changing return type of derived function

This is a discussion on changing return type of derived function within the C++ Programming forums, part of the General Programming Boards category; Hi, I am writing a code and got a problem with deriving the class. I have got a class A ...

  1. #1
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    changing return type of derived function

    Hi,
    I am writing a code and got a problem with deriving the class.
    I have got a class A (with pure virtual functions) which got a (pure virtual) function operator[].
    also I got classes B and C which inherits A.
    if B is a class which is a vector of complex numbers and C is a bitArray than what should operator[] return.
    in the situation of B it should return a reference to complex number.
    In the case of C it should return a proxy class which holds the bit.

    thus, how can i define operator[] in A so both B and C can overwrite the operator[].
    It is important that everyone who uses A knows that it supports operator[].

    thanks in advance.

  2. #2
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    I don't think you can do that.

    You'll have to have the same type for it to work correctly with inheritance. No escape from that.

    --
    Mats
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    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
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    can't i say that i just returning a reference to 'no matter what'?
    like a reference to Object in Java.
    something like void& (i just don't know if void& exists). ?
    thanks.

  4. #4
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    You can certainly return a reference to some base-class that in turn have virtual member functions, and return a derived class of that. But there is no way to say "i'm just returning 'any' object here" in C++ - you have to give it a type of some sort.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  5. #5
    Code Goddess Prelude's Avatar
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    >like a reference to Object in Java.
    C++ doesn't have a top level type (unless you go out of your way to make one), nor does it have dynamic typing.

    >in the situation of B it should return a reference to complex number.
    >In the case of C it should return a proxy class which holds the bit.
    Why not use a proxy class across the board so that you can take advantage of polymorphism?
    My best code is written with the delete key.

  6. #6
    and the hat of sweating
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    You could return a void* pointer instead of a reference, but I'd highly recommend against doing that since you'd lose all type safety by doing that.

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    The proxy class approach is typically the safest in this situation, I would think.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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