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## Chapter 1. Boost.Functional/Factory 1.0

### Tobias Schwinger

### Glen Fernandes

Copyright © 2007, 2008 Tobias Schwinger

Copyright © 2019 Glen Joseph Fernandes

Distributed under the Boost Software License, Version 1.0.

## 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.

## 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 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.

## Reference

### value_factory

##### Description

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

##### Header

    #include <boost/functional/value_factory.hpp>

##### Synopsis

    namespace boost {

    template<class T>
    class value_factory;

    } // boost

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`

##### Expression Semantics

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`.

##### Limits

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

### factory

##### 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).

##### Header

    #include <boost/functional/factory.hpp>

##### 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

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`

##### Expression Semantics

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.

##### Limits

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

## Changes

#### 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`

#### 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.

## 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.

## References

1. Design Patterns, Gamma et al. - Addison Wesley Publishing, 1995
2. Standard Template Library Programmer's Guide, Hewlett-Packard Company, 1994
3. Boost.Bind, Peter Dimov, 2001-2005
4. Boost.Function, Douglas Gregor, 2001-2004

Last revised: December 02, 2021 at 06:52:06 GMT |