According to Wikipedia, a callback is a reference to a piece of executable code that is passed as an argument to other code. Callback is widely used in C++, you can find it in STL algorithm, single handler of Linux, Windows API, etc..
In C++, callback can be implemented using function pointer and function object(functor).
Function pointer is a pointer that points to executable code in the memory.Usually, there are two types of function pointer: ordinary function pointer and class member function pointer(pointer to static method is treated the same as ordinary function pointer).
Function object(functor) is a class with function call operator "()" overloaded.
Function pointer V.S. functor:
1. When passed as a parameter, you pass the function pointer itself if you are using function pointer, if you functor, you passed the object.
2. The functor can preserve the state of the object between calls.
Ordinary function pointer V.S. Member function pointer:
1. The size of the pointer is different. In my environment(Ubuntu 10.04 X86_64) the size of ordinary function pointer is 8 byte while member function pointer occupies 16 bytes.
2. When invoking the callback, you need to passed the parameter, and if you are using functor, you have to passed one addition parameter -- the object.
I implemented a unified functor generator class that will take both ordinary function pointer and member function pointer and return a functor.
Here are some code example:
The code can be check out at https://github.com/Jasonhcwong/blog.
In C++, callback can be implemented using function pointer and function object(functor).
Function pointer is a pointer that points to executable code in the memory.Usually, there are two types of function pointer: ordinary function pointer and class member function pointer(pointer to static method is treated the same as ordinary function pointer).
Function object(functor) is a class with function call operator "()" overloaded.
Function pointer V.S. functor:
1. When passed as a parameter, you pass the function pointer itself if you are using function pointer, if you functor, you passed the object.
2. The functor can preserve the state of the object between calls.
Ordinary function pointer V.S. Member function pointer:
1. The size of the pointer is different. In my environment(Ubuntu 10.04 X86_64) the size of ordinary function pointer is 8 byte while member function pointer occupies 16 bytes.
2. When invoking the callback, you need to passed the parameter, and if you are using functor, you have to passed one addition parameter -- the object.
I implemented a unified functor generator class that will take both ordinary function pointer and member function pointer and return a functor.
Here are some code example:
// test ordinary function
functor g1;
g1 = &test1;
g1(12.123);
// test member function
functor g3(&Base::static_b);
functor g4(&Base::d9);
functor g5(&Base::virtual_c);
g3(29) == 29);
g4(&b, 1,2,3,4,5,6,7,8,9);
g5(&c, 5);
functor functorCmp = &mycmp;
std::sort(myvector2.begin(), myvector2.end(), functorCmp);
The code can be check out at https://github.com/Jasonhcwong/blog.
Reference:
1. Boost function, http://www.boost.org/doc/libs/1_49_0/doc/html/function/
2. Member Function Pointers and the Fastest Possible C++ Delegates, http://www.codeproject.com/Articles/7150/Member-Function-Pointers-and-the-Fastest-Possible
3. Wikipedia
