RFR: 8319117: GrowableArray: Allow for custom initializer instead of copy constructor

[Quan Anh Mai]

On Tue, 31 Oct 2023 10:01:26 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi, 
>> 
>> When using at_put and at_put_grow you can provide a value which will be supplied to the constructor of each element. In other words, you can intialize each element through a copy constructor.
>> 
>> I suggest that we also provide a function equivalent where the function is provided a pointer to the memory to be initialized. This can be used for `NONCOPYABLE` classes, for example.
>> 
>> This is implemented using a SFINAE pattern because `nullptr` introduces ambiguity if you use static overload.
>> 
>> Currently running tier1-tier4.
>
>> I think a more preferable approach is to do emplace-like filling
>> 
>> ```
>> template <class... Args>
>> E& at_grow(int i, Args... args) {
>>     assert(0 <= i, "negative index %d", i);
>>     if (i >= this->_len) {
>>         if (i >= this->_capacity) {
>>             grow(i);
>>         }
>>         for (int j = this->_len; j <= i; j++) {
>>             _data[j].~E();
>>             new (&_data[j]) E(args...);
>>         }
>>         this->_len = i + 1;
>>     }
>>     return _data[i];
>> }
>> ```
> 
> I think you might be right. If I understand this correctly we can pick between copy construction (having `Args` be equal to `E`) and "regular" construction depending on the arguments provided?
> 
> @stefank, @dean-long. Re: tests, yes, I should add tests. The goal here is to avoid copy construction, and I chose to provide a function so that you can yourself pick how to initialize the memory. I think @merykitty's solution might be preferable to mine.

@jdksjolen Yes you are right, note that the idiom for standard C++ looks like this

    template <class... Args>
    void emplace(Args&&... args) {
        call(std::forward<Args>(args)...);
    }

And I'm not really sure what this would become without move semantics.

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PR Comment: https://git.openjdk.org/jdk/pull/16409#issuecomment-1786906761