How to fix a C++ cyclical dependency with forward declaration in template class header

Question

For teaching purposes in my applied oriented object courses, we are asked to develop a fully featured C++ application without using the STL nor any string manipulation functions from cstring (SDL for GUI will be involved in a later stage).

While redeveloping simple String and list container hierarchy classes, I encountered a cyclical dependency issue. Previously, I fixed these kind of issues using forward declaration. However, this time, things do not go as expected and this problem kept me busy for a few evenings now.

Here is a simple UML diagram of the issue I have.

Every class has their own .cpp and .hpp files, except for BaseListItemNotFoundException I declare with a using statement above the BaseList class declaration.

class BaseListItemNotFoundException: BaseException {
    using BaseException::BaseException;
};

Even if this doesn't add any added pieces of info (IMHO), let me precise BaseList and HeplList classes are actually template classes defined using .ipp and .hpp.

I omitted some other classes involved to restrict the environment to a minimal working example (iterator and Cell generic class hierarchy used as payload for the lists). Header protections using define and ifndef conditions have been removed for clarity.

Here are a snipped of the files:

BaseList.hpp:

#include <cstddef>
#include <iostream>
#include "Cell.hpp"

class HeplString; // Forward declaration

#include "BaseException.hpp"
class BaseListItemNotFoundException: BaseException {
    using BaseException::BaseException;
};

template<class T>
class BaseList {
    // code
};

HeplList.hpp:

#include <cstddef>
#include "BaseList.hpp"
#include "Cell.hpp"

template<class T>
class HeplList : public BaseList<T> {
    // code
};

#include "HeplList.ipp"

HeplString.hpp:

#include <cstddef>
#include <iostream>
#include <ostream>
#include <fstream>
#include "HeplList.hpp"

class HeplString {
    // code
};

BaseException.hpp:

#include "HeplString.hpp"
#include "BaseList.hpp"

class BaseException {
    // code
};

The main issue I have with this example is errors like this one:

src/tests/../BaseException.hpp:9:20: error: field ‘msg’ has incomplete type ‘HeplString’
         HeplString msg;
                    ^~~
In file included from src/tests/../HeplList.hpp:5,
                 from src/tests/../HeplString.hpp:9,
                 from src/tests/test.cpp:2:
src/tests/../BaseList.hpp:9:7: note: forward declaration of ‘class HeplString’
 class HeplString;
       ^~~~~~~~~~

I don't understand what I'm doing wrong here. Reading other similar issues didn't help.

My git repository with the full code is available here, if needed: https://github.com/wget/hepl-2-cpp

Why does `BaseException` need to know about `BaseList` and `HeplString`? — Mooing Duck, Dec 05 '18 at 19:45
`BaseException` is internally using an `HeplString` in order to store the custom error message. Therefore, `BaseException` is implicitly making use of `BaseList` via the use of `HeplString`. — wget, Dec 05 '18 at 19:48
You seem to come full circle here. The ring *must* be broken somewhere. You can never create a data member with only a forward declaration, but you *can* create a pointer or reference to it. This means the instance will have to live somewhere else though. — ravnsgaard, Dec 05 '18 at 19:56
@ravnsgaard Using a reference is something I have seen as well among the proposed solutions, but with the current use case, I don't know how I could implement this in an elegant OOP manner :-/ — wget, Dec 05 '18 at 20:07
@wget Hmm... Untested, but... `BaseException` *needs* `HeplString`, so you cannot just forward declare that, but `HeplString` can make do with a forward declaration of `HeplList`. You are only returning it from one member function; that only requires a declaration, not a definition. — ravnsgaard, Dec 05 '18 at 20:23
Hint: The definition of `HeplString` _class_ probably doesn't require the definition of anything else. Only it's method definitions. That class can be your root. — Mooing Duck, Dec 05 '18 at 21:21
@ravnsgaard Unfortunately I have an explode() method in my class HeplString which manipulates the HeplList object directly, so I cannot even not forward declare it. I followed this example to understand better the situation: https://stackoverflow.com/a/553869/3514658 — wget, Dec 11 '18 at 15:27
@MooingDuck same explanation as above. I'm manipulating `HeplList`, so I do actually require inclusion. How can I solve this? I tried to break the loop by using references, but this is not working. Also subclassing `HeplString` and craft a class called `HeplStringExploded` to replace `HeplList` doesn't help either (unless I'm proceeding badly here). — wget, Dec 11 '18 at 15:30
You need to keep your declarations and definitions clearly separated. The `HeplString` *declaration* only needs a forward declaration of `HeplList`; the *definition* needs the full definition of `HeplList`. — ravnsgaard, Dec 11 '18 at 16:09
@ravnsgaard Like I said, this is exactly what I did :). I removed `#include "HeplList.hpp"` from `HeplString.hpp` and replaced it by the forward declaration `template class HeplList;`, but the `HeplString.cpp` complains with these error: https://gist.github.com/wget/2d9d156953e3e29abd94e023c54a0a98 — wget, Dec 14 '18 at 12:49
@ravnsgaard These are the two files concerned by this issue: https://github.com/wget/hepl-2-cpp/blob/master/src/HeplList.hpp and https://github.com/wget/hepl-2-cpp/blob/master/src/HeplString.hpp — wget, Dec 14 '18 at 12:51
@ravnsguard Thanks for your latest comment. Rethinking to that sentence after a good night put me on track. :) — wget, Dec 18 '18 at 14:34

score 1 · Accepted Answer · answered Dec 18 '18 at 14:28

1

Add #include "BaseException.hpp" to BaseList.hpp
Add #include "HeplList.hpp" to HeplString.cpp
Add the forward declaration template<class T> class HeplList; to HeplString.hpp
Now, you may need to modify some of your other classes which weren't including the BaseList.hpp header because they were relying on the header HeplString.hpp to do this for them.

answered Dec 18 '18 at 14:28

wget

195
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15

The commit that fixed my issue: https://github.com/wget/hepl-2-cpp/commit/ac144652c5e0146f6dce1294503dfeee16812e6c – wget Dec 18 '18 at 14:31

How to fix a C++ cyclical dependency with forward declaration in template class header

1 Answers1