Can I override an overload of an operator and return a different type?

class A{
  public: 
    virtual char &operator[](int);
  protected:
    ..
};
class B:A{
  public:
    A* &operator[](int);
  protected:
}

Can I change the return type when I overload an overload of an operator?

thanks!

//EDIT Okay, so now that we established that this wont work how can I build a work around?

Lets say I have classes A,B,C, and D.

class A{
  public: 
  private:
    char &operator[](int);
  protected:
    ..
};
class开发者_运维百科 B:A{
  public: 
    virtual char &operator[](int);
};
class C: A{
  public:
  private:
    A::&operator[](int);
}
class D: A{
  public:
  private:
     A::&operator[](int);
}

Can I do something like this? If so is this the correct syntax?

Not like this, no.

The return type of an override must be either

• the same type as the return type of the virtual function being overridden, or
• a derived class of the return type of the virtual function being overridden (this is called a "covariant return type").

So, if a virtual A::operator[] returned an A*, then a B::operator[] override could return a B*.

The reason that a polymorphic function can't return different types in different classes isn't because someone on the C++ committee decided that it was "taboo", but because any code that used that function's return value couldn't compile.

By creating an inheritance heirarchy, you're able to access derived objects through a base pointer or reference:

class A
{
public:
    virtual char operator[](int);
};
class B : public A
{
public:
    virtual char operator[](int);
};

A *a;

std::cout << "Do you want to make an A or a B?";

char type;
std::cin >> type;

if (type == 'A')
    a = new A();
else
    a = new B();

char c = (*a)[0];

Note that on the last line, the compiler won't know what type of object a is pointing to, since that's determined at runtime. This is fine, because no matter what type of object a is pointing to, operator[] is still going to return a character. But what if that operator were allowed to return a different type in class B?

class Sequence
{
    ...
};

class A
{
public:
    virtual char operator[](int);
};
class B : public A
{
public:
    virtual Sequence operator[](int);
};

A *a = new B();
char c = (*a)[0];

Obviously, that last line makes no sense when a is an object of type B. In that case, you're trying to assign a Sequence to a character. Likewise, Sequence c = (*a)[0]; wouldn't make sense if a were an object of type A.

The proper solution is to make A a member of classes C and D rather than a base class. Since C and D need a different signature for a public method, they're clearly not equivalent to an A in every context, so trying to use inheritance here really makes no sense.

OK looking at your last comment about Sequence (A), GeneSequence (B) and GenomeSequence (C)

you want the operator to sometimes return a char, and sometimes return a Sequence

well then, as you can see, it is incorrect for the superclass A to declare the type of &operator [] as char.

It should use a template as a type parameter, to specify whether it is a collection of chars or Sequences, right?