1/* 2 * Copyright 2007 Haiku, Inc. All rights reserved. 3 * Distributed under the terms of the MIT License. 4 * 5 * Authors: 6 * Niels Sascha Reedijk <niels.reedijk@gmail.com> 7 * 8 * Corresponds to: 9 * headers/os/support/Flattenable.h rev 39675 10 * src/kits/support/Flattenable.cpp rev 12963 11 */ 12 13 14/*! 15 \file Flattenable.h 16 \ingroup support 17 \ingroup libbe 18 \brief Provides the BFlattenable interface 19*/ 20 21 22/*! 23 \class BFlattenable 24 \ingroup support 25 \ingroup libbe 26 \brief Interface for classes that can flatten and unflatten themselves to 27 a stream of bytes. 28 29 It is convenient that objects can be stored as a flat stream of bytes. In 30 this way, they can be written to disk, exchanged between applications or send 31 over networks. This ability, which is known in many other programming 32 languages as marshalling, is not native in C++. The Haiku API has created a 33 universal interface that classes have if they are able to be flattened. This 34 class defines the interface. This class does nothing on its own, and 35 therefore contains pure virtuals. By inheriting this class and inmplementing 36 the methods in your own class, you will be able to use your objects as 37 flattenable objects throughout the Haiku API. 38 39 Flattened objects can be used for example when sending messages within an 40 application or between applications. The BMessage class uses the interface 41 to store and transmit custom classes. 42 43 If you want to be able to flatten your objects, you will need to implement 44 various methods. Flatten() and Unflatten() are where the magic happen. These 45 methods handle the actual flattening and unflattening. To identify flattened 46 data in for example BMessage, the object has a type_code. Type codes are 47 four byte long integers. You can choose to flatten to one of the existing 48 types, if you are certain that you are compatible to those, but you'll 49 usually define your own type. Your best option is by using a multicharacter 50 constant, such as 'STRI'. Implement TypeCode() to return the type you 51 support. Implement FlattenedSize() to make sure that other objects can 52 provide the right buffers. Implement IsFixedSize() to return whether your 53 objects always store to a fixed size. 54 55 See the following example: 56\code 57type_code CUSTOM_STRING_TYPE = 'CUST'; 58 59class CustomString : public BFlattenable 60{ 61public: 62 char data[100]; 63 64 // From BFlattenable 65 bool IsFixedSize() const { return false; }; 66 type_code TypeCode() const { return CUSTOM_STRING_TYPE; }; 67 ssize_t FlattenedSize() const { return strlen(data); }; 68 69 status_t Flatten(void* buffer, ssize_t size) const 70 { 71 if ((strlen(data) + 1) < size) 72 return B_BAD_VALUE; 73 memcpy(buffer, data, size); 74 return B_OK; 75 }; 76 77 status_t Unflatten(type_code code, const void* buffer, ssize_t size) 78 { 79 if (code != CUSTOM_STRING_TYPE) 80 return B_BAD_TYPE; 81 if (size > 100) 82 return B_NO_MEMORY; 83 memcpy(data, buffer, size); 84 return B_OK; 85 }; 86}; 87\endcode 88 89 Have a look at TypeConstants.h for a list of all the types that the Haiku 90 API defines. 91 92 The Haiku API has a second interface for storing objects, which is with 93 BArchivable. BArchivable is for more complex cases. Instead of one flat 94 datastream, it stores an object in a BMessage. In that way you can reflect 95 internals of a class better. It also provides an interface for instantiating 96 objects, that is, for objects to restore themselves from a BMessage. In 97 essence, BArchivable is more suitable for objects that are alive. In short 98 BFlattenable is for data objects, BArchivable is for 'live' objects. 99 100 Other classes in the API that support flattening and unflattening are for 101 example BMessage, which enables you to conveniently write flattened data 102 to disk. Another example is BPath. Because of that you can store paths and 103 send them over via messages. Throughout the Haiku API you will find classes 104 that provide the flattening interface. 105*/ 106 107 108/*! 109 \fn virtual bool BFlattenable::IsFixedSize() const 110 \brief Pure virtual that should return whether or not flattened objects of 111 this type always have a fixed size. 112*/ 113 114 115/*! 116 \fn virtual type_code BFlattenable::TypeCode() const 117 \brief Pure virtual that returns the type_code this class flattens to. 118 119 \return Either one of the existing typecodes found in TypeConstants.h 120 if your class actually is compatible to those formats, or a 121 custom four-byte integer constant if not. 122*/ 123 124 125/*! 126 \fn virtual ssize_t BFlattenable::FlattenedSize() const 127 \brief Pure virtual that should return the size of the flattened object in 128 bytes. 129*/ 130 131 132/*! 133 \fn virtual status_t BFlattenable::Flatten(void* buffer, ssize_t size) const 134 \brief Pure virtual that should flatten the object into the supplied 135 \a buffer. 136 137 Please make sure that you check that the supplied buffer is not a \c NULL 138 pointer. Also make sure that the size of the flattened object does isn't 139 larger than the size of the buffer. 140 141 \param buffer The buffer to flatten in. 142 \param size The size of the buffer. 143 144 \retval B_OK The object was flattened. 145 \retval B_NO_MEMORY The buffer was smaller than required. 146 \retval B_BAD_VALUE The buffer was a \c NULL pointer. 147*/ 148 149 150/*! 151 \fn bool BFlattenable::AllowsTypeCode(type_code code) const 152 \brief Get whether or not the supplied type_code is supported. 153 154 This default implementation checks the \a code argument against the type_code 155 returned by TypeCode(). 156 157 \param code The type_code constant you want to check for. 158 159 \returns Whether or not the supplied type_code is supported. 160 \retval true The type_code is supported. 161 \retval false The type_code is not supported. 162*/ 163 164 165/*! 166 \fn virtual status_t BFlattenable::Unflatten(type_code code, 167 const void* buffer, ssize_t size) 168 \brief Pure virtual that should unflatten the buffer and put the contents 169 into the current object. 170 171 Make sure that the supplied buffer is not \c NULL and that you actually 172 support the typecode. 173 174 \param code The type_code this data is. 175 \param buffer The buffer to unflatten the data from. 176 \param size The size of the data. 177 178 \returns A status code. 179 \retval B_OK The object is unflattened. 180 \retval B_BAD_VALUE The \a buffer pointer is \c NULL or the data is invalid. 181 \retval B_BAD_TYPE You don't support data with this \a code. 182*/ 183 184 185/*! 186 \fn virtual BFlattenable::~BFlattenable() 187 \brief Destructor. Does nothing. 188*/ 189