How different is Objective-C from C

Question

What are the main differences between Objective-C and C   in terms of the syntax  features  paradigms  frameworks and libraries    Important  My goal is not to start a performance war between the two languages  I only want real hard facts  In fact  my question is not related to performance  Please give sources for anything that may seem subjective

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They re completely different  Objective C has more in common with Smalltalk than with C    well  except for the syntax  really

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While they are both rooted in C  they are two completely different languages   A major difference is that Objective-C is focused on runtime-decisions for dispatching and heavily depends on its runtime library to handle inheritance and polymorphism  while in C   the focus usually lies on static  compile time  decisions   Regarding libraries  you can use plain C libraries in both languages - but their native libraries are completely different   Of interest though is that you can mix both languages  with some limitations   The result is called Objective-C

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Obj-C has much more dynamic capabilities in the language itself  whereas C   is more focused on compile-time capabilities with some dynamic capabilities   In  C   parametric polymorphism is checked at compile-time  whereas in Obj-C  parametric polymorphism is achieved through dynamic dispatch and is not checked at compile-time   Obj-C is very dynamic in nature  You can add methods to a class during run-time  Also  it has introspection at run-time to look at classes  In C    the definition of class can t change  and all introspection must be done at compile-time  Although  the dynamic nature of Obj-C could be achieved in C   using a map of functions or something like that   it is still more verbose than in Obj-C   In C    there is a lot more checks that can be done at compile time  For example  using a variant type like a union  the compiler can enforce that all cases are written or handled  So you don t forget about handling the edge cases of a problem  However  all these checks come at a price when compiling  Obj-C is much faster at compiling than C

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As others have said  Objective-C is much more dynamic in terms of how it thinks of objects vs  C   s fairly static realm   Objective-C  being in the Smalltalk lineage of object-oriented languages  has a concept of objects that is very similar to that of Java  Python  and other  standard   non-C   object-oriented languages   Lots of dynamic dispatch  no operator overloading  send messages around   C   is its own weird animal  it mostly skipped the Smalltalk portion of the family tree   In some ways  it has a good module system with support for inheritance that happens to be able to be used for object-oriented programming   Things are much more static  overridable methods are not the default  for example

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Short list of some of the major differences    C   allows multiple inheritance  Objective-C doesn t  Unlike C    Objective-C allows method parameters to be named and the method signature includes only the names and types of the parameters and return type  see bbum s and Chuck s comments below   In comparison  a C   member function signature contains the function name as well as just the types of the parameters return  without their names   C   uses bool  true and false  Objective-C uses BOOL  YES and NO  C   uses void  and nullptr  Objective-C prefers id and nil  Objective-C uses  selectors   which have type SEL  as an approximate equivalent to function pointers  Objective-C uses a messaging paradigm  a la Smalltalk  where you can send  messages  to objects through methods selectors  Objective-C will happily let you send a message to nil  unlike C   which will crash if you try to call a member function of nullptr Objective-C allows for dynamic dispatch  allowing the class responding to a message to be determined at runtime  unlike C   where the object a method is invoked upon must be known at compile time  see wilhelmtell s comment below   This is related to the previous point    Objective-C allows autogeneration of accessors for member variables using  properties   Objective-C allows assigning to self  and allows class initialisers  similar to constructors  to return a completely different class if desired  Contrast to C    where if you create a new instance of a class  either implicitly on the stack  or explicitly through new  it is guaranteed to be of the type you originally specified  Similarly  in Objective-C other classes may also dynamically alter a target class at runtime to intercept method calls  Objective-C lacks the namespace feature of C    Objective-C lacks an equivalent to C   references  Objective-C lacks templates  preferring  for example  to instead allow weak typing in containers  Objective-C doesn t allow implicit method overloading  but C   does  That is  in C   int foo  void  and int foo  int  define an implicit overload of the method foo  but to achieve the same in Objective-C requires the explicit overloads -  int  foo and -  int  foo  int  intParam  This is due to Objective-C s named parameters being functionally equivalent to C   s name mangling  Objective-C will happily allow a method and a variable to share the same name  unlike C   which will typically have fits  I imagine this is something to do with Objective-C using selectors instead of function pointers  and thus method names not actually having a  value   Objective-C doesn t allow objects to be created on the stack - all objects must be allocated from the heap  either explicitly with an alloc message  or implicitly in an appropriate factory method   Like C    Objective-C has both structs and classes  However  where in C   they are treated as almost exactly the same  in Objective-C they are treated wildly differently - you can create structs on the stack  for instance    In my opinion  probably the biggest difference is the syntax  You can achieve essentially the same things in either language  but in my opinion the C   syntax is simpler while some of Objective-C s features make certain tasks  such as GUI design  easier thanks to dynamic dispatch   Probably plenty of other things too that I ve missed  I ll update with any other things I think of  Other than that  can highly recommend the guide LiraNuna pointed you to  Incidentally  another site of interest might be this   I should also point out that I m just starting learning Objective-C myself  and as such a lot of the above may not quite be correct or complete - I apologise if that s the case  and welcome suggestions for improvement   EDIT  updated to address the points raised in the following comments  added a few more items to the list

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Off the top of my head    Styles - Obj-C is dynamic  C   is typically static Although they are both OOP  I m certain the solutions would be different  Different object model  C   is restricted by its compile-time type system     To me  the biggest difference is the model system  Obj-C lets you do messaging and introspection  but C   has the ever-so-powerful templates   Each have their strengths

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Objective-C is a more perfect superset of C  In C and Objective-C implicit casting from void  to a struct pointer is allowed   Foo  bar   malloc sizeof Foo      C   will not compile unless the void pointer is explicitly cast   Foo  bar    Foo  malloc sizeof Foo      The relevance of this to every day programming is zero  just a fun trivia fact

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