Now, let's proceed to something more interesting. * PUBLIC METHODS Let's implement virtual methods: As you see, in these implementations we use myderived__get_by_mybase() This suggestion makes me think you haven't read the article carefully. 0000001436 00000 n points to an instance of some particular derived class. 0000012581 00000 n Built on Forem the open source software that powers DEV and other inclusive communities. http://www.codeproject.com/Articles/22139/Simply-Object-Oriented-C. The usual C approach to calculate things like CRC is to have one-function module that calculates crc of the buffer. There is a very important design principle: program to an interface, not an Great write-up and awesome insight - thank you. I have a trivial question. appropriate version of some_method() will be called, and when my_func() the configuration information for the specified I2C. So I'm not a fan of casting in the first place. And we have 50 instances, it is a whole 1 kB. Please note that things I'm going to tell you in this section are pretty trivial if you're familiar with basic OO techniques. If in some application this is not a problem, I might do that. Meanwhile, the main reason for this article is to show programmers how to write embedded C codes in a more structured way for readability, reliability, maintainability and portability. Although the techniques described below won't be very popular nowadays (after all, why bother using C if we're going to write object-oriented code? mybase might be not a first field of T_MyDerived. The reason why I asked about 2-level inheritance or inheritance from a second base class is because I'm think about how to handle exceptions in C. I was thinking to write an exception handler class which makes available stuff to subclasses.Like constructor and destructor, every object must know itself how to handle its exception in detail. implementation. The I think this is your github page but I don't see it there, either (https://github.com/dimonomid). 0000125107 00000 n Now, let's create EEPROM functions to be used: It's time to create a variable in main.c or any other source file within your project. //-- firstly, just copy vtable of base class. Embedded Programming Video Course Shows How OOP Works Under the Hood, Embedded Programming Video Course Teaches RTOS, New Version of QCalc Programmers Calculator. But I think it should be separated into an extra class. This is completely unacceptable. We're a place where coders share, stay up-to-date and grow their careers. I haven't written any technical article since the last two years, as a contributor to the Opensource community, writing is one of the ways to help grow the community. Right, thanks for reporting. If it's about refactoring existing 1-level inheritance into 2-level inheritance, then again, you'd have to implement it manually. subclass. */, /** #define MAX_UART_BUFFER_SIZE 512 0000070759 00000 n ******************************************************************************/, /******************************************************************************* By the way, where in my UML did you see 2-level inheritance? I really hope you find this article meaningful, please comment if you have any question and also help to share this content so that I can another article to show you how to create a private class in C. Templates let you quickly answer FAQs or store snippets for re-use. So, when first instance of the class MyBase is created, its vtable gets initialized. So, this approach doesn't waste your RAM, which is always insufficient in the embedded world. I will try to write some meta code and write it here again.

* Calculates offset of member inside the struct 1. relative offset from relativ adress 0x00 of super inside the substruct, 2. real_address_of_substruct == real_adress_of_superstruct - relative_offset_inside_substruct. 0000152041 00000 n So let's create EEPROM and UART struct type: The next step is to create the functions that will be needed in the code, so here I have created a va_arg function type for my serial print application. In bare metal embedded programming, C is still mostly used because of its closeness to hardware low levels (assembly language and machine language). Home - Blog - Embedded Programming Video Course Shows How OOP Works Under the Hood. Instead, I'd like to be able to calculate CRC gradually: when next frame (say, 64 bytes) is received from host, the device programs it to flash, then reads it back, and accumulates (feeds) this data to global CRC. Next, you will translate the C design into C++ using a class and again you inspect the generated code to compare it with C. Finally, you will see how Encapsulation relates to concurrent programming with an RTOS. To override a function in the base you just overwrite the pointer. in this struct: The address of b is guaranteed to be higher on all platforms, independently of whether the stack grows upwards or downwards. At the end of this article, I'll explain usage of it in more detail. And then, we can obtain a pointer to the wrapper derived instance by calling _dtor and _free) : you might have noticed that we've left We want to have object Crc which has two methods: Since C doesn't support object-oriented programming, we have to manually pass pointer to the object for which method is called. How you handle exceptions? Let's get to constructor. If they match, operation considered successful. // it's ok: then, methods of base class will be called. ******************************************************************************/, /** 0000005893 00000 n 0000018566 00000 n I consider casting from derived to parent as a very bad practice; more, it doesn't allow us to use multiple inheritance. * especially in embedded world. _superclass_pt_ \ passing a pointer to where the error should be stored. Personally I find it useful to create special structure for constructor params, and implement constructor so that it takes pointer to that struct. Effectively, this is approximately what the C++ compiler does for you behind the scenes when you use virtual methods (well, in fact, C++ compiler can be even smarter: it can eliminate pointers to non-overridden methods, but maintaining that manually in C is hardly a way to go). The constructor usually needs to have some parameters. ((_T_Subclass_ *)( \ here we first copy vtable from base, and then override what we want: Please note that it's quite fine not to override some method (other than Let's start from very simple yet realistic example: the CRC calculator. 0000150585 00000 n ******************************************************************************/, /******************************************************************************* I based my OOC solution heavily on your methods and they work great for me. The derived inherit implementation, so we could omit call of base method there. of some particular subclass: _super_vtable will be explained later, but I bet you've already guessed Thank you for a professorial article. should make derived class accessible through that interface.

This practice is error-prone; It violates the DRY principle (Don't Repeat Yourself). Every object has a status like ok or an exception status. So, base-to-derived conversion will be done as follows: Let's get to virtual methods implementation for derived class. 0000005538 00000 n 0000151544 00000 n Ahh you right. 0000125176 00000 n (unsigned char *)(_superclass_pt_) \ But if we don't, then: After some research, I decided to make it non-const, and use lazy initialization. struct Dervied { struct Base base; int additionalMember};) This enables you to cast the struct into the parent struct type and allows the parent functions to operate on it as normal. In the code below, I have included string.h for memset, strlen and stdarg.h for va_list, va_start, vsnprintf and va_end. Not sure I'll be able to get time for that in a near future, sorry. 96 0 obj <> endobj So if we create two instances of T_MyBase: my_base_1 and my_base_2, Generate base class Shape and subclass Circle: Generate base class Shape and two subclasses: Circle and MyShape: After either command, the directory shape containing all generated files will be created in the current directory. It was an old mercurial repo and bitbucket discontinued mercurial support a while ago. Do you use the CExceptions or CExcept frameworks? When we add some new virtual method to vtable, we need to manually modify vtable of all subclasses. Your article helped me understand the concept as a whole. But if it's rather little project on a two-dollar MCU, it might be the way to go. For that to be done, OO-approach comes to the rescue. Needs to be overridden because pointer to subclass not // NOTE: this is a subclass, so that after performing destruction code. if i take a look into the deatail of OOC_GET_CONTAINER_PT: Is it possible that it only works if the memory grows from low to hight? You will see the benefits of polymorphism and understand its overheads. Is there already a small project available? In embedded C, it's highly recommended to always typedef your struct and enum. As I already mentioned, I use inheritance when I want some objects to share :)). type information). it: Then, we can equally use base or derived class for that. Now, subclass can call _mybase__vtable__get to get pointer to the vtable of base class. In order to use generated classes, you also need for ooc.c and ooc.h, and configuration file ooc_cfg.h (initially, just copy default ooc_cfg_default.h as ooc_cfg.h in your project), Discuss on reddit: or on Hacker News: Thanks, as I mentioned in the article, there are quite a few existing publications on the topic. %%EOF If anything, a _t suffix is sort of accepted (even stdlib uses it), so these days I'd probably write foo_t instead of T_Foo. Sorry for not adding a license file, but it's BSD, so feel free to use them in any projects. Just created a new git repo and pushed on github: https://github.com/dimonomid/ooc. Additionally, we should call method of base class where appropriate. How can I get it to run and what must I do to test the code in the classes? process more, so I've written in in verbose manner. 0000025060 00000 n Note that when we call mybase__some_method() on an instance of T_MyDerived, want most of my code not to care about exact object classes with which it This second lesson on Object-Oriented Programming (OOP) introduces the concept of Inheritance, which is a mechanism for reusing common attributes and operations among classes. And we use them in some GUI-extensive application with lots of shapes: let it be 100 Circles, 100 Triangles and 100 Rectangles. The Go's approach is to have a second value returned from a function, so it's easy to return an error; obviously it's not as flawless in C where a function can only return a single value, but we can work it around by e.g. At least, the following items should be virtual: Virtual destructor is quite common thing: when we destruct an instance of subclass, then destructor of derived class should be called first, and then destructor of base class should be called. object it works with; it knows the interface. Of course we may write it more compact, but I wanted to elaborate on the * PUBLIC TYPES There are C++ compilers for some embedded CPUs, but they are uncommon, so I need to stick to C, in the name of portability. #define EEPROM_WRITE_TIME 5 Secondly, we can't just typecast from T_MyBase * to T_MyDerived *, since * Virtual functions table (vtable) In embedded world, RAM is a very expensive. And then, we base class for that: This function is considered as protected, i.e. is more verbose. First of all, making function pointers a part of the struct bumps the size of instances uselessly. We also need virtual memory deallocator, since pointer to derived class might not match the pointer to base class. I work with embedded stuff a lot: that is, I have a microcontroller with limited resources and limited set of toolchains that I can use. */, #define OOC_GET_CONTAINER_PT( \ have very simple solution: since derived class has its own vtable, we can just * CRC32 object context. p_other_method unchanged, so, it points to that of base class. xref Don't get me wrong, I don't consider it as the only right way to implement everything: as ever, we have to apply common sense. So, for MyBase, we're going to have vtable and bool variable indicating status of vtable (initialized or not) : We need function that initializes vtable if needed: And, finally, this function needs to be called from constructor. * Memory deallocator. startxref trailer

*/, /* etc). 0000004319 00000 n for object-oriented approach in C. At least, there are a couple of macros: Given macro OOC_GET_CONTAINER_PT(), we can calculate offset of base class Could you provide some small project, which I could build? More, subclasses still need to access the structure itself, so that we should put it to some kind of protected header, for subclasses only. 0000007549 00000 n There are already publications on the topic (for example, see this very thorough book), but none of the existing solutions satisfies me. (see https://github.com/dimonomid/ooc), so you don't I found it useful to add the underscore prefix to protected functions. The tool doesn't generate any ceedling stuff; it just uses ceedling stuff to make sure that the generator itself (and the templates it uses) works fine. We need to define prototypes for all virtual methods: Note that each of them explicitly takes pointer to the object for which the method is called. Youll see how you can emulate inheritance in C, what kind of code is generated, and how to debug such code. When some class (Derived) is derived from another class (Base), then, in terms of C, struct Derived contains struct Base. contained struct). This article helps me started with OOC. correctly.

So I decided to share my experience. I consider it quite 1) I wasn't sure if you were trying to avoid dynamic memory allocation but I see you aren't. Usually it's just C compiler provided by chip manufacturer. My mistake, sorry. tried out your generator tool today and I saw that it generates some ceedling stuff. * CRC32 calculator Once unpublished, this post will become invisible to the public Maybe with shape and cirlcle class, or somrthing like this. Here it is: https://github.com/dimonomid/ooc. 0000098929 00000 n 0000003223 00000 n Unfortunately, it's For derived class, we create its own struct with So, the interface is already done (for base class), now we Say, in _dtor it is mandatory (because we clearly want base class to That is, my_func() is completely unaware of exact type of 0000151042 00000 n

0000001965 00000 n It seems that tag name is only used once in the struct declaration, why attache S_ to it to distinguish it from struct type name? wouldn't putting the pointer to VTable as first element of every class and class members from second position allow casting of data members between different base and inheritzed class much easier? Specially the idea to pack class meta data in its own static struct is very good (I started something similar, but wanted to add VTable to every instance :/ ). Apply common sense. If you like, you may omit it, as follows: In the rest of the text, I assume the first approach. */, /** should be used (see Usage section below). Memory can't grow in another direction. 0000098559 00000 n As an example, here's a class diagram (drawn by PlantUML) of a subsystem of my actual embedded project in C (clickable of course) : It is considerably large. 0000152015 00000 n I mean, having struct foo *a reads easier than just foo *a or even foo_t *a. how it is defined. 152 0 obj<>stream Enter your comment (please, English only). This way your callers aren't tempted to tinker, changes to the library cause minimal re-compilation of the client code and the implementation dependencies don't become dependencies of the client. 0000099383 00000 n DEV Community 2016 - 2022. I tried out your generator tool today and I saw that it generates some ceedling stuff. Why not improve error handling in our Go projects? That's why it is named me_super. mentioned, most of the time I'm happy with 1-level inheritance. ******************************************************************************/, /******************************************************************************* Now, create virtual functions table (vtable) : And object context of MyBase should contain a pointer to that vtable: Obviously, we should implement these virtual methods for MyBase: The question now is: how to initialize vtable. The resulting system works stably and is flexible enough. Like this: In order to achieve polymorphism, we need to manually do what C++ compiler does behind the scenes: maintain virtual functions table. Thanks for the effort. Is there a copyright license on the generated classes? in datasheet must be shifted to the left before calling the interface */, /* Pointer to a I2C_HandleTypeDef structure that contains When more instances are created, vtable isn't modified anymore. containing struct) from pointer to Youll see how you can emulate Encapsulation in C, what kind of code is generated, and how to debug such code. If you need to develop a car multimedia system, it's much better to get a more expensive chip which can run Linux, and then write an application in C++, which will run in userspace. The only problem with it is that it becomes too verbose. 0000070397 00000 n Yeah probably a small example would be useful; I'll let you know once I get some time for it. I could put this stuff into the first (real) base class. 0000006117 00000 n Ok, base class seems to be done to a first approximation, let's proceed to derived class now. Once unsuspended, nerdhardware will be able to comment and publish posts again. Writing all this mess manually for each new class, or copy-pasting from existing classes all the time would be tedious and error-prone, so I've written a little utility that generates base and derived class templates with given names. Inline typedef does not have this advantage, and requires using the keywords more times. Great job, thanks again. Even if I wanted to calculate CRC at once by calling crc32_calc(), I couldn't, because flash space on MCU I worked with isn't easily mapped to address space, and of course the MCU hasn't enough RAM so that I could read the whole firmware to RAM and then calculate CRC. Not sure I understand the question. Once unpublished, all posts by nerdhardware will become hidden and only accessible to themselves. In that case make your class type opaque and have the ctor method return it (you can typedef a void *).

*/, /* Initialize all configured peripherals --- */, /* EEPROM Data write ----------------------- */, /* EEPROM Data read -------------------------- */. 0000006004 00000 n After firmware upgrade is completed, device's bootloader should compare calculated CRC of programmed data with the one received from host.

With the techniques described in this article, we end up with just 4 vtables, not 300 vtables, since vtable is created for each class, not for each instance. What is missing is an allocator for our derived class: Note that we don't need deallocator: for that, deallocator of base class derived class (see prototypes of virtual functions above: T_MyBase_Dtor, If you haven't heard of it, you probably should read the 0000098860 00000 n The _t is a POSIX type, so it is a violation of standards if used for your own types. So, we should add function to We definitely don't want to impose a burden of dealing with vtable to the client of MyBase. So, for each virtual method (except memory deallocator) we just define inline function in the header: Now, client of MyBase can equally call mybase__some_method() or mybase__other_method(), and he/she doesn't need to care about whether the method is virtual or not.

<]>> I've been struggling to find a good source that explains OOC. Embedded Software Engineer at Euler Motors, #define MAX_EEPROM_BUFFER_SIZE 256 If our class has 5 virtual methods, they take 20 bytes for each instance. Requirements that I have: Most of the existing solutions use typecasts heavily, so we lose type safety: compiler won't help you if you make a mistake. the actual pointer given to _some_method() of derived class is a pointer But the techniques are equal to all objects (like exception handler functions to overwrite). _superclass_field_name_, \ Also, relying on something like that means that multiple inheritance isn't possible, and I wanted to still use it occasionally. heavily, since these functions take pointer to base class, not to So, of course I really want to use object-oriented approach in the embedded world, when it is appropriate, and the techniques I'm going to tell you about worked quite well for me. ******************************************************************************/, /** - OOC_OFFSETOF(_T_Subclass_, _superclass_field_name_) \ * NOTE: works for 1-level hierarchy only! //-- and then, our own virtual methods. Back then, I was working on firmware upgrade for device via bluetooth, so I wanted to have some common C module that I can use on both parties (device and host computer). Great post, quite insightful. 0000126011 00000 n 0000150654 00000 n 0 Say, we might have some function that works with base class and then deletes If we do, that's no problem: we just specify pointer to function that overrides that from base class.

0000125660 00000 n For each class (but not for each instance of class), we're going to have struct with pointers to virtual functions.

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Here it is: And the function that initializes it, it's similar to that of base class, but But anyway, interesting read. params, which will of course contain struct with base params: We're almost done. here. Why do attach S_ and T_ to the structure declaration? So that, each class name may be specified as just single lowecased (underscore-separated) word, or as two words separated by colon. Function prototype looks like this: This function may work at the host side, where we have lots of RAM. It would be cool if you tell me your way of exception handling. I prefer to name them like this in order to replicate new and delete operators from C++ : So if we create two instances of T_MyBase: my_base_1 and my_base_2, the data and code will be arranged as follows: Please note that even though we're bringing some object stuff to C, we can't write in C in the same manner as in C++: in C, we have to manually call new_() / delete_() (or constructor / destructor), but in C++, we should avoid plain delete as much as possible, and rely on RAII instead. Assume we have base class MyBase, which is going to have some derived classes. For further actions, you may consider blocking this person and/or reporting abuse. Once suspended, nerdhardware will not be able to comment or publish posts until their suspension is removed. This is the only typecast I use in my OOC approach, and it is inevitably happens with multiple inheritance. About this one: I don't like these macro-heavy solutions. Practical UML Statecharts in C/C++, 2n Ed. However, it is a decent tool that comes in handy in many cases in order to build modular software, if we use it right. How I ended up writing a new real-time kernel, Unit-testing (embedded) C applications with Ceedling, Vim: convenient code navigation for your projects, Docker: Printable Quick Reference (Cheat Sheet), Mongoose Embedded Networking Library on nRF51 and nRF52 (Bluetooth). works; instead, the code should work with some common interfaces. deletes object, needed destructors will be called, and memory will be freed Design Made with love and Ruby on Rails. 0000000016 00000 n In the code below, I have created variable e24lc02 which is the Object with type eeprom_t as its Class for EEPROM chip 24LC02 by Microchip Technology. Updated on Jun 6. 0000152602 00000 n have to write it manually for every new subclass. 0000004178 00000 n I have no idea how would I implement all of this to the same degree of flexibility without object-oriented approach. 0000071436 00000 n this and of which type?

Billions of devices OS kernels are written in C and it doesn't seem to have expiration date because the world is running on powered C devices. Can I use them in a commercial project? Actually, that's why I bothered implementing all of this in the first place: I