-------- Original Message -------- Subject: Re: [Boost-Users] Four Questions on boost::shared_ptr Date: Tue, 05 Feb 2002 10:08:25 +0900 From: hicks <hicks@kgk.co.jp> To: Boost-Users@yahoogroups.com, Duane Murphy <duanemurphy@mac.com> References: <B8838630.133F2%darin@bentspoon.com> <20020204170343.12718@mail.murphyslogic.com> Hi

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You should normally use std::vector unless you have some special reason to use boost:: scoped_array and boost::shared_array. Those classes are provided both for completeness, and because occasionally there are such special reasons. But not often.

I consider user vector<> often. However there is a cost. For a fixed size allocation of a buffer there is only the data allocated. If you use a vector<> the vector also includes (for most implementations) the current capacity, the current size, and the allocator.

There is another sometimes very big overhead incurred by std::vector, and not incurred by by new T[]/ delete[]. Most implementions of std::vector will enter a loop and call a destructor on each element of the array, even if there is no machine code associated with that destructor. At least Borlands implemention of delete[] automatically detects this condition and does not insert a loop to call the destructors. This could be avoided if std::vector used a traits type to determine whether to enter a loop calling destructors. Craig Hicks KGK Tokyo Duane Murphy wrote:

--- At Mon, 4 Feb 2002 00:28:32 -0800, Darin Adler wrote:

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On 2/3/02 11:54 PM, "Jon Wang" <webmaster@boost.100ok.com> <mailto:webmaster@boost.100ok.com> wrote:

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1. Mr Stroustrup said that there wasn't a need for auto_array, for "a

better

...

...

solution is to use a vector". So, should we use vector or scoped_array/shared_array?

You should normally use std::vector unless you have some special reason to use boost:: scoped_array and boost::shared_array. Those classes are provided both for completeness, and because occasionally there are such special reasons. But not often.

I consider user vector<> often. However there is a cost. For a fixed size allocation of a buffer there is only the data allocated. If you use a vector<> the vector also includes (for most implementations) the current capacity, the current size, and the allocator.

I would be interested in finding a class that is somewhere between allocating my own buffers and vector<>. Low overhead for simple allocations; no resizing or growing. Even in this case, in order to support iterator semantics the size is going to be needed.

Sigh.

I suppose a case like this might be when scoped_array would be used?

...Duane

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