful)
• dynamic_castltTOgt(from)
• runtime check (from must have at least one
  virtual function). Often a sign of bad design.
  Why dont you know the type. dynamic_castltderived
  gt(base)
• static_castltTOgt(from)convert only if reasonable
• const_castltTOgt(from)remove either const or
  volatile qualifiers
• (TO)c-style cast -- avoid it, like static_cast
  OR reinterpret_cast optionally combined with
  const_cast, and can be more forceful than
  reinterpret_cast.

21
Compiler settings(use PDB for all)
22
Linker settings(include debug info)
23
Linker settings(do not separate types)