Virtual Functions

[S6k9GNh0]

I'm rather confused as to how virtual functions work. I made this example:

Header (sandbox.h):

#ifndef SANDBOX_H
#define SANDBOX_H

class Sandbox
{
public:
	virtual ~Sandbox() = 0;
	virtual void print_something() =0;
	virtual int return_something() =0;
}

#endif 

Implementation (sandbox.cpp):

#include "sandbox.h"
#include <iostream>

class SandboxImpl : public Sandbox
{
private:
	virtual ~SandboxImpl();
	virtual void print_something()
	{
		std::cout << "Printing something..." << std::endl;
	}
	
	virtual int return_something()
	{
		return 20;
	}
}

Compiler basically says "screw you" with 500 errors (exactly) and 16 warnings. Did I misunderstand something?

[quirkyusername]

You've made those functions pure virtual (with the =0 on the end), when you declare a function as pure virtual you are telling the compiler that all classes derived from that class must define their own implementation of the virtual functions, and you don't give them a body in the base class.

Oops sorry didn't realise it was derived from Sandbox.

What Xander said...

[anonymous23323124]

Well you've missed two semicolons at the end of the class definitions, but I'm guessing that's just the version on here.

I made it compile by doing to following:
1) add the semicolons
2) make the base class's functions public
3) implement the two destructors

Surely it's a problem to make the base class's destructor pure virtual as the inherited destructor calls the base destructor?
EDIT: Nope, it doesn't work... my original statement was correct, not my updated one ;)

[kev82]

The missing semicolons on lines sandbox.h:10 and sandbox.cpp:17 won't help.

I'm not sure if there are any issues with a pure virtual destructor - I've never used one. SandboxImpl doesn't have a destructor defined which will cause a linker error. I'm also not sure what happens if a virtual function is private/protected in a derived class, but public in the base.

[anonymous23323124]

Update:
According to an article by Herb Sutter (http://www.gotw.ca/gotw/031.htm ), a destructor can be pure virtual. However, this is only to make the class abstract in the absence of other pure virtual functions, and even a pure virtual destructor must have a body.

[S6k9GNh0]

I see. Thanks for the help.

On a second note, why did GCC go wack? I didn't try this with VC++ but what caused it to give such a messed up error?

[anonymous23323124]

Had you missed the semicolons? In larger projects, that tends to cause a lot of errors. The problems with the destructor would probably only actually emerge at link time, not compile time.

EDIT: Also, were the functions in the child class supposed to be private? That struck me as somewhat odd, and if they weren't then that would doubtless cause more errors.

[S6k9GNh0]

I was looking for the pImpl idiom... my bad >.>.