[Webkit-unassigned] [Bug 70629] class X : public RefCounted<Y> should cause a compilation error

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=70629


Caio Marcelo de Oliveira Filho <cmarcelo at webkit.org> changed:

           What    |Removed                     |Added
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                 CC|                            |cmarcelo at webkit.org




--- Comment #1 from Caio Marcelo de Oliveira Filho <cmarcelo at webkit.org>  2012-04-27 06:50:56 PST ---
I've tried some solutions using traits and compile asserts without full success, I could prevent some wrong uses but not all. Unless we are willing to put a macro to include the check in the class X itself, I don't see how we can fully check by compile asserts.

The ideal solution would be to make RefCounted constructor private and make it friend to its template parameter.

template<typename T> class RefCounted : public RefCountedBase {
    friend T; // Note C++11 syntax.
    RefCounted() { ... }
    ...
};

But this support was added only in C++11. There are tricks that work for current GCC and MSVC that allow us to achieve it without C++11

template <typename T>
struct Identity {
    typedef T type;
};

template<typename T> class RefCounted : public RefCountedBase {
    friend class Identity<T>::type; // Apparently MSVC doesn't need 'class' here.
    RefCounted() { ... }
    ...
};

Is it worth it to add this prevention even if it can't cover all the compilers? My feeling is that if the check works in some EWSs, we could benefit even if the contributor compiler don't catch the problem.

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