How can i hold reference in boost::shared_ptr using boost::bind without definition of explicit function?

Question

I want to hold reference to object so it doesn't get deleted in bind function, but without using helper function.

struct Int
{
   int *_int;
   ~Int(){ delete _int; }
};

void holdReference(boost::shared_ptr<Int>, int*) {} // helper

boost::shared_ptr<int> fun()
{
   boost::shared_ptr<Int> a ( new Int ); 
   // I get 'a' from some please else, and want to convert it
   a->_int = new int;

   return boost::shared<int>( a->_int, boost::bind(&holdReference, a, _1) );

}

Is there a way to declare holdReference function in place? Like with lambda expressions or sth? (without using this nasty holdReference function, that have to be declared outside the scope of fun function) I had few tries but non of them compiled :)

Ok, here is more detailed example:

#include <boost/shared_ptr.hpp>
#include <boost/bind.hpp>

// the case looks more or less like this
// this class is in some dll an I don't want to use this class all over my project
// and also avoid coppying the buffer
class String_that_I_dont_have 
{
    char * _data; // this is initialized in 3rd party, and released by their shared pointer

public:
    char * data() { return _data; }
};


// this function I created just to hold reference to String_that_I_dont_have class 
// so it doesn't get deleted, I want to get rid of this
void holdReferenceTo3rdPartyStringSharedPtr( boost::shared_ptr<String_that_I_dont_have>, char *) {}


// so I want to use shared pointer to char which I use quite often 
boost::shared_ptr<char> convert_function( boost::shared_ptr<String_that_I_dont_have> other) 
// 3rd party is using their own shared pointers, 
// not the boost's ones, but for the sake of the example ...
{
    return boost::shared_ptr<char>( 
        other->data(), 
        boost::bind(
            /* some in place here instead of holdReference... */
            &holdReferenceTo3rdPartyStringSharedPtr   , 
            other, 
            _1
        )
    );
}

int main(int, char*[]) { /* it compiles now */ }

// I'm just looking for more elegant solution, for declaring the function in place

Answer 1

I am a bit confused by what you are trying to do here.

Is fun() supposed to return boost::shared_ptr<int> or boost::shared_ptr<Int>???

I don't think that you want to create a shared_ptr<int> that is a shared pointer around a raw pointer that is directly owned by the Int object, as the Int object will delete the _int whenever it goes out of scope (even though in the example it did not 'new' it!).

You need to come up with a clear ownership/responsibility model.

Perhaps you can provide a different, more realistic, example of what you are trying to achieve?

Answer 2

You may be looking for the "shared ownership" constructor, this allows ref counting an interior pointer.

struct Int
{
   int *_int;
   ~Int(){ delete _int; }
};

boost::shared_ptr<int> fun()
{
   boost::shared_ptr<Int> a ( new Int ); 
   // I get 'a' from some please else, and want to convert it
   a->_int = new int;

   // only refcount the 'a' object but expose the interior _int pointer
   return boost::shared_ptr<int>(a, a->_int);
}