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Advanced c++, c++ concepts, Data Structure

Function pointers and functor

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Function pointers and functor :

Syntax for Accessing function pointer in c :

int getRes(void);

int main()
{
int (*current_res)(void);
current_ans = &getRes;
// HERE
printf(“%d”, current_res());

}

Syntax – Array of function pointers :

int (*funcPtrArr2[100])(float, int, char) = {NULL};

Functor:

It’s a function object. A class object is called a functor if it has a provision of function operator. This gives class object function semantics.

A basic functor :
class MyFunctor
{
public:
int operator()(int x) { return x * 2;}
}

MyFunctor twice;
int x = twice(5);
Drawbacks of using function pointers in C:

1. A function contains no instance state, mainly because there is no such thing as multiple instances of a function; there will only ever be one instance and that is global. Although, it is possible to declare a local static within the function that can retain state between calls, but since a function only has one instance there can only be one instance of the static member and that must be shared between function calls.

A function with local static state

MyDataClass &
foo()
{
   static MyDataClass data;
   // Do some work
   return data;
}
2.If you try to maintain state in the function by using a local static variable the function will not be reentrent, so it cannot be safely called on multiple threads without the additional overhead of thread synchronisation to ensure access to the local static data has mutual exclusion semantics. This effectively means the function can only allow one thread into it at any one time and that will create quite a bottle neck of thread contention (multiple threads all fighting for access to a single resource).
Furthermore, if access is required to the static state local variable after the function has finished ,the caller must continue to block access until the state has either been copied for use or it is no longer required otherwise .It’ll be read by one thread whileanother is potentially trying to modify it, resulting in a race condition.
A function with local static state using a mutex in attempt to make the function reentrant

MyDataClass &
foo(Mutex m)
{
   ScopedLock sl(m); // Ensure no other thread can get in here
   static MyDataClass data;
   // Do some work
   return data;
   // Note once the scoped lock ends here another thread could enter and modify data before the caller has a chance to copy it, so even this isn’t a very good solution, really the mutex should  be locked by the caller.
 }

3. Function pointers do not work very well with templates, if only one signature of the function exists then things will work otherwise the compiler will complain of template instantiation ambiguities that can only be resolved through ugly function pointer casting.

A function pointer casting to resolve template instantiation ambiguities

void callback_func(void *){}
void callback_func(char *){}
template
void consumer(funcT callback_func)
{
}
int main()
{
   consumer(callback_func); // ERROR: Which one?
   consumer((void (*)(void*))callback_func);
   consumer((void (*)(char*))callback_func);
}
An example of logging functor:
https://writeulearn.com/template/