Re: problems with constructor throwing exception

"peter koch larsen" <>
23 Sep 2006 10:45:08 -0400
<> skrev:


I have a base class with a constructor that can throw a memory
exception. This could happen only when the last of the members is
being initialized in the initializer list. Moreover, the usage model
is to always use placement new syntax with a new operator. Also, none
of the data members in the base class nor derived class has a
destructor! I manage cleanup of resources in the destructors of the 2

I wanted to check the following assumptions when the base class causes
a memory error:

(1) neither the destructor for the base class nor the derived class are
invoked in this scenario.
(2) Since none of the data members of the base class nor derived class
have destructors, there are no destructors to invoke for these.
(3) The memory error is caused by allocating an object from a pool in
the base class, so since there is no destructor for the pool, and the
destructor of the base class is never invoked, there is a potential
leak here.
(4) The placement new operator defined for the base class also has a
placement delete operator (forced by the compiler). My guess is that
the compiler calls my placement new operator first (which gets memory
from the heap operand of the placement variable), then tries to
construct, does its own internal catch, if it fails to construct, then
it sees that no constructors were completed, so does not call any
destructors, and finally, the internal code calls the placement delete
operator to complement the placement new call.

I am especially concerned with (4). If the compiler does not
internally generate a catch clause that calls placement delete upon
encountering an exception in the base class constructor, then this is a
leak that I have no control over. Can someone explain the rollback
scenario or lack thereof for placement new, and if the compiler does
have a rollback scenario, where is this code generated? Is it
generated inline in each constructor call. If it is generated inline
in each constructor call with a catch then whereever I use:

    MyDerivedClass* obj = new (myheap) MyDerivedClass();

I am not going to get any register optimizations in the body of the
code that makes the above call!

My conclusion is that I am best served by using a 2-phase construction,
in which the actual constructor cannot throw an exception, and then
there is a subsequent call made to a Construct() function.

That can never be correct. You probably have a case of bad design, but
normally you handle those cases by introducing safe intermediates - eg:

class bad
    class foo* pf;
    class bar* pb;
public: bad();

// dangerous way to do it:
   pf = new foo();
   pb = new bar();

// safe way to do it:
   std::auto_ptr<foo> hpf(new foo());
   std::auto_ptr<bar> hpb(new bar());
   pf = hpf.release();
   pb = hpb.release();


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