Re: [boost] How to get the size of an C-array?

Subject: Re: [boost] How to get the size of an C-array?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2009-10-15 16:19:44

vicente.botet wrote:

>> I'm not sure I understand you here. Are you saying that other threads 
>> may be accessing the array elements during the destruction?
> 
> Yes. This is the case I try to solve.

Why not locking the mutex (or whatever you use to synchronize the 
threads) in the destructor? It will block if another thread is working 
with the object.

>> All in all, the overall scheme is as follows:
>>
>>         static void* operator new[] (std::size_t size)
>>         {
>>             void* p = std::malloc(sizeof(info_t) + gap_size + size);
>>             if (p)
>>             {
>>                 new (p) info_t();
>>                 static_cast< info_t* >(p)->m_Count = size / sizeof(T);
> 
> Are you sure the size given as parameter consider only the user info? 
 > Doesn't this size includes any additional part needed by the standard
 > library to know if the the allocation corresponds to a single element
 > or to an array and whatever is needed to call the destructors of all
 > the array elements?

Hmm... Looks like you're right here. The Standard doesn't say what the 
size will be and at least gcc does add a little to the size. However, if 
sizeof(T) is big enough, you may not notice this extra increase in size.

Well, unless you want to tweak your code for every compiler/platform to 
take that additional info into account, I don't see how this can be 
solved. You might want to use an additional argument to the operator 
new[] to pass the array size twice, but this essentially eliminates the 
advantage of using the regular operator new. Still, you will be able to 
use scoped_array, shared_array, etc.