Re: [boost] [Interprocess] Named pipe interface proposal

Subject: Re: [boost] [Interprocess] Named pipe interface proposal
From: Jason Roehm (jasonr_at_[hidden])
Date: 2013-08-11 21:25:21

On 08/11/2013 08:56 PM, Marshall Clow wrote:
>> Accessing an element of an empty vector is undefined behavior. Calling resize() to reserve space also causes elements to be initialized, which is often not needed when calling an API that indicates the number of bytes written.
>>
>> What I often do is the following:
>>
>> std::vector<char> v;
>> v.reserve(1024);
>> v.push_back(0);
>> read(&v[0], v.capacity());
>>
>> That can be simplified by a wrapper class.
> Rob --
>
> Could you have meant "resize()" instead of "reserve()" here?
>

I think he meant reserve() here, for the reason indicated above the code 
example. Using std::vector::resize() causes any newly-added elements to 
be initialized, which may be a waste of effort if you're just passing 
that vector to a function whose job is to fill it with new data. Using 
reserve() instead will allocate the required memory but won't initialize 
it. I agree that the intent of the above code perhaps isn't readily 
apparent (and if you're going to use it that way, it would make sense to 
wrap the code to hide that implementation detail), but I can see the 
argument for using it. I've had some applications in the past where 
large (multi-GB) buffers were being allocated using std::vector 
instances, and in some cases, the time it took to default-initialize all 
of the elements was non-trivial (or even a bottleneck!)

Jason