Programming Forums > Application Development > C++

titaniumdecoy · Jul 1st, 2008, 10:16 PM

Another C++ question.

I am trying to write a Board class so that I can access pieces in an internal Piece *pieces[8][8]; structure using index operators. I have a helper class Board::PieceDict which is constructed with a pointer to a particular row of pieces so that I can get a particular piece by writing Board b; Piece& p = b[0][0];.

Here is my implementation of Board::operator[]:

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const Board::PieceDict& Board::operator[] (unsigned int x) 
{
    PieceDict *pd = new PieceDict(pieces[x]);
    return *pd;
}

When I call, on a line by itself, board[0]; I get this error:

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Board.c: In function ‘std::ostream& ChessGame::operator<<(std::ostream&, const ChessGame::Board&)’:
Board.c:44: error: passing ‘const ChessGame::Board’ as ‘this’ argument of ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int)’ discards qualifiers
make: *** [Board.o] Error 1

Any advice is appreciated.

Also, is there a way to automatically free the PieceDict object that will only be used temporarily?

titaniumdecoy · Jul 1st, 2008, 10:57 PM

Got it. It turns out the Board::operator[] function didn't appreciate being called on a const Board& since the former did not have a suffix of const. I hate the cryptic error messages that the C++ compiler generates.

My last question remains unanswered:

Quote:

Originally Posted by titaniumdecoy

Also, is there a way to automatically free the PieceDict object that will only be used temporarily?

I am not sure how to deal with deleting the 'new' object returned by board[i] since I intend to use the object in a statement like if (board[i][j] == NULL).

EDIT: Almost all of the problems I have encountered so far have come from a) #includeing a file that should (not) have been included, or b) declaring functions/parameters/return values as const that should not have been declared as such. Are there definitive rules for dealing with these issues?

grumpy · Jul 2nd, 2008, 3:54 AM

Why do you have operator[], which returns a const reference, creating a new object? You could also simply do this;

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const Board::PieceDict& Board::operator[] (unsigned int x) const
{
    return pieces[x];
}

The additional const qualifier also allows the operator [] to be used for a const Board object. No need to worry about releasing dynamically created objects.

titaniumdecoy · Jul 2nd, 2008, 12:47 PM

I get this error:

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Board.c: In member function ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int) const’:
Board.c:23: error: invalid initialization of reference of type ‘const ChessGame::Board::PieceDict&’ from expression of type ‘ChessGame::Piece* const*’

I got it to work as follows, although I'm not sure whether it is a better/worse solution:

(Toggle Plain Text)

ChessGame::Board::PieceDict ChessGame::Board::operator[] (unsigned int x)
{
	return pieces[x];
}

When I try to add a suffix of const to the function, it doesn't compile. I think this is because pieces is being passed as a non-const reference to the constructor of PieceDict. I don't know whether I should try to change everything to const, since eventually I want to return the underlying Piece* and it seems like that would require a const_cast.

grumpy · Jul 3rd, 2008, 5:01 AM

Quote:

Originally Posted by titaniumdecoy

When I try to add a suffix of const to the function, it doesn't compile. I think this is because pieces is being passed as a non-const reference to the constructor of PieceDict.

By this, I assume you mean that pieces is not an array of PieceDict (i.e. piece[x] is not a PieceDict so one needs to be constructed from piece[x]).

This means PieceDict needs a constructor of the form

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   PieceDict(const WhateverTypeThePieceArrayElementsAre &);

and the operator[] should not return a reference (returning a reference to a temporary from a function can cause the caller to give undefined results, as that temporary will no longer be guaranteed to exist.

The alternative is to change the operator[] so it is of the form;

(Toggle Plain Text)

    const WhateverTypeThePieceArrayElementsAre  &operator[](unsigned int) const;

Quote:

Originally Posted by titaniumdecoy

I don't know whether I should try to change everything to const, since eventually I want to return the underlying Piece* and it seems like that would require a const_cast.

Depends on what you want to do with that "underlying Piece*". If you need to change it, then it should not be returned as a const.

However, you're the one who is having trouble with discarding const: that means you have something declared const, but you are attempting to return it as a non-const which (implicitly) will allow that const thing to be changed.

In short, you need to make up your mind.

Making a non-static member function const is a declaration that the function will not change the object it acts on. It is not allowed to do any operation that permits changing that object (eg passing it by non-const reference to another function, calling a non-const member function, etc).

A compiler complaining about discarding const means that you have something the compiler considers const, but your are forcing the compiler to consider it as non-const.

titaniumdecoy · Jul 3rd, 2008, 12:28 PM

That makes sense, thanks.

titaniumdecoy · Jul 3rd, 2008, 3:01 PM

This is probably a stupid question, but I can't get it to work.

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std::ostream& operator<< (std::ostream& os, const ChessGame::Board& board);

When I add the const (in bold) keyword to the function above, I can no longer print a non-const Board. This seems wrong, since the function does not change Board. This is the error I get:

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Undefined symbols:
  "ChessGame::operator<<(std::basic_ostream<char, std::char_traits<char> >&, ChessGame::Board&)", referenced from:
      _main in Chess.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [chess] Error 1

titaniumdecoy · Jul 3rd, 2008, 8:14 PM

Turns out it was just a problem with my Makefile.

Jul 1st, 2008, 10:16 PM	#1
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	error: passing as 'this' discards qualifiers Another C++ question. I am trying to write a Board class so that I can access pieces in an internal `Piece pieces[8][8];` structure using index operators. I have a helper class Board::PieceDict which is constructed with a pointer to a particular row of pieces so that I can get a particular piece by writing `Board b; Piece& p = b[0][0];`. Here is my implementation of Board::operator[]: (Toggle Plain Text) const Board::PieceDict& Board::operator[] (unsigned int x) { PieceDict pd = new PieceDict(pieces[x]); return pd; } const Board::PieceDict& Board::operator[] (unsigned int x) { PieceDict pd = new PieceDict(pieces[x]); return pd; } When I call, on a line by itself, `board[0];` I get this error: (Toggle Plain Text) Board.c: In function ‘std::ostream& ChessGame::operator<<(std::ostream&, const ChessGame::Board&)’: Board.c:44: error: passing ‘const ChessGame::Board’ as ‘this’ argument of ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int)’ discards qualifiers make: [Board.o] Error 1 Board.c: In function ‘std::ostream& ChessGame::operator<<(std::ostream&, const ChessGame::Board&)’: Board.c:44: error: passing ‘const ChessGame::Board’ as ‘this’ argument of ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int)’ discards qualifiers make: * [Board.o] Error 1 Any advice is appreciated. Also, is there a way to automatically free the PieceDict object that will only be used temporarily?

Jul 2nd, 2008, 3:54 AM	#3
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,207 Rep Power: 5	Re: error: passing as 'this' discards qualifiers Why do you have operator[], which returns a const reference, creating a new object? You could also simply do this; (Toggle Plain Text) const Board::PieceDict& Board::operator[] (unsigned int x) const { return pieces[x]; } const Board::PieceDict& Board::operator[] (unsigned int x) const { return pieces[x]; } The additional const qualifier also allows the operator [] to be used for a const Board object. No need to worry about releasing dynamically created objects.

Jul 2nd, 2008, 12:47 PM	#4
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	Re: error: passing as 'this' discards qualifiers I get this error: (Toggle Plain Text) Board.c: In member function ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int) const’: Board.c:23: error: invalid initialization of reference of type ‘const ChessGame::Board::PieceDict&’ from expression of type ‘ChessGame::Piece* const’ Board.c: In member function ‘const ChessGame::Board::PieceDict& ChessGame::Board::operator[](unsigned int) const’: Board.c:23: error: invalid initialization of reference of type ‘const ChessGame::Board::PieceDict&’ from expression of type ‘ChessGame::Piece const’ I got it to work as follows, although I'm not sure whether it is a better/worse solution: (Toggle Plain Text) ChessGame::Board::PieceDict ChessGame::Board::operator[] (unsigned int x) { return pieces[x]; } ChessGame::Board::PieceDict ChessGame::Board::operator[] (unsigned int x) { return pieces[x]; } When I try to add a suffix of const to the function, it doesn't compile. I think this is because pieces is being passed as a non-const reference to the constructor of PieceDict. I don't know whether I should try to change everything to const, since eventually I want to return the underlying Piece and it seems like that would require a const_cast.

Jul 3rd, 2008, 12:28 PM	#6
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	Re: error: passing as 'this' discards qualifiers That makes sense, thanks.

Jul 3rd, 2008, 3:01 PM	#7
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	Re: error: passing as 'this' discards qualifiers This is probably a stupid question, but I can't get it to work. (Toggle Plain Text) std::ostream& operator<< (std::ostream& os, const ChessGame::Board& board); std::ostream& operator<< (std::ostream& os, const ChessGame::Board& board); When I add the const (in bold) keyword to the function above, I can no longer print a non-const Board. This seems wrong, since the function does not change Board. This is the error I get: (Toggle Plain Text) Undefined symbols: "ChessGame::operator<<(std::basic_ostream<char, std::char_traits<char> >&, ChessGame::Board&)", referenced from: _main in Chess.o ld: symbol(s) not found collect2: ld returned 1 exit status make: * [chess] Error 1 Undefined symbols: "ChessGame::operator<<(std::basic_ostream<char, std::char_traits<char> >&, ChessGame::Board&)", referenced from: _main in Chess.o ld: symbol(s) not found collect2: ld returned 1 exit status make: * [chess] Error 1

Jul 3rd, 2008, 8:14 PM	#8
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	Re: error: passing as 'this' discards qualifiers Turns out it was just a problem with my Makefile.

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Programming Forums > Application Development > C++

error: passing as 'this' discards qualifiers