 Smart Pointer Programming Techniques
Smart Pointer Programming TechniquesUsing incomplete classes for implementation hiding
            The "Pimpl" idiom
            Using abstract classes for implementation hiding
            Preventing delete px.get()
            Using a shared_ptr to hold a pointer to an array
            Encapsulating allocation details, wrapping factory 
                functions
            Using a shared_ptr to hold a pointer to a statically 
                allocated object
            Using a shared_ptr to hold a pointer to a COM object
            Using a shared_ptr to hold a pointer to an object 
                with an embedded reference count
            Using a shared_ptr to hold another shared 
                ownership smart pointer
            Obtaining a shared_ptr from a raw pointer
            Obtaining a shared_ptr (weak_ptr) 
                to this in a constructor
            Obtaining a shared_ptr to this
            Using shared_ptr as a smart counted handle
            Using shared_ptr to execute code on block 
                exit
            Using shared_ptr<void> to hold an arbitrary 
                object
            Associating arbitrary data with heterogeneous shared_ptr
                instances
            Using shared_ptr as a CopyConstructible mutex lock
            Using shared_ptr to wrap member function calls
            Delayed deallocation
            Weak pointers to objects not managed by a shared_ptr
        
A proven technique (that works in C, too) for separating interface from implementation is to use a pointer to an incomplete class as an opaque handle:
class FILE; FILE * fopen(char const * name, char const * mode); void fread(FILE * f, void * data, size_t size); void fclose(FILE * f);
It is possible to express the above interface using shared_ptr, 
            eliminating the need to manually call fclose:
class FILE; shared_ptr<FILE> fopen(char const * name, char const * mode); void fread(shared_ptr<FILE> f, void * data, size_t size);
This technique relies on shared_ptr's ability to execute a custom 
            deleter, eliminating the explicit call to fclose, and on the fact 
            that shared_ptr<X> can be copied and destroyed when X
            is incomplete.
A C++ specific variation of the incomplete class pattern is the "Pimpl" idiom. 
            The incomplete class is not exposed to the user; it is hidden behind a 
            forwarding facade. shared_ptr can be used to implement a "Pimpl":
// file.hpp:
class file
{
private:
    class impl;
    shared_ptr<impl> pimpl_;
public:
    file(char const * name, char const * mode);
    // compiler generated members are fine and useful
    void read(void * data, size_t size);
};
        // file.cpp:
#include "file.hpp"
class file::impl
{
private:
    impl(impl const &);
    impl & operator=(impl const &);
    // private data
public:
    impl(char const * name, char const * mode) { ... }
    ~impl() { ... }
    void read(void * data, size_t size) { ... }
};
file::file(char const * name, char const * mode): pimpl_(new impl(name, mode))
{
}
void file::read(void * data, size_t size)
{
    pimpl_->read(data, size);
}
        The key thing to note here is that the compiler-generated copy constructor, 
            assignment operator, and destructor all have a sensible meaning. As a result, 
                file is CopyConstructible and Assignable, 
            allowing its use in standard containers.
Another widely used C++ idiom for separating inteface and implementation is to 
            use abstract base classes and factory functions. The abstract classes are 
            sometimes called "interfaces" and the pattern is known as "interface-based 
            programming". Again, shared_ptr can be used as the return type of 
            the factory functions:
// X.hpp:
class X
{
public:
    virtual void f() = 0;
    virtual void g() = 0;
protected:
    ~X() {}
};
shared_ptr<X> createX();
        -- X.cpp:
class X_impl: public X
{
private:
    X_impl(X_impl const &);
    X_impl & operator=(X_impl const &);
public:
    virtual void f()
    {
      // ...
    }
    virtual void g()
    {
      // ...
    }
};
shared_ptr<X> createX()
{
    shared_ptr<X> px(new X_impl);
    return px;
}
        A key property of shared_ptr is that the allocation, construction, deallocation, 
            and destruction details are captured at the point of construction, inside the 
            factory function. Note the protected and nonvirtual destructor in the example 
            above. The client code cannot, and does not need to, delete a pointer to X; 
            the shared_ptr<X> instance returned from createX
            will correctly call ~X_impl.
delete px.get()It is often desirable to prevent client code from deleting a pointer that is 
            being managed by shared_ptr. The previous technique showed one 
            possible approach, using a protected destructor. Another alternative is to use 
            a private deleter:
class X
{
private:
    ~X();
    class deleter;
    friend class deleter;
    class deleter
    {
    public:
        void operator()(X * p) { delete p; }
    };
public:
    static shared_ptr<X> create()
    {
        shared_ptr<X> px(new X, X::deleter());
        return px;
    }
};
        shared_ptr to hold a pointer to an arrayA shared_ptr can be used to hold a pointer to an array allocated 
            with new[]:
shared_ptr<X> px(new X[1], checked_array_deleter<X>());
Note, however, that shared_array is 
            often preferable, if this is an option. It has an array-specific interface, 
            without operator* and operator->, and does not 
            allow pointer conversions.
shared_ptr can be used in creating C++ wrappers over existing C 
            style library interfaces that return raw pointers from their factory functions 
            to encapsulate allocation details. As an example, consider this interface, 
            where CreateX might allocate X from its own private 
            heap, ~X may be inaccessible, or X may be incomplete:
X * CreateX(); void DestroyX(X *);
The only way to reliably destroy a pointer returned by CreateX is 
            to call DestroyX.
Here is how a shared_ptr-based wrapper may look like:
shared_ptr<X> createX()
{
    shared_ptr<X> px(CreateX(), DestroyX);
    return px;
}
        Client code that calls createX still does not need to know how the 
            object has been allocated, but now the destruction is automatic.
shared_ptr to hold a pointer to a statically 
                allocated objectSometimes it is desirable to create a shared_ptr to an already 
            existing object, so that the shared_ptr does not attempt to 
            destroy the object when there are no more references left. As an example, the 
            factory function:
shared_ptr<X> createX();
in certain situations may need to return a pointer to a statically allocated X
            instance.
The solution is to use a custom deleter that does nothing:
struct null_deleter
{
    void operator()(void const *) const
    {
    }
};
static X x;
shared_ptr<X> createX()
{
    shared_ptr<X> px(&x, null_deleter());
    return px;
}
        The same technique works for any object known to outlive the pointer.
shared_ptr to hold a pointer to a COM ObjectBackground: COM objects have an embedded reference count and two member 
            functions that manipulate it. AddRef() increments the count. Release()
            decrements the count and destroys itself when the count drops to zero.
It is possible to hold a pointer to a COM object in a shared_ptr:
shared_ptr<IWhatever> make_shared_from_COM(IWhatever * p)
{
    p->AddRef();
    shared_ptr<IWhatever> pw(p, mem_fn(&IWhatever::Release));
    return pw;
}
        Note, however, that shared_ptr copies created from pw will 
            not "register" in the embedded count of the COM object; they will share the 
            single reference created in make_shared_from_COM. Weak pointers 
            created from pw will be invalidated when the last shared_ptr
            is destroyed, regardless of whether the COM object itself is still alive.
As explained in the mem_fn documentation, 
            you need to #define 
                BOOST_MEM_FN_ENABLE_STDCALL first.
shared_ptr to hold a pointer to an object 
                with an embedded reference countThis is a generalization of the above technique. The example assumes that the 
            object implements the two functions required by intrusive_ptr,
            intrusive_ptr_add_ref and intrusive_ptr_release:
template<class T> struct intrusive_deleter
{
    void operator()(T * p)
    {
        if(p) intrusive_ptr_release(p);
    }
};
shared_ptr<X> make_shared_from_intrusive(X * p)
{
    if(p) intrusive_ptr_add_ref(p);
    shared_ptr<X> px(p, intrusive_deleter<X>());
    return px;
}
        shared_ptr to hold another shared 
                ownership smart pointerOne of the design goals of shared_ptr is to be used in library 
            interfaces. It is possible to encounter a situation where a library takes a shared_ptr
            argument, but the object at hand is being managed by a different reference 
            counted or linked smart pointer.
It is possible to exploit shared_ptr's custom deleter feature to 
            wrap this existing smart pointer behind a shared_ptr facade:
template<class P> struct smart_pointer_deleter
{
private:
    P p_;
public:
    smart_pointer_deleter(P const & p): p_(p)
    {
    }
    void operator()(void const *)
    {
        p_.reset();
    }
    
    P const & get() const
    {
        return p_;
    }
};
shared_ptr<X> make_shared_from_another(another_ptr<X> qx)
{
    shared_ptr<X> px(qx.get(), smart_pointer_deleter< another_ptr<X> >(qx));
    return px;
}
        One subtle point is that deleters are not allowed to throw exceptions, and the 
            above example as written assumes that p_.reset() doesn't throw. If 
            this is not the case, p_.reset() should be wrapped in a try {} 
                catch(...) {} block that ignores exceptions. In the (usually 
            unlikely) event when an exception is thrown and ignored, p_ will 
            be released when the lifetime of the deleter ends. This happens when all 
            references, including weak pointers, are destroyed or reset.
Another twist is that it is possible, given the above shared_ptr instance, 
            to recover the original smart pointer, using 
                    get_deleter:
void extract_another_from_shared(shared_ptr<X> px)
{
    typedef smart_pointer_deleter< another_ptr<X> > deleter;
    if(deleter const * pd = get_deleter<deleter>(px))
    {
        another_ptr<X> qx = pd->get();
    }
    else
    {
        // not one of ours
    }
}
        shared_ptr from a raw pointerSometimes it is necessary to obtain a shared_ptr given a raw 
            pointer to an object that is already managed by another shared_ptr 
            instance. Example:
void f(X * p)
{
    shared_ptr<X> px(???);
}
        Inside f, we'd like to create a shared_ptr to *p.
In the general case, this problem has no solution. One approach is to modify f
            to take a shared_ptr, if possible:
void f(shared_ptr<X> px);
The same transformation can be used for nonvirtual member functions, to convert 
            the implicit this:
void X::f(int m);
would become a free function with a shared_ptr first argument:
void f(shared_ptr<X> this_, int m);
If f cannot be changed, but X uses intrusive counting, 
            use make_shared_from_intrusive described 
            above. Or, if it's known that the shared_ptr created in f
            will never outlive the object, use a null deleter.
shared_ptr (weak_ptr) 
                to this in a constructorSome designs require objects to register themselves on construction with a central authority. When the registration routines take a shared_ptr, this leads to the question how could a constructor obtain a shared_ptr to this:
class X
{
public:
    X()
    {
        shared_ptr<X> this_(???);
    }
};
        In the general case, the problem cannot be solved. The X instance 
            being constructed can be an automatic variable or a static variable; it can be 
            created on the heap:
shared_ptr<X> px(new X);
but at construction time, px does not exist yet, and it is 
            impossible to create another shared_ptr instance that shares 
            ownership with it.
Depending on context, if the inner shared_ptr this_ doesn't 
            need to keep the object alive, use a null_deleter as explained 
                here and here. If X is 
            supposed to always live on the heap, and be managed by a shared_ptr, 
            use a static factory function:
class X
{
private:
    X() { ... }
public:
    static shared_ptr<X> create()
    {
        shared_ptr<X> px(new X);
        // use px as 'this_'
        return px;
    }
};
        shared_ptr to thisSometimes it is needed to obtain a shared_ptr from this
            in a virtual member function under the assumption that this is 
            already managed by a shared_ptr. The transformations 
                described in the previous technique cannot be applied.
A typical example:
class X
{
public:
    virtual void f() = 0;
protected:
    ~X() {}
};
class Y
{
public:
    virtual shared_ptr<X> getX() = 0;
protected:
    ~Y() {}
};
// --
class impl: public X, public Y
{
public:
    impl() { ... }
    virtual void f() { ... }
    virtual shared_ptr<X> getX()
    {
        shared_ptr<X> px(???);
        return px;
    }
};
        The solution is to keep a weak pointer to this as a member in impl:
class impl: public X, public Y
{
private:
    weak_ptr<impl> weak_this;
    impl(impl const &);
    impl & operator=(impl const &);
    impl() { ... }
public:
    static shared_ptr<impl> create()
    {
        shared_ptr<impl> pi(new impl);
        pi->weak_this = pi;
        return pi;
    }
    virtual void f() { ... }
    virtual shared_ptr<X> getX()
    {
        shared_ptr<X> px(weak_this);
        return px;
    }
};
        The library now includes a helper class template 
                    enable_shared_from_this that can be used to encapsulate the 
            solution:
class impl: public X, public Y, public enable_shared_from_this<impl>
{
public:
    impl(impl const &);
    impl & operator=(impl const &);
public:
    virtual void f() { ... }
    virtual shared_ptr<X> getX()
    {
        return shared_from_this();
    }
}
        Note that you no longer need to manually initialize the weak_ptr member 
            in enable_shared_from_this. 
            Constructing a shared_ptr to impl takes care of that.
shared_ptr as a smart counted handleSome library interfaces use opaque handles, a variation of the incomplete class technique described above. An example:
typedef void * HANDLE; HANDLE CreateProcess(); void CloseHandle(HANDLE);
Instead of a raw pointer, it is possible to use shared_ptr as the 
            handle and get reference counting and automatic resource management for free:
typedef shared_ptr<void> handle;
handle createProcess()
{
    shared_ptr<void> pv(CreateProcess(), CloseHandle);
    return pv;
}
        shared_ptr to execute code on block exitshared_ptr<void> can automatically execute cleanup code when 
            control leaves a scope.
f(p), where p is a pointer:shared_ptr<void> guard(p, f);
f(x, y):shared_ptr<void> guard(static_cast<void*>(0), bind(f, x, y));
For a more thorough treatment, see the article "Simplify Your Exception-Safe Code" by Andrei Alexandrescu and Petru Marginean, available online at http://www.cuj.com/experts/1812/alexandr.htm?topic=experts.
shared_ptr<void> to hold an arbitrary 
                objectshared_ptr<void> can act as a generic object pointer similar 
            to void*. When a shared_ptr<void> instance 
            constructed as:
shared_ptr<void> pv(new X);
is destroyed, it will correctly dispose of the X object by 
            executing ~X.
This propery can be used in much the same manner as a raw void* is 
            used to temporarily strip type information from an object pointer. A shared_ptr<void>
            can later be cast back to the correct type by using 
                    static_pointer_cast.
shared_ptr
                instancesshared_ptr and weak_ptr support operator<
            comparisons required by standard associative containers such as std::map. 
            This can be used to non-intrusively associate arbitrary data with objects 
            managed by shared_ptr:
typedef int Data; std::map< shared_ptr<void>, Data > userData; // or std::map< weak_ptr<void>, Data > userData; to not affect the lifetime shared_ptr<X> px(new X); shared_ptr<int> pi(new int(3)); userData[px] = 42; userData[pi] = 91;
shared_ptr as a CopyConstructible mutex lockSometimes it's necessary to return a mutex lock from a function, and a 
            noncopyable lock cannot be returned by value. It is possible to use shared_ptr
            as a mutex lock:
class mutex
{
public:
    void lock();
    void unlock();
};
shared_ptr<mutex> lock(mutex & m)
{
    m.lock();
    return shared_ptr<mutex>(&m, mem_fn(&mutex::unlock));
}
        Better yet, the shared_ptr instance acting as a lock can be 
            encapsulated in a dedicated shared_lock class:
class shared_lock
{
private:
    shared_ptr<void> pv;
public:
    template<class Mutex> explicit shared_lock(Mutex & m): pv((m.lock(), &m), mem_fn(&Mutex::unlock)) {}
};
        shared_lock can now be used as:
shared_lock lock(m);
Note that shared_lock is not templated on the mutex type, thanks to 
                shared_ptr<void>'s ability to hide type information.
shared_ptr to wrap member function callsshared_ptr implements the ownership semantics required from the Wrap/CallProxy
            scheme described in Bjarne Stroustrup's article "Wrapping C++ Member Function 
            Calls" (available online at http://www.stroustrup.com/wrapper.pdf). 
            An implementation is given below:
template<class T> class pointer
{
private:
    T * p_;
public:
    explicit pointer(T * p): p_(p)
    {
    }
    shared_ptr<T> operator->() const
    {
        p_->prefix();
        return shared_ptr<T>(p_, mem_fn(&T::suffix));
    }
};
class X
{
private:
    void prefix();
    void suffix();
    friend class pointer<X>;
    
public:
    void f();
    void g();
};
int main()
{
    X x;
    pointer<X> px(&x);
    px->f();
    px->g();
}
        In some situations, a single px.reset() can trigger an expensive 
            deallocation in a performance-critical region:
class X; // ~X is expensive
class Y
{
    shared_ptr<X> px;
public:
    void f()
    {
        px.reset();
    }
};
        The solution is to postpone the potential deallocation by moving px 
            to a dedicated free list that can be periodically emptied when performance and 
            response times are not an issue:
vector< shared_ptr<void> > free_list;
class Y
{
    shared_ptr<X> px;
public:
    void f()
    {
        free_list.push_back(px);
        px.reset();
    }
};
// periodically invoke free_list.clear() when convenient
        Another variation is to move the free list logic to the construction point by using a delayed deleter:
struct delayed_deleter
{
    template<class T> void operator()(T * p)
    {
        try
        {
            shared_ptr<void> pv(p);
            free_list.push_back(pv);
        }
        catch(...)
        {
        }
    }
};
        shared_ptrMake the object hold a shared_ptr to itself, using a null_deleter:
class X
{
private:
    shared_ptr<X> this_;
    int i_;
public:
    explicit X(int i): this_(this, null_deleter()), i_(i)
    {
    }
    // repeat in all constructors (including the copy constructor!)
    X(X const & rhs): this_(this, null_deleter()), i_(rhs.i_)
    {
    }
    // do not forget to not assign this_ in the copy assignment
    X & operator=(X const & rhs)
    {
        i_ = rhs.i_;
    }
    weak_ptr<X> get_weak_ptr() const { return this_; }
};
        When the object's lifetime ends, X::this_ will be destroyed, and 
            all weak pointers will automatically expire.
$Date$
Copyright © 2003 Peter Dimov. Distributed under the Boost Software License, Version 1.0. See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt.