GrowableArray<JavaVMOption> - how it works?

[Kim Barrett]

> On Dec 1, 2020, at 1:31 PM, Dmitry Samersoff <dms at samersoff.net> wrote:
> 
> Hello Everybody,
> 
> While experimenting with the use of templates within hotspot in order to reduce VM size, I found a thing that I can't understand.
> 
> What I'm missing here?
> 
> arguments.cpp:
> 
> GrowableArray<JavaVMOption> *options = new (ResourceObj::C_HEAP, mtArguments)
> 
> Where JavaVMOption is the plain C structure {char*, void*} that comes from jni.h
> 
> but
> 
> template<class E>
> class GrowableArray : public GenericGrowableArray {
> 
> contains couple of functions like one below:
> 
>  int  find_from_end(const E& elem) const {
>    for (int i = _len-1; i >= 0; i--) {
>      if (_data[i] == elem) return i;
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>    }
>    return -1;
>  }
> 
> How it supposed to work?
> 
> If I instantiate GrowableArray<JavaVMOption> explicitly it can't be compiled at al.
> 
> Thank you!
> -Dmitry

Templates have requirements for the template parameters.

Attempting to use a type that doesn't meet the requirements is going
to fail to compile, sometimes with a really large error dump (the
so-called error novel). Concepts should eventually help with that, but
we don't have those in C++14.

GrowableArray doesn't document its requirements (HotSpot templates
rarely have such documentation, which I think is unfortunate), but
it's probably similar to the requirements for std::vector; see C++14
23.2.1 and 23.2.3.

This is all basic C++ template knowledge.

In the specific example given, E must be EqualityComparable.
JavaMVOption isn't.