Does a pure virtual destructor suffice to make a class abstract?

Question

I hope this is not becoming a 'duplicate', as there are so many questions about (pure) virtual destructures (yes, I do know about them).

I want to build an 'interface' (-> abstract class) with a bunch of methods, which can be reimplemented, but don't need to. So I have like:

class IBase
{
public:
  virtual bool doThis() { return true; }
  virtual bool doThat() { return true; }
}

And I'm giving a bunch of implementations, some use doThis some use doThat. That's why the 'interface' methods are just virtual and not pure. Like:

class Derived1 : public IBase
{
public:
  bool doThis() override { return doSomething(); }
}

class Derived2 : public IBase
{
public:
  bool doThat() override { return doSomethingElse(); }
}

The problem: This IBase class is instantiable, which it must not, because it doesn't do anything...

My question(s): Does it suffice to define a pure virtual destructor virtual ~IBase() = 0 to make it uninstantiable? And/or do I need to delete the standard constructor IBase() = delete?

Maybe I eventually became code-blind for thinking about it for too long, so I'll excuse in advance.

Edit: My initial greeting got cut (by me or SO), so I'll greet y'all now or never: Hey folks!

Answer 1

personally I would avoid making the destructor a pure virtual. In C++ you can define a function that has been declared as virtual. See the following example:

class IBase
{
public:
IBase() = default;
virtual ~IBase() = default;

virtual bool doThis() = 0;

virtual bool doThat() = 0;
};

bool IBase::doThis()
{
    return true;
}

bool IBase::doThat()
{
    return true;
}

class Derived2 : public IBase
{
public:
bool doThat() override { return false; }

bool doThis() override { return IBase::doThis(); } // call parent implementation
};

int main()
{
    Derived2 a;
    std::cout << a.doThat() << std::endl;
    std::cout << a.doThis() << std::endl;
}

However, from a design point of view, a class that derives from a an interface should implement all its method. I therefore suggest you rethinking about your solution, if you can.