[webkit-dev] Forward.h's Vector

JF Bastien jfbastien at apple.com
Wed Sep 13 08:11:14 PDT 2017

Hello WebCritters,

I’m moving some code around, and one particular header I have is included everywhere in JSC so I’d like it to be lightweight and include as few other headers as possible. It unfortunately uses WTF::Vector, but it only does so as a pointer:

void ohNoes(Vector<Noms>*);

It seems like something a forward declaration would fix nicely, and oh joy and wonder we have Forward.h just for this! Here’s what it does:

template<typename T, size_t inlineCapacity, typename OverflowHandler, size_t minCapacity, typename Malloc> class Vector;

That’s nice and great for T, but all the other template parameters are SOL because Vector is actually declared with default template parameters:

template<typename T, size_t inlineCapacity = 0, typename OverflowHandler = CrashOnOverflow, size_t minCapacity = 16, typename Malloc = FastMalloc>
class Vector : private VectorBuffer<T, inlineCapacity, Malloc> {

The extra painful thing is that, contrary to what I originally thought, one cannot declare Vector in Forward.h and then define it in Vector.h with the same defaults twice! Ideally the compiler would just yell at a mismatch, but instead it says error: template parameter redefines default argument (thanks clang, that’s exactly what I’m trying to do, just tell me if you catch a mismatch and ODR away otherwise).

Here’s what I propose:

Change the forward declaration in Forward.h to contain the default template parameters (and forward-declare CrashOnOverflow).
Remove these default template parameters from Vector.h.
Include Forward.h in Vector.h.
Optionally, if the WebCritters think it useful, leave a comment just above the Vector definition redirecting to Forward.h for defaults (or more fancy, use size_t inlineCapacity /*=0*/ style like LLVM’s codebase does, and have a tool that checks for consistency).
Optionally, try to fix C++20 so this isn’t A Thing anymore. Note that my hopes are low because of modules (why fix it if modules will magically make this not a thing).

Alternatively, I could just include Vector.h and be done with it.



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