haiku/docs/user/support/Flattenable.dox

/*
 * Copyright 2007 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk <niels.reedijk@gmail.com>
 *
 * Corresponds to:
 *		headers/os/support/Flattenable.h	rev 19972
 *		src/kits/support/Flattenable.cpp	rev 12963
 */


/*!
	\file Flattenable.h
	\ingroup support
	\ingroup libbe
	\brief Provides the BFlattenable interface
*/


/*!
	\class BFlattenable
	\ingroup support
	\ingroup libbe
	\brief Interface for classes that can flatten and unflatten themselves to
		   a stream of bytes.

	It is convenient that objects can be stored as a flat stream of bytes. In
	this way, they can be written to disk, exchanged between applications or send
	over networks. This ability, which is known in many other programming
	languages as marshalling, is not native in C++. The Haiku API has created a
	universal interface that classes have if they are able to be flattened. This
	class defines the interface. This class does nothing on its own, and
	therefore contains pure virtuals. By inheriting this class and inmplementing
	the methods in your own class, you will be able to use your objects as
	flattenable objects throughout the Haiku API.

	Flattened objects can be used for example when sending messages within an
	application or between applications. The BMessage class uses the interface
	to store and transmit custom classes. 

	If you want to be able to flatten your objects, you will need to implement
	various methods. Flatten() and Unflatten() are where the magic happen. These
	methods handle the actual flattening and unflattening. To identify flattened
	data in for example BMessage, the object has a type_code. Type codes are
	four byte long integers. You can choose to flatten to one of the existing
	types, if you are certain that you are compatible to those, but you'll
	usually define your own type. Your best option is by using a multicharacter
	constant, such as 'STRI'. Implement TypeCode() to return the type you
	support. Implement FlattenedSize() to make sure that other objects can
	provide the right buffers. Implement IsFixedSize() to return whether your
	objects always store to a fixed size.

	See the following example:
\code
type_code CUSTOM_STRING_TYPE = 'CUST';

class CustomString : public BFlattenable
{
public:
    char data[100];

    // From BFlattenable
    bool IsFixedSize() const { return false; };
    type_code TypeCode() const { return CUSTOM_STRING_TYPE; };
    ssize_t FlattenedSize() const { return strlen(data); };

    status_t Flatten(void* buffer, ssize_t size) const
    {
        if ((strlen(data) + 1) < size)
            return B_BAD_VALUE;
        memcpy(buffer, data, size);
        return B_OK;
    };

    status_t Unflatten(type_code code, const void* buffer, ssize_t size)
    {
        if (code != CUSTOM_STRING_TYPE)
            return B_BAD_TYPE;
        if (size > 100)
            return B_NO_MEMORY;
        memcpy(data, buffer, size);
        return B_OK;
    };
};
\endcode

	Have a look at TypeConstants.h for a list of all the types that the Haiku
	API defines.

	The Haiku API has a second interface for storing objects, which is with
	BArchivable. BArchivable is for more complex cases. Instead of one flat
	datastream, it stores an object in a BMessage. In that way you can reflect
	internals of a class better. It also provides an interface for instantiating
	objects, that is, for objects to restore themselves from a BMessage. In
	essence, BArchivable is more suitable for objects that are alive. In short
	BFlattenable is for data objects, BArchivable is for 'live' objects.

	Other classes in the API that support flattening and unflattening are for
	example BMessage, which enables you to conveniently write flattened data
	to disk. Another example is BPath. Because of that you can store paths and
	send them over via messages. Throughout the Haiku API you will find classes
	that provide the flattening interface.
*/


/*!
	\fn virtual bool BFlattenable::IsFixedSize() const
	\brief Pure virtual that should return whether or not flattened objects of 
		   this type always have a fixed size.
*/


/*!
	\fn virtual type_code BFlattenable::TypeCode() const
	\brief Pure virtual that returns the type_code this class flattens to.

	\return Either one of the existing typecodes found in TypeConstants.h
		if your class actually is compatible to those formats, or a
		custom four-byte integer constant if not.
*/


/*!
	\fn virtual ssize_t BFlattenable::FlattenedSize() const
	\brief Pure virtual that should return the size of the flattened object in
		   bytes.
*/


/*!
	\fn virtual status_t BFlattenable::Flatten(void* buffer, ssize_t size) const
	\brief Pure virtual that should flatten the object into the supplied
		   \a buffer.

	Please make sure that you check that the supplied buffer is not a \c NULL
	pointer. Also make sure that the size of the flattened object does isn't
	larger than the size of the buffer.

	\param buffer The buffer to flatten in.
	\param size The size of the buffer.

	\retval B_OK The object was flattened.
	\retval B_NO_MEMORY The buffer was smaller than required.
	\retval B_BAD_VALUE The buffer was a \c NULL pointer.
*/


/*!
	\fn bool BFlattenable::AllowsTypeCode(type_code code) const
	\brief Get whether or not the supplied type_code is supported.

	This default implementation checks the \a code argument against the type_code
	returned by TypeCode().

	\param code The type_code constant you want to check for.

	\returns Whether or not the supplied type_code is supported.
	\retval true The type_code is supported.
	\retval false The type_code is not supported.
*/


/*!
	\fn virtual status_t BFlattenable::Unflatten(type_code code,
		const void* buffer, ssize_t size)
	\brief Pure virtual that should unflatten the buffer and put the contents
		   into the current object.

	Make sure that the supplied buffer is not \c NULL and that you actually
	support the typecode.

	\param code The type_code this data is.
	\param buffer The buffer to unflatten the data from.
	\param size The size of the data.

	\returns A status code.
	\retval B_OK The object is unflattened.
	\retval B_BAD_VALUE The \a buffer pointer is \c NULL or the data is invalid.
	\retval B_BAD_TYPE You don't support data with this \a code. 
*/


/*!
	\fn virtual BFlattenable::~BFlattenable()
	\brief Destructor. Does nothing.
*/