doc/smart_ptr/techniques.adoc

////
Copyright 2003, 2017 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
////

[[techniques]]
[appendix]
# Smart Pointer Programming Techniques
:toc:
:toc-title:
:idprefix: techniques_

[#techniques_incomplete]
## Using incomplete classes for implementation hiding

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`&#8217;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.

## The "Pimpl" idiom

A {cpp} 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.

## Using abstract classes for implementation hiding

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`.

## Preventing `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;
    }
};
```

## Encapsulating allocation details, wrapping factory functions

`shared_ptr` can be used in creating {cpp} 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.

[#techniques_static]
## Using a shared_ptr to hold a pointer to a statically allocated object

Sometimes 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.

## Using a shared_ptr to hold a pointer to a COM Object

Background: 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 link:../../../../libs/bind/mem_fn.html#Q3[explained] in the `mem_fn` documentation, you need to `#define BOOST_MEM_FN_ENABLE_STDCALL` first.

[#techniques_intrusive]
## Using a shared_ptr to hold a pointer to an object with an embedded reference count

This is a generalization of the above technique. The example assumes that the object implements the two functions required by `<<intrusive_ptr,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;
}
```

## Using a shared_ptr to hold another shared ownership smart pointer

One 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`&#8217;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 `<<shared_ptr_get_deleter,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
    }
}
```

[#techniques_from_raw]
## Obtaining a shared_ptr from a raw pointer

Sometimes 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 `<<techniques_intrusive,make_shared_from_intrusive>>` described above. Or, if it's known that the `shared_ptr` created in `f` will never outlive the object, use <<techniques_static,a null deleter>>.

## Obtaining a shared_ptr (weak_ptr) to this in a constructor

Some 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 <<techniques_static,here>> and <<techniques_weak_without_shared,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;
    }
};
```

## Obtaining a shared_ptr to this

Sometimes 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 <<techniques_from_raw,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,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.

## Using shared_ptr as a smart counted handle

Some library interfaces use opaque handles, a variation of the <<techniques_incomplete,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;
}
```

## Using shared_ptr to execute code on block exit

`shared_ptr<void>` can automatically execute cleanup code when control leaves a scope.

* Executing `f(p)`, where `p` is a pointer:
+
```
shared_ptr<void> guard(p, f);
```

* Executing arbitrary code: `f(x, y)`:
+
```
shared_ptr<void> guard(static_cast<void*>(0), bind(f, x, y));
```

## Using shared_ptr<void> to hold an arbitrary object

`shared_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 `<<shared_ptr_static_pointer_cast,static_pointer_cast>>`.

## Associating arbitrary data with heterogeneous `shared_ptr` instances

`shared_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;
```

## Using `shared_ptr` as a `CopyConstructible` mutex lock

Sometimes 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>`&#8217;s ability to hide type information.

## Using shared_ptr to wrap member function calls

`shared_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();
}
```

## Delayed deallocation

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(...)
        {
        }
    }
};
```

[#techniques_weak_without_shared]
## Weak pointers to objects not managed by a shared_ptr

Make 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.