Function Object:
In simple term we can define as :A Function Object, or Functor (the two terms are synonymous) is simply any object that can be called as if it is a function. An ordinary function is a function object, and so is a function pointer; more generally, so is an object of a class that defines operator().
A very simple example can be :-
class DeleteObject{
public:
template<typename T>
void operator() ( const T& ptr) const { delete ptr; }
};
vector<Drive *> vec; // our supposed definition of vector
Above functor can be used with for_each as :
for_each(vec.begin(), vec.end(), DeleteObject());
here on each element of vec, delete operator will be invoked.
and every thing is fine, assuming after for_each vec.clear() is called.
if we change the definition of DeleteObject as follow:
class DeleteObject{
public:
template<typename T>
void operator() ( const T* ptr) const {
cout << "del "<< endl; delete ptr;
}
};
and perform the same operation as above, things work fine again.
Now we change our definition as :
class DeleteObject{
public:
template<typename T>
void operator() ( const T& ptr) const { delete ptr; }
template<typename T>
void operator() ( const T* ptr) const {
cout << "del "<< endl; delete ptr;
}
};
and perform the same operation as above, things work fine again and void operator() ( const T& ptr) get invoked and it worked fine.
Now my question that is puzzling me how compiler decide which one to invoked from:
void operator() ( const T& ptr) const;
void operator() ( const T* ptr) const ;
and what difference these make?
any suggestion and response will be highly appriciated.
