if d-tor is called, does it means the object is gc-ed?

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You can already access this information using, iirc, std.gc.getStats(). 
  Just pull the usedsize and poolsize (used and allocated memory, 
respectively.)

-[Unknown]


>> I think the std.gc module should also provide some APIs like
>>
>> ulong std.gc.allocatedMemory();  // in bytes
>> ulong std.gc.availableMemory();
>>
>> So we can monitor the memory usage within the program itself.
> 
> This certainly wouldn't be difficult to implement.