C++ - Parameters of a pure virtual function

Question

If I have a class say

class Base {
 public:
  virtual void func() = 0;
};

That is the base for two other classes

class DerivedA1 : public Base {
 public:
  virtual bool func(string command);
};

class DerivedB1 : public Base {
 public:
  virtual void func();
}

class DerivedA2 : public DerivedA1 {
 public:
  bool func(string command);     //This one implements it differently
};                                 //its base class.

Is the above allowed? I declared func() with no parameters but then I'm using it with parameters. I have a similar situation in my code that I can't post because this is it's part of a school assignment, and am getting an error similar to

error: no matching function for call to Base::fucn(std::string&)
note: candidate is: virtual bool Base::move();
note: candidate expects 0 arguments, provided  1

I want func() to be used differently in its different derived classes. How can I fix this problem?

No, it isn't allowed. Overrides of virtual functions of the base class must have the exact same signature. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Apr 2, 2016 at 18:35
There is a concept called contravariance which allows the derived class to change the method's argument (not arbitrarily though), but that does not apply here. You could use default arguments: en.cppreference.com/w/cpp/language/default_arguments — mike
– mike, Commented Apr 2, 2016 at 18:36
You can make another class for the parameter. virtual void func(MyBaseParemeterClass*) = 0; — QuentinUK
– QuentinUK, Commented Apr 2, 2016 at 18:37
@mike: if default parameters on virtual functions are not identical for all overrides, calling them behaves differently depending on the used type. I find this too brittle and confusing, so I try to avoid default parameters in such situations. — Benjamin Bannier
– Benjamin Bannier, Commented Apr 2, 2016 at 18:59

Richard Hodges · Accepted Answer · 2016-04-02 18:38:02Z

DerivedA1::func(string) is hiding Base::func(). It's allowed, but won't override the behaviour of Base::func. clang will warn you about this because it's almost always an error

class DerivedA1 : public Base {
 public:
  virtual bool func(string command);
};

This is fine:

class DerivedB1 : public Base {
 public:
  virtual void func();
}

This is overriding DerivedA1::func(string). A virtual function which only exists on a DervivedA1 interface or something derived from it. It does not exist on Base

class DerivedA2 : public DerivedA1 {
 public:
  bool func(string command);     //This one implements it differently
};

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