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Subject: Re: [boost] [Interprocess] Named pipe interface proposal
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2013-08-11 22:32:30

On Aug 11, 2013, at 6:25 PM, Jason Roehm <jasonr_at_[hidden]> wrote:
> 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.

Yes, but - it will initialize the contents of the vector by calling (the moral equivalent of) char::char() - which is a no-op - and will generate no code.

> 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!)

The problem with calling reserve (instead of resize) is that v.size() is still 1, and so the vector believes that it only contains a single character.
Accessing the elements of the vector (past the first element) is technically undefined behavior.

-- Marshall

Marshall Clow Idio Software <mailto:mclow.lists_at_[hidden]>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki

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