C++:Correctly initializing constructor of abstract class

Question

What is the correct way to initialize a constructor of an abstract class? I commented out the constructor in the cc file and it seemed to be working, I would like to understand why.

Board. h Board.h is an abstract class, as it has 2 pure virtual function NOT included here.

class Board {
public:
   Board(size_t width, size_t height)
    : width(width), height(height) {}

   virtual ~Board() {}
....

protected:
//this is line 54
size_t height;
size_t width;
};

My localBoard.h

#include <Board.h>
class LocalBoard : public Board {

public:

  LocalBoard(size_t width, size_t height) :Board(width),Board(height) {}

  ~LocalBoard() {}

 ...
};

LocalBoard.cc

#include <board/LocalBoard.h>
    // commenting this out fixed the error
    //LocalBoard(size_t width, size_t height) {}

error: multiple initializations of constructor

On an another note may someone help me understand what the following warning means, what consequences it has for my program and how to fix it? I think it related to the constructor again.

  ./include/board/Board.h: In constructor ‘Board::Board(size_t, size_t)’:
  ./include/board/Board.h:54:9: warning: ‘Board::width’ will be              initialized    after [-Wreorder]
   size_t width;

Board(width, height) - but your book would've told you this ;) — UKMonkey, Oct 25 '17 at 08:29
class Board as provided here is not an abstract class, It would be better to add a pure virtual function here to make it abstract. — Werner Henze, Oct 25 '17 at 08:35
@WernerHenze you can't see the whole class - how do you know it doesn't have a pure virtual function already - infact the question explictly states it has 2? — UKMonkey, Oct 25 '17 at 08:36
"On another note": Please don't do that! One question per question please. If you want to ask that as a separate question, that is fine (but do check it hasn't already been asked first.) — Martin Bonner supports Monica, Oct 25 '17 at 08:45
You have `Board(width)` but you don't have a ctor for `Board` that takes only 1 argument?? — Disillusioned, Oct 25 '17 at 08:45
@WernerHenze Why are you mentioning me? I only said the OP should not ask a second question (on initialization order warning). — Martin Bonner supports Monica, Oct 25 '17 at 11:16
@UKMonkey I did not say that Board is not Abstract. I just said that if the OP thinks that it is important that Board is Abstract, then it is a good idea to also Show that in the code and not only mention it in the question. It's just one additional line of code. — Werner Henze, Oct 25 '17 at 12:09

Werner Henze · Accepted Answer · 2017-10-25T08:49:51.490

0

You already provided an inline implementation of class LocalBoard's ctor in this line:

LocalBoard(size_t width, size_t height) :Board(width),Board(height) {}

You cannot give the same ctor twice (once inline and once in the cc file).

You need to write either

class LocalBoard {
    LocalBoard(size_t width, size_t height) :Board(width, height) {}
};

or

class LocalBoard {
    LocalBoard(size_t width, size_t height);
}

// probably in the cc file:
LocalBoard::LocalBoard(size_t width, size_t height) : Board(width, height)
{
}

Please note that I changed your two base class ctor calls Board(width),Board(height) to one base class ctor call Board(width, height).

Regards the warning you can look at What's the point of g++ -Wreorder?.

edited Oct 25 '17 at 08:49

answered Oct 25 '17 at 08:42

Werner Henze

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Probably worth pointing out that the OP's inline implementation is wrong - you can't call the base class constructor twice - you have to call it once with two parameters. – Martin Bonner supports Monica Oct 25 '17 at 08:43
@MartinBonner Good Point, done! – Werner Henze Oct 25 '17 at 08:45

C++:Correctly initializing constructor of abstract class

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