/* Minimal object-oriented facilities for C. Copyright (C) 2006 Free Software Foundation, Inc. Written by Bruno Haible , 2006. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ /* This file defines minimal facilities for object-oriented programming style in ANSI C. The facilities allow to define classes with single inheritance and "virtual" methods. Strict type checking is provided in combination with a C++ compiler: The code compiles in ANSI C with less strict type checking; when compiled with a C++ compiler, strict type checking is done. In contrast to [OOC] and [OOPC], this implementation concentrates on the bare essentials of an object-oriented programming style. It does not provide features that are "sometimes useful", but not essential. Features: - Combination of fields and methods into a single object. YES - Description of objects of same shape and same behaviour by a class. YES - Single inheritance. YES - Multiple inheritance. NO - Operator overloading (compile-time polymorphism). NO - Virtual methods (run-time polymorphism). YES - Information hiding: private/protected/public. private fields - Static fields and methods. NO - Constructors, destructors. NO - 'new', 'delete'. NO - Exception handling. NO - Garbage collection. NO - Templates / Generic classes with parameters. NO - Namespaces. NO - Hidden 'this' pointer in methods. NO - Declaring or implementing several classes in the same file. NO Rationale for NO: - Multiple inheritance is not supported because programming languages like Java and C# prove that they are not necessary. Modern design patterns use delegation more often than composition; this reduces the pressure to use multiple inheritance. Multiple inheritance of "interfaces" (classes without fields) might be considered, though. - Operator overloading is not essential: The programmer can rename methods so that they carry unambiguous method names. This also makes the code more readable. - Virtual methods are supported. Non-virtual methods are not: they constitute an assumption about the possible subclasses which is more often wrong than right. In other words, non-virtual methods are a premature optimization - "the root of all evil", according to Donald E. Knuth. - Information hiding: 'protected' is not supported because it is always inappropriate: it prohibits the use of the delegation design pattern. 'private' is implemented on fields. There are no 'public' fields, since the use of getters/setters allows for overriding in subclasses and is more maintainable (ability to set breakpoints). On the other hand, all methods are 'public'. 'private` methods are not supported because methods with static linkage can be used instead. - Static fields and methods are not supported because normal variables and functions with static or extern linkage can be used instead. - Constructors and destructors are not supported. The programmer can define 'init' and 'do_free' methods himself. - 'new', 'delete' are not supported because they only provide the grouping of two lines of code into a single line of code. - Exception handling is not supported because conventions with a return code can be used instead. - Garbage collection is not supported. Without it the programmer's life is harder, but not impossible. The programmer has to think about ownership of objects. - Templates / Generic classes with parameters are not supported because they are mostly used for container classes, and container classes can be implemented in a simpler object-oriented way that requires only a very limited form of class inheritance. - Namespaces are not implemented, because they can be simulated by a consistent naming convention. - A hidden 'this' pointer in methods is not implemented. It reduces the transparency of the code (because what looks like a variable access can be an access through 'this') and is simply not needed. - Declaring or implementing several classes in the same file is not supported, because it is anyway good practice to define each class in its own .oo.h / .oo.c file. Syntax: The syntax resembles C++, but deviates from C++ where the C++ syntax is just too braindead. A root class is declared in a .oo.h file: struct rootfoo { methods: int method1 (rootfoo_t x, ...); ... }; and in the corresponding .oo.c file: struct rootfoo { fields: int field1; ... }; A subclass is declared in a .oo.h file as well: struct subclass : struct rootfoo { methods: int method2 (subclass_t x, ...); ... }; and in the corresponding .oo.c file: struct subclass : struct rootfoo { fields: int field2; ... }; This defines: - An incomplete type 'struct any_rootfoo_representation' or 'struct subclass_representation', respectively. It denotes the memory occupied by an object of the respective class. The prefix 'any_' is present only for a root class. - A type 'rootfoo_t' or 'subclass_t' that is equivalent to a pointer 'struct any_rootfoo_representation *' or 'struct subclass_representation *', respectively. - A type 'struct rootfoo_implementation' or 'struct subclass_implementation', respectively. It contains a virtual function table for the corresponding type. - A type 'struct rootfoo_representation_header' or 'struct subclass_representation_header', respectively, that defines the part of the memory representation containing the virtual function table pointer. - Functions 'rootfoo_method1 (rootfoo_t x, ...);' ... 'subclass_method1 (subclass_t x, ...);' ... 'subclass_method2 (subclass_t x, ...);' ... that invoke the corresponding methods. They are realized as inline functions if possible. - A declaration of 'rootfoo_typeinfo' or 'subclass_typeinfo', respectively, each being a typeinfo_t instance. - A declaration of 'ROOTFOO_SUPERCLASSES' or 'SUBCLASS_SUPERCLASSES', respectively, each being an initializer for an array of typeinfo_t. - A declaration of 'ROOTFOO_SUPERCLASSES_LENGTH' or 'SUBCLASS_SUPERCLASSES_LENGTH', respectively, each denoting the length of that initializer. - A declaration of 'rootfoo_vtable' or 'subclass_vtable', respectively, being an instance of 'struct rootfoo_implementation' or 'struct subclass_implementation', respectively. - A header file "rootfoo.priv.h" or "subclass.priv.h" that defines the private fields of 'struct rootfoo_representation' or 'struct subclass_representation', respectively. A class implementation looks like this, in a .oo.c file: struct subclass : struct rootfoo { fields: int field2; ... }; int subclass::method1 (subclass_t x, ...) { ... } [optional] int subclass::method2 (subclass_t x, ...) { ... } ... At the place of the second "struct subclass" definition, the type 'struct subclass_representation' is expanded, and the macro 'super' is defined, referring to the vtable of the superclass. For root classes, 'super' is not defined. Also, 'subclass_typeinfo' is defined. Each method subclass::method_i defines the implementation of a method for the particular class. Its C name is subclass__method_i (not to be confused with subclass_method_i, which is the externally visible function that invokes this method). Methods that are not defined implicitly inherited from the superclass. At the end of the file, 'subclass_vtable' is defined, as well as 'subclass_method1 (subclass_t x, ...);' ... 'subclass_method2 (subclass_t x, ...);' ... if they were not already defined as inline functions in the header file. Object representation in memory: - Objects have as their first field, called 'vtable', a pointer to a table to data and function pointers that depend only on the class, not on the object instance. - One of the first fields of the vtable is a pointer to the 'superclasses'; this is a NULL-terminated array of pointers to typeinfo_t objects, starting with the class itself, then its superclass etc. [OOC] Axel-Tobias Schreiner: Object-oriented programming with ANSI-C. 1993. [OOPC] Laurent Deniau: Object Oriented Programming in C. 2001. */ #ifndef _MOO_H #define _MOO_H /* Get size_t, abort(). */ #include /* An object of this type is defined for each class. */ typedef struct { const char *classname; } typeinfo_t; /* IS_INSTANCE (OBJ, ROOTCLASSNAME, CLASSNAME) tests whether an object is instance of a given class, given as lower case class name. */ #define IS_INSTANCE(obj,rootclassname,classname) \ (((const struct rootclassname##_representation_header *)(const struct any_##rootclassname##_representation *)(obj))->vtable->superclasses_length \ >= classname##_SUPERCLASSES_LENGTH \ && ((const struct rootclassname##_representation_header *)(const struct any_##rootclassname##_representation *)(obj))->vtable->superclasses \ [((const struct rootclassname##_representation_header *)(const struct any_##rootclassname##_representation *)(obj))->vtable->superclasses_length \ - classname##_SUPERCLASSES_LENGTH] \ == & classname##_typeinfo) /* This instance test consists of two comparisons. One could even optimize this to a single comparison, by limiting the inheritance depth to a fixed limit, for example, say, depth <= 10. The superclasses list would then need to be stored in reverse order, from the root down to the class itself, and be filled up with NULLs so that the array has length 10. The instance test would look like this: #define IS_INSTANCE(obj,rootclassname,classname) \ (((const struct rootclassname##_representation_header *)(const struct any_##rootclassname##_representation *)(obj))->vtable->superclasses \ [classname##_SUPERCLASSES_LENGTH - 1] \ == & classname##_typeinfo) but the classname##_superclasses_length would no longer be available as a simple sizeof expression. */ #endif /* _MOO_H */