GrowableArray<JavaVMOption> - how it works?

[Dmitry Samersoff]

Hello Kim,

Thank you!

As soon as we have c++14 enabled for jdk16, are there any plans to 
cleanup template usage in hs - add static_asserts etc?

-Dmitry


On 02/12/2020 04:19, Kim Barrett wrote:
>> 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.
>