Is it safe to forward declare a template class even if I know which generic I'll be using?

Question

This compiles, but I'm wondering if it's unsafe.

In my header I forward declare this

class QStringList;
template<> class QList<QStringList>;

class MyClass {
     ...
     static void somethin(const QList<QStringList> &thelist);
}

then in my implementation i do this

#include <QStringList>
#include <QList>
#include "MyClass.h"
void MyClass::somethin(const QList<QStringLisT> &thelist) { ... }

Is this guaranteed to be safe no matter "what?"

Thanks

Your forward declared class will need to have a definition for this to do anything, but I don't know why this would be *unsafe*? — Tony The Lion, Feb 06 '13 at 20:36
I'm not sure, but what particular reason is there that you can't just incldue QStringList and QList before your MyClass declaration? — Mats Petersson, Feb 06 '13 at 20:37
@Mats. There is no reason I can't include them, I just realized I didn't know if this would work or not and saw it compile, but I'm not sure if it's OK. — Boumbles, Feb 06 '13 at 20:39
see http://stackoverflow.com/questions/4757565/c-forward-declaration — Kyle C, Feb 06 '13 at 20:44
I assume you mean for the definition to use a reference, or for the definition to use a pointer, for `thelist`. — GManNickG, Feb 06 '13 at 21:07

score 0 · Accepted Answer · answered Feb 06 '13 at 21:11

somethin is declared to take a reference and defined to accept a pointer so that should not compile (unless it is an overload you have removed in the example.

As for the forward declaration of QList it should be fine since pointers and references such as in somethin only require forward declarations but beware that if the actual definition takes additional template arguments (possibly defaulted) then you must include those additional arguments in the forward declaration. That is one of the reasons why you should never forward declare types and functions in the standard library since they could have implementation defined additional parameters (template or otherwise) which can conflict.

Is it safe to forward declare a template class even if I know which generic I'll be using?

1 Answers1