Public Member Functions | |
| virtual bool | AllowsTypeCode (type_code code) const |
| Return whether or not the supplied type_code is supported. | |
| virtual status_t | Flatten (void *buffer, ssize_t size) const =0 |
| Pure virtual that should flatten the object into the supplied buffer. | |
| virtual ssize_t | FlattenedSize () const=0 |
| Pure virtual that should return the size of the flattened object in bytes. | |
| virtual bool | IsFixedSize () const=0 |
| Pure virtual that should return whether or not flattened objects of this type always have a fixed size. | |
| virtual type_code | TypeCode () const=0 |
| Pure virtual that should return which type_code this class flattens to. | |
| virtual status_t | Unflatten (type_code code, const void *buffer, ssize_t size)=0 |
| Pure virtual that should unflatten the buffer and put the contents into the current object. | |
| virtual | ~BFlattenable () |
| Destructor. Does nothing. | |
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:
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; }; };
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.
| bool BFlattenable::AllowsTypeCode | ( | type_code | code | ) | const [virtual] |
Return whether or not the supplied type_code is supported.
This default implementation checks the code argument against the type_code returned by TypeCode().
| code | The type_code constant you want to check for. |
| true | The type_code is supported. | |
| false | The type_code is not supported. |
| virtual status_t BFlattenable::Flatten | ( | void * | buffer, | |
| ssize_t | size | |||
| ) | const [pure virtual] |
Pure virtual that should flatten the object into the supplied buffer.
Please make sure that you check that the supplied buffer is not a NULL pointer. Also make sure that the size of the flattened object does isn't larger than the size of the buffer.
| buffer | The buffer to flatten in. | |
| size | The size of the buffer. |
| B_OK | The object was flattened. | |
| B_NO_MEMORY | The buffer was smaller than required. | |
| B_BAD_VALUE | The buffer was a NULL pointer. |
| type_code BFlattenable::TypeCode | ( | ) | const [pure virtual] |
Pure virtual that should return which type_code this class flattens to.
| status_t BFlattenable::Unflatten | ( | type_code | code, | |
| const void * | buffer, | |||
| ssize_t | size | |||
| ) | [pure virtual] |
Pure virtual that should unflatten the buffer and put the contents into the current object.
Make sure that the supplied buffer is not NULL and that you actually support the typecode.
| code | The type_code this data is. | |
| buffer | The buffer to unflatten the data from. | |
| size | The size of the data. |
| B_OK | The object is unflattened. | |
| B_BAD_VALUE | The buffer pointer is NULL or the data is invalid. | |
| B_BAD_TYPE | You don't support data with this code. |