Delete raw pointer argument to boost::bind

Question

Lets say I have heap allocated A*, which I want to pass as argument to boost::bind. boost::bind is saved for later processing in some STL like container of boost::functions's.

I want to ensure A* will be destroyed at destruction of the STL container.

To demostrate:

A* pA = new A();

// some time later
container.push_back(boost::bind(&SomeClass::HandleA, this, pA);

// some time later
container is destroyed => pA is destroyed too

How can it be done?

EDIT

Maybe what I want is not that realistic.

I have raw pointer and function which receives the raw pointer. The call is delayed by means of boost::bind. At this point I want automatic memory management in case boost::bind want executed. I'm lazy, so I want to use "ready" smart-pointer solution.

std::auto_ptr looks like a good candidate, however ...

auto_ptr<A> pAutoA(pA);
container.push_back(boost::bind(&SomeClass::HandleA, this, pAutoA);

doesn't compile (see here)

auto_ptr<A> pAutoA(pA);
container.push_back(boost::bind(&SomeClass::HandleA, this, boost::ref(pAutoA));

pAutoA is destroyed, deleting underlying pA.

EDIT 02

In the mentioned container I will need to store misc "callbacks" with different arguments. Some of them are raw pointers to object. Since the code is old, I not always can change it.

Writing own wrapper for storing callbacks in container is last resort (while maybe the only one), hence bounty.

Answer 1

The idea of @pmjordan was already going in the right direction. You replied that you can't use shared_ptr, because you can't take ownership back from it once constructed. But that is not entirely correct: with shared_ptr's custom deleter mechanism, you can. This is how:

Assume these toy defintions for your A and f(A*):

struct A {
    ~A() { std::cout << "~A()" << std::endl; }
};

void f( A * a ) {
    std::cout << "in f(A*)" << std::endl;
    delete a;
}

1. Write a deleter that can be "switched off":

   struct opt_delete {
       bool m_delete;
       opt_delete() : m_delete( true ) {}
       template <typename T>
       void operator()( T * t ) {
           if ( m_delete ) delete t;
       }
   };
   

2. Then you can write a take() function that takes ownership of the shared_ptr payload again:

   template <typename T>
   T * take( const boost::shared_ptr<T> & sp ) {
       opt_delete * d = boost::get_deleter<opt_delete>( sp );
       assert( d );
       assert( d->m_delete == true );
       d->m_delete = false;
       return sp.get();
   }
   

   (this will leave the payload in the remaining shared_ptr instances, but for your case, that's ok, and the assert()s cover the cases when it's not).

3. Now you can manually wrap f(A*) like this:

   void f_sp( const boost::shared_ptr<A> & a ) {
       f( take( a ) );
   }
   

4. And finally, test the two scenarios:

   int main( int argc, char * argv[] ) {
   
       const boost::shared_ptr<A> a( new A, opt_delete() );
   
       const boost::function<void()> func =
           boost::bind( &f_sp, a );
   
       if ( argc >= 2 && *argv[1] == '1' ) // call 'func'
           func();
       else
           ; // don't
   
       return 0;
   }
   

Executing the test program with a 1 argument will print

in f(A*)
~A()

and without (or any other argument), it will print

~A()

You can extend the test harness to put func into a container first, but it'll still be safe. The only thing that isn't safe in the case is calling the func copies more than once (but then you'll trigger the second assertion in take()).

EDIT: Note that this mechanism isn't thread-safe. To make it thread-safe, you need to supply opt_delete with a mutex to synchronise operator() with take().

Answer 2

I assume you mean you have some function, let's call it f() which takes an A*, which you then proxy with boost::bind? Can you change this function to accept a Boost/TR1 shared_ptr<A> instead? Using a shared_ptr (or, less likely, a C++98 std::auto_ptr) should solve your lifecycle problem.

Alternatively, if you can't change f itself, you could create a wrapper which accepts a shared_ptr<A>, pulls out the raw pointer and calls f with it. If you find yourself writing a lot of these wrappers, you may be able to create a template for generating them, assuming the function signatures are similar.