Re: Static virtual functions?

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Fri, 21 Mar 2008 12:11:01 +0100
<47e397c8$0$4754$> wrote:

I appear to need a static virtual member function, which it seems I
can't have. I'm trying to do something like the following:

class Base {
  virtual void writetofile() = 0;
  virtual void readfromfile() = 0;
  (something here) char *name(); };

class Deriv1 : public Base {
  virtual void writetofile();
  virtual void readfromfile();
  (ditto) char *name() { return "Deriv1"; } };

class Deriv2 : public Base {
  virtual void writetofile();
  virtual void readfromfile();
  (ditto) char *name() { return "Deriv2"; } };

Now I want to write them to a file doing something like:

write (x -> name); x -> writetofile();

which requires name to be a virtual function. But I also want to do:

if (strcmp(type, Deriv1::name) == 0) { x = new Deriv1; x ->
readfromfile(); }
if (strcmp(type, Deriv2::name) == 0) { x = new Deriv2; x ->
readfromfile(); }

which requires name to be a static member function.

Any suggestions? I want to avoid having to duplicate the names in
different places.

class Deriv1 : public Base {
   static const char* const name;
   virtual const char *get_name() { return name; } };

const char* const Deriv1::name = "Deriv1";

Now you can get the name as constant from the class type as well as from
a class instance.

However, it seems that you want to implement your own (de)serialization.
There are solutions for this job around. You might want to give them a look.
First of all somewhere you should have an object type repository to
dispatch the deserialization. Virtual functions won't help you since you
do not have a typed object at this time. This is the job you did quite
ineffectively by the strcmp(type, Deriv1::name cascade. There are
significantly more sophisticated and faster solutions for this purpose.
Have a look at

Furthermore, C++ programs should not contain char* objects. They are
nearly always risky. You should avoid char* wherever possible and use
std::string or const char* instead.


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