Re: Why use *protected* virtual functions for Template Method
Ulrich Eckhardt wrote:
Barry wrote:
private member function in the base class is not visible in the derived
class, only visible function can be overridden
Definitely no! You can not access the baseclass' private function, but you
can still override it, provided it is virtual. Also, visibility is not
affected by private/protected/public, only accessibility is.
My very big mistake, the *!* states :-)
code example, the virtual functions are overridden by private
functions in a derived class.
(Please I don't want this thread to stray into "should virtual
functions be non-public": this has been already covered in past
threads)
See this case:
class Ostream {
public:
Ostream& operator<< (int i) {
write(&i, sizeof(int));
return *this;
}
Ostream& operator<< (char c) {
write(&c, sizeof(char));
return *this;
}
protected:
virtual size_t write(void const* buf, size_t size) = 0;
};
class FileOstream : public Ostream {
private: // protected if you don't wanna seal FileOstream
virtual size_t write(void const* buf, size_t) {
// your implementation here
}
};
There is no reason not to make write() private. Further, you don't "seal"
the class if you make it private, a class derived from FileOstream can
override the function again.
Oops, I did it again :-)
But if I say "you don't wanna make FileOstream::write" accessible
outside of FileOstream including the derived ?
There's still one more question here, as I use Java,
the derived class must override the base's abstract method, with no less
accessibility, say protected in the base, then protected or public
must be chosen for the derived.
Is this a software design idiom?
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