What is a C delegate

Question

What is the general idea of a delegate in C    What are they  how are they used and what are they used for   I d like to first learn about them in a  black box  way  but a bit of information on the guts of these things would be great too   This is not C   at its purest or cleanest  but I notice that the codebase where I work has them in abundance  I m hoping to understand them enough  so I can just use them and not have to delve into the horrible nested template awfulness   These two The Code Project articles explain what I mean but not particularly succinctly    Member Function Pointers and the Fastest Possible C   Delegates The Impossibly Fast C   Delegates

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Very simply  a delegate provides functionality for how a function pointer SHOULD work  There are many limitations of function pointers in C    A delegate uses some behind-the-scenes template nastyness to create a template-class function-pointer-type-thing that works in the way you might want it to   ie - you can set them to point at a given function and you can pass them around and call them whenever and wherever you like   There are some very good examples here     http   www codeproject com Articles 7150 Member-Function-Pointers-and-the-Fastest-Possible http   www codeproject com Articles 11015 The-Impossibly-Fast-C-Delegates http   www codeproject com Articles 13287 Fast-C-Delegate

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Windows Runtime equivalent of a function object in standard C    One can use the whole function as a parameter  actually that is a function pointer   It is mostly used in conjunction with events  The delegate represents a contract that event handlers much fulfill  It facilitate how a function pointer can work for

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An option for delegates in C   that is not otherwise mentioned here is to do it C style using a function ptr and a context argument  This is probably the same pattern that many asking this question are trying to avoid  But  the pattern is portable  efficient  and is usable in embedded and kernel code   class SomeClass       in someMember      int SomeFunc  int        static void EventFunc  void  this    int a  int b  int c                SomeClass  this    static cast lt  SomeClass  gt   this              this - gt SomeFunc  a            this - gt someMember   b   c            void ScheduleEvent  void   delegateFunc   void   int  int  int   void  delegateContext                SomeClass  someObject   new SomeObject                ScheduleEvent  SomeClass  EventFunc  someObject

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You have an incredible number of choices to achieve delegates in C    Here are the ones that came to my mind     Option 1   functors   A function object may be created by implementing operator    struct Functor           Normal class struct members       int operator   double d     Arbitrary return types and parameter list                  return  int  d   1                Use  Functor f  int i   f 3 14       Option 2  lambda expressions  C  11 only      Syntax is roughly   capture  parameter list  - gt  return type  block     Some shortcuts exist auto func      int i  - gt  double   return 2 i 1 15     double d   func 1       Option 3  function pointers  int f double d          typedef int   MyFuncT   double d   MyFuncT fp    amp f  int a   fp 3 14       Option 4  pointer to member functions  fastest solution   See Fast C   Delegate  on The Code Project    struct DelegateList        int f1 double d           int f2 double d          typedef int  DelegateList    DelegateType  double d    DelegateType d    amp DelegateList  f1  DelegateList list  int a    list  d  3 14       Option 5  std  function   or boost  function if your standard library doesn t support it   It is slower  but it is the most flexible    include  lt functional gt  std  function lt int double  gt  f    can be set to about anything in this answer     Usually more useful as a parameter to another functions     Option 6  binding  using std  bind   Allows setting some parameters in advance  convenient to call a member function for instance   struct MyClass       int DoStuff double d      actually a DoStuff MyClass  this  double d      std  function lt int double d  gt  f   std  bind  amp MyClass  DoStuff  this  std  placeholders   1      auto f   std  bind       in C  11     Option 7  templates  Accept anything as long as it matches the argument list   template  lt class FunctionT gt  int DoSomething FunctionT func        return func 3 14

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The need for C   delegate implementations are a long lasting embarassment to the C   community  Every C   programmer would love to have them  so they eventually use them despite the facts that    std  function   uses heap operations  and is out of reach for serious embedded programming   All other implementations make concessions towards either portability or standard conformity to larger or lesser degrees  please verify by inspecting the various delegate implementations here and on codeproject   I have yet to see an implementation which does not use wild reinterpret casts  Nested class  prototypes  which hopefully produce function pointers of the same size as the one passed in by the user  compiler tricks like first forward declare  then typedef then declare again  this time inheriting from another class or similar shady techniques  While it is a great accomplishment for the implementers who built that  it is still a sad testimoney on how C   evolves  Only rarely is it pointed out  that now over 3 C   standard revisions  delegates were not properly addressed   Or the lack of language features which allow for straightforward delegate implementations   With the way C  11 lambda functions are defined by the standard  each lambda has anonymous  different type   the situation has only improved in some use cases  But for the use case of using delegates in  DLL  library APIs  lambdas alone are still not usable  The common technique here  is to first pack the lambda into a std  function and then pass it across the API

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A delegate is a class that wraps a pointer or reference to an object instance  a member method of that object s class to be called on that object instance  and provides a method to trigger that call   Here s an example   template  lt class T gt  class CCallback   public      typedef void  T   fn   int anArg         CCallback T amp  trg  fn op            m rTarget trg            m Operation op                   void Execute  int in                  m rTarget  m Operation   in           private       CCallback        CCallback  const CCallback amp          T amp  m rTarget      fn m Operation       class A   public      virtual void Fn  int i                    int main  int   argc    char     argv           A a      CCallback lt A gt  cbk  a   amp A  Fn        cbk Execute  3

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