C++ Shared pointers. How can I change the underlying object's pointer for all copies?

Question

I have a strange situation with virtual classes and I need design help.

1. The objects do work on destruction,
2. The objects are stored in a vector, but I need to get the object or a reference to it out of the vector, change it, and have that change reflected in ALL 'copies' of the object,
3. I'd like the objects to be copyable.

I have a solution, but I am looking for a better one. This is the code I've written that accomplishes what I am looking for, but it depends on pointers to pointers to pointers.

I feel like if I could just directly manipulate the underlying data pointer of the shared pointer, I could get away with one less layer of abstraction.

I would like to do this with one InterfaceWrapper, not two.

#include <stdio.h>
#include <memory>
#include <vector>

class Interface
{
  public:
    virtual void WriteIt() = 0;
    virtual ~Interface() { }
};

class Foo : public Interface
{
  void WriteIt() { printf ("Foo\n"); }
};

class Bar : public Interface
{
  void WriteIt() { printf ("Bar\n"); }
};

// This class wraps Interface so we can call WriteIt on desctruction
// I'd like to do this in the Interface class, but you can't call virtual methods during destruction.
class InterfaceWrapper : public std::unique_ptr<Interface>
{
  public:
  InterfaceWrapper(Interface * i) : std::unique_ptr<Interface>(i) { }
  ~InterfaceWrapper() { (*this)->WriteIt(); }
};

// This class provides counted destruction to InterfaceWrapper
class InterfaceWrapper2
{
  public:
    InterfaceWrapper2 () : _ptr(new InterfaceWrapper(new Foo)) { }

    void MakeBar() { _ptr->reset(new Bar); }

  private:
    std::shared_ptr<InterfaceWrapper> _ptr;
};

int main (void)
{
  std::vector<InterfaceWrapper2> thing_vector;

  // The default implementation will, print "Foo" on destruction.
  InterfaceWrapper2 thing;

  // That destructor should only happen once for all copies of 'thing'
  thing_vector.push_back(thing);

  // MakeBar changes the virtual implementation so it prints "Bar" instead of "Foo"
  thing.MakeBar();

  // When destructors happen, the program should just print "Bar" once.

  return 0;
}

Anything is welcome, but I particularly interested in solutions which work on C++03 using boost (my example is C++11 but my 'real' code is C++03 using boost::shared_ptr).

CLARIFICATION

I am basically looking for a better way to implement InterfaceWrapper2 in my example code. main() is the best explanation of what I am trying to accomplish. Just remember the behavior is stuck in those virtual classes.

Answer 1

Is this what you're after?

Update 3

If you want to keep the Interface machinery, the following is a pretty succinct way to write the wrapper by just composing standard library features:

class InterfaceWrapper {
    using UI = std::unique_ptr<Interface>;

    std::shared_ptr<UI> _sui {new UI{new Foo}, [](UI*p){ (*p)->WriteIt(); delete p; }};

public:
    void MakeBar() { _sui->reset(new Bar); }
};

See it Live On Coliru

Update 2

1. After realizing that an std::function<> is already a dynamic, single-function, mutable interface that can be bound to any stateful functor, I thought of the following simplified version:

   Live On Coliru

    #include <memory> #include <iostream> #include <vector> struct dynamic_release { template <typename F> dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~dynamic_release() { _f(); } private: std::function<void()> _f; }; void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } int main(void) { using InterfaceWrapper = std::shared_ptr<dynamic_release>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once } 

2. To enable optimizations in case you want to achieve more efficiency with state-less functors too, add a basic_dynamic_release template that allows to use different functor types (like, eg void(*)() ):

   Live On Coliru

    #include <memory> #include <iostream> namespace detail { template <typename InterfaceCallable> struct basic_dynamic_release { basic_dynamic_release() = default; template <typename F> basic_dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> basic_dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~basic_dynamic_release() { _f(); } private: InterfaceCallable _f; }; } using dynamic_release = detail::basic_dynamic_release<std::function<void()>>; #include <vector> void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } int main(void) { using InterfaceWrapper = std::shared_ptr<detail::basic_dynamic_release<void(*)(void)>>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once } 

3. To allow for default constructed instances to have a well defined implementation of the interface, adding a factory (this is to make it very generic):

   Live On Coliru

    #include <memory> #include <iostream> namespace detail { template <typename T> struct default_construction final { T operator()() const { return {}; } }; template <typename InterfaceCallable, typename Factory = default_construction<InterfaceCallable> > struct basic_dynamic_release { basic_dynamic_release() = default; template <typename F> basic_dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> basic_dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~basic_dynamic_release() { _f(); } private: InterfaceCallable _f = Factory()(); }; using dynamic_interface = std::function<void()>; template <typename Factory = default_construction<dynamic_interface> > using dynamic_release = basic_dynamic_release<dynamic_interface, Factory>; } #include <vector> void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } struct foo_default { detail::dynamic_interface operator()() const { return do_foo; } }; int main(void) { using InterfaceWrapper = std::shared_ptr<detail::dynamic_release<foo_default> >; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = &do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once } 

---

Old answer

The old answer favoured static polymorphism using boost::variant , at the cost of being somewhat more complex the manage, but with greater flexibility:

I opted to replace the dynamic polymorphism with static polymorphism, which removes the extra allocation, which also takes with it the lifetime management (what used to be the unique_ptr ).

I think this makes resulting solution a bit simplified, and at the same time more generic (naturally provides some extension points).

Live On Coliru

 #include <boost/variant.hpp> #include <memory> #include <iostream> namespace nature { // detail namespace template <typename> struct Nature; template<> struct Nature<struct FooTag> { void do_it() { std::cout << "Foo" << "\\n"; } }; template<> struct Nature<struct BarTag> { void do_it() { std::cout << "Bar" << "\\n"; } }; using FooNature = Nature<FooTag>; using BarNature = Nature<BarTag>; using AnyNature = boost::variant<FooNature, BarNature>; struct Holder { AnyNature held; ~Holder() { DoIt()(held); } private: struct DoIt : boost::static_visitor<> { void operator()(AnyNature& any) const { return boost::apply_visitor(*this, any); } template <typename N> void operator()(N& nature) const { return nature.do_it(); } }; }; } #include <vector> int main(void) { using InterfaceWrapper = std::shared_ptr<nature::Holder>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); // FooNature is default thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing->held = nature::BarNature {}; // prints nothing thing_vector.push_back(thing); } // prints "Bar" once } 

Prints

 Foo Bar 

Answer 2

Borrowing from sehe's brilliant insight , this version gives you something that is closer to what you can do within C++.2003:

class InterfaceWrapper {
    typedef std::unique_ptr<Interface> UI;
    std::shared_ptr<UI> p_;
public:
    InterfaceWrapper () : p_(std::make_shared<UI>(new Foo)) {}
    ~InterfaceWrapper () { if (p_.unique()) (*p_)->WriteIt(); }
    void MakeBar() { p_->reset(new Bar); }
    const UI & operator -> () { return *p_.get(); }
};

Instead of a custom deleter, I chose to add a conditional within the destructor of InterfaceWrapper . This allowed the constructor to use std::make_shared .