functional/factory/doc/factory.qbk

[/
Copyright 2007,2008 Tobias Schwinger

Copyright 2019 Glen Joseph Fernandes
(glenjofe@gmail.com)

Distributed under the Boost Software License, Version 1.0.
(http://www.boost.org/LICENSE_1_0.txt)
]

[library Boost.Functional/Factory
[quickbook 1.5]
[version 1.0]
[authors [Schwinger, Tobias], [Fernandes, Glen]]
[copyright 2007 2008 Tobias Schwinger]
[copyright 2019 Glen Joseph Fernandes]
[license Distributed under the Boost Software License, Version 1.0.]
[purpose Function object templates for object creation.]
[category higher-order]
[category generic]]

[def __boost__bind__
[@http://www.boost.org/libs/bind/bind.html `boost::bind`]]

[def __boost__forward_adapter__
[@http://www.boost.org/libs/functional/forward/doc/index.html
`boost::forward_adapter`]]

[def __boost_function__
[@http://www.boost.org/doc/html/function.html Boost.Function]]

[def __smart_pointer__
[@http://www.boost.org/libs/smart_ptr/index.html Smart Pointer]]

[def __smart_pointers__
[@http://www.boost.org/libs/smart_ptr/index.html Smart Pointers]]

[def __boost__shared_ptr__
[@http://www.boost.org/libs/smart_ptr/shared_ptr.htm `boost::shared_ptr`]]

[def __allocator__ [@https://www.boost.org/sgi/stl/Allocators.html Allocator]]

[def __std_allocators__
[@https://www.boost.org/sgi/stl/Allocators.html Allocators]]

[def __boost__factory__ `boost::factory`]

[def __boost__value_factory__ `boost::value_factory`]

[def __value_factory__ `value_factory`]

[section Brief Description]

The template __boost__factory__ lets you encapsulate a `new` expression as a
function object, __boost__value_factory__ encapsulates a constructor invocation
without `new`.

```
__boost__factory__<T*>()(arg1,arg2,arg3)
// same as new T(arg1,arg2,arg3)

__boost__value_factory__<T>()(arg1,arg2,arg3)
// same as T(arg1,arg2,arg3)
```

Before C++11 the arguments to the function objects have to be LValues. A
factory that also accepts RValues can be composed using the
__boost__forward_adapter__ or __boost__bind__. In C++11 or higher the
arguments can be LValues or RValues.

[endsect]

[section Background]

In traditional Object Oriented Programming a Factory is an object implementing
an interface of one or more methods that construct objects conforming to known
interfaces.

```
// assuming a_concrete_class and another_concrete_class are derived
// from an_abstract_class

struct a_factory {
    virtual an_abstract_class* create() const = 0;
    virtual ~a_factory() { }
};

struct a_concrete_factory
    : a_factory {
    an_abstract_class* create() const {
        return new a_concrete_class();
    }
};

struct another_concrete_factory
    : a_factory {
    an_abstract_class* create() const {
        return new another_concrete_class();
    }
};

// [...]

int main()
{
    boost::ptr_map<std::string, a_factory> factories;

    // [...]

    factories.insert("a_name",
        std::unique_ptr<a_factory>(new a_concrete_factory));
    factories.insert("another_name",
        std::unique_ptr<a_factory>(new another_concrete_factory));

    // [...]

    std::unique_ptr<an_abstract_class> x(factories.at(some_name).create());

    // [...]
}
```

This approach has several drawbacks. The most obvious one is that there is lots
of boilerplate code. In other words there is too much code to express a rather
simple intention. We could use templates to get rid of some of it but the
approach remains inflexible:

* We may want a factory that takes some arguments that are forwarded to the
constructor,
* we will probably want to use smart pointers,
* we may want several member functions to create different kinds of objects,
* we might not necessarily need a polymorphic base class for the objects,
* as we will see, we do not need a factory base class at all,
* we might want to just call the constructor - without `new` to create an
object on the stack, and
* finally we might want to use customized memory management.

Experience has shown that using function objects and generic Boost components
for their composition, Design Patterns that describe callback mechanisms
(typically requiring a high percentage of boilerplate code with pure Object
Oriented methodology) become implementable with just few code lines and without
extra classes.

Factories are callback mechanisms for constructors, so we provide two class
templates, __boost__value_factory__ and __boost__factory__, that encapsulate
object construction via direct application of the constructor and the `new`
operator, respectively.

We let the function objects forward their arguments to the construction
expressions they encapsulate. Over this __boost__factory__ optionally allows
the use of smart pointers and __std_allocators__.

Compile-time polymorphism can be used where appropriate,

```
template<class T>
void do_something()
{
    // [...]
    T x = T(a, b);

    // for conceptually similar objects x we neither need virtual
    // functions nor a common base class in this context.
    // [...]
}
```

Now, to allow inhomogeneous signatures for the constructors of the types passed
in for `T` we can use __value_factory__ and __boost__bind__ to normalize
between them.

```
template<class ValueFactory>
void do_something(ValueFactory make_obj = ValueFactory())
{
    // [...]
    typename ValueFactory::result_type x = make_obj(a, b);

    // for conceptually similar objects x we neither need virtual
    // functions nor a common base class in this context.
    // [...]
}

int main()
{
    // [...]

    do_something(boost::value_factory<X>());
    do_something(boost::bind(boost::value_factory<Y>(), _1, 5, _2));
    // construct X(a, b) and Y(a, 5, b), respectively.

    // [...]
}
```

Maybe we want our objects to outlive the function's scope, in this case we have
to use dynamic allocation;

```
template<class Factory>
whatever do_something(Factory new_obj = Factory())
{
    typename Factory::result_type ptr = new_obj(a, b);

    // again, no common base class or virtual functions needed,
    // we could enforce a polymorphic base by writing e.g.
    //     boost::shared_ptr<base>
    // instead of
    //     typename Factory::result_type
    // above.
    // Note that we are also free to have the type erasure happen
    // somewhere else (e.g. in the constructor of this function's
    // result type).

    // [...]
}

// [... call do_something like above but with boost::factory instead
// of boost::value_factory]
```

Although we might have created polymorphic objects in the previous example, we
have used compile time polymorphism for the factory. If we want to erase the
type of the factory and thus allow polymorphism at run time, we can use
__boost_function__ to do so. The first example can be rewritten as follows.

```
typedef boost::function<an_abstract_class*()> a_factory;

// [...]

int main()
{
    std::map<std::string, a_factory> factories;

    // [...]

    factories["a_name"] = boost::factory<a_concrete_class*>();
    factories["another_name"] = boost::factory<another_concrete_class*>();

    // [...]
}
```

Of course we can just as easy create factories that take arguments and/or
return __smart_pointers__.

[endsect]

[section:reference Reference]

[section value_factory]

[heading Description]

Function object template that invokes the constructor of the type `T`.

[heading Header]

```
#include <boost/functional/value_factory.hpp>
```

[heading Synopsis]

```
namespace boost {

template<class T>
class value_factory;

} // boost
```

[variablelist Notation
[[`T`][an arbitrary type with at least one public constructor]]
[[`a0`...`aN`][argument values to a constructor of `T`]]
[[`F`][the type `value_factory<F>`]]
[[`f`][an instance object of `F`]]]

[heading Expression Semantics]

[table
[[Expression][Semantics]]
[[`F()`][creates an object of type `F`.]]
[[`F(f)`][creates an object of type `F`.]]
[[`f(a0`...`aN)`][returns `T(a0`...`aN)`.]]
[[`F::result_type`][is the type `T`.]]]

[heading Limits]

Before C++11, the maximum number of arguments supported is 10. Since C++11 an
arbitrary number of arguments is supported.

[endsect]

[section factory]

[heading Description]

Function object template that dynamically constructs a pointee object for the
type of pointer given as template argument. Smart pointers may be used for the
template argument, given that `pointer_traits<Pointer>::element_type` yields
the pointee type.

If an __allocator__ is given, it is used for memory allocation and the
placement form of the `new` operator is used to construct the object. A
function object that calls the destructor and deallocates the memory with a
copy of the Allocator is used for the second constructor argument of `Pointer`
(thus it must be a __smart_pointer__ that provides a suitable constructor,
such as __boost__shared_ptr__).

If a third template argument is `factory_passes_alloc_to_smart_pointer`, the
allocator itself is used for the third constructor argument of `Pointer`
(__boost__shared_ptr__ then uses the allocator to manage the memory of its
separately allocated reference counter).

[heading Header]

```
#include <boost/functional/factory.hpp>
```

[heading Synopsis]

```
namespace boost {

enum factory_alloc_propagation {
    factory_alloc_for_pointee_and_deleter,
    factory_passes_alloc_to_smart_pointer
};

template<class Pointer,
    class Allocator = void,
    factory_alloc_propagation Policy = factory_alloc_for_pointee_and_deleter>
class factory;

} // boost
```

[variablelist Notation
[[`T`][an arbitrary type with at least one public constructor]]
[[`P`][pointer or smart pointer to `T`]]
[[`a0`...`aN`][argument values to a constructor of `T`]]
[[`F`][the type `factory<P>`]]
[[`f`][an instance object of `F`]]]

[heading Expression Semantics]

[table
[[Expression][Semantics]]
[[`F()`][creates an object of type `F`.]]
[[`F(f)`][creates an object of type `F`.]]
[[`f(a0`...`aN)`]
[dynamically creates an object of type `T` using `a0`...`aN` as arguments for
the constructor invocation.]]
[[`F::result_type`][is the type `P` with top-level cv-qualifiers removed.]]]

[heading Limits]

Before C++11, the maximum number of arguments supported is 10. Since C++11 an
arbitrary number of arguments is supported.

[endsect]

[endsect]

[section Changes]

[heading Boost 1.72.0]

Glen Fernandes rewrote the implementations of `factory` and `value_factory` to
provide the following features:

* Support r-value arguments when available
* Support arbitrary number of arguments via variadic templates when available
* Support allocators that are final
* Support allocators that use fancy pointers
* Support for disabled exceptions (`BOOST_NO_EXCEPTIONS`)
* Improved compilation times

The following features have been removed:

* Increasing limits for C++03 compilers through
  `BOOST_FUNCTIONAL_VALUE_FACTORY_MAX_ARITY`
* Using `boost::none_t` in place of `void` through
  `BOOST_FUNCTIONAL_FACTORY_SUPPORT_NONE_T`

[heading Boost 1.58.0]

In order to remove the dependency on Boost.Optional, the default parameter for
allocators has been changed from `boost::none_t` to `void`. If you have code
that has stopped working because it uses `boost::none_t`, a quick fix is to
define `BOOST_FUNCTIONAL_FACTORY_SUPPORT_NONE_T`, which will restore support,
but this will be removed in a future release. It should be be relatively easy
to fix this properly.

[endsect]

[section Acknowledgements]

Tobias Schwinger for creating this library.

Eric Niebler requested a function to invoke a type's constructor (with the
arguments supplied as a Tuple) as a Fusion feature. These Factory utilities are
a factored-out generalization of this idea.

Dave Abrahams suggested Smart Pointer support for exception safety, providing
useful hints for the implementation.

Joel de Guzman's documentation style was copied from Fusion.

Peter Dimov for sharing his insights on language details and their evolution.

[endsect]

[section References]

# [@http://en.wikipedia.org/wiki/Design_Patterns Design Patterns],
Gamma et al. - Addison Wesley Publishing, 1995

# [@https://boost.org/sgi/stl/ Standard Template Library Programmer's Guide],
Hewlett-Packard Company, 1994

# [@http://www.boost.org/libs/bind/bind.html Boost.Bind],
Peter Dimov, 2001-2005

# [@http://www.boost.org/doc/html/function.html Boost.Function],
Douglas Gregor, 2001-2004

[endsect]