RE: [Dev-C++] Optimized memory allocation

[Louis G. <lg...@ya...>]

Ioannis,
What's the difference betweem the 3rd edition and the special edition ?
Which one do you recommend for someone who knows the basics of C++?

--- Ioannis Vranos <iv...@em...> wrote:
> #include <vector>
>  
>  
> // ...
>  
> using std::vector;
>  
> // ...
>  
> vector<my_class>array;
>  
>  
>  
> // Reserves space for 100 "my_class"es, does not create any objects.
> array.reserve(100);
>  
>  
> // Creates a my_class at the end copying the value using the assignment
> operator.
> // At push_back i passed a temporary here.
> array.push_back(my_class(7) );
>  
>  
> // Equivalent to realloc(). array[0] remains my_class(7)
> // and 9 more objects get created with the default constructor.
>  
> array.resize(10);
>  
>  
> array.erase(array.begin()+6);
>  
>  
> // Vector (random access) iterators behave like pointers
> vector<my_class>::iterator p=array.begin();
>  
>  
> //array.end() points one after the end
>  
> array.erase(p+3);
>  
>  
> // array.size() returns the number of objects
> //array.capacity() the number of reserved object space
>  
>  
>  
> It is a long story, you have to read vectors. :) I suggest "The C++
> Programming Language" 3rd Edition or Special Edition by Bjarne
> Stroustrup
> (the creator of C++). Keep in mind that you must have finished some
> introductory book first. Check this:
>  
> http://www.research.att.com/~bs/3rd.html
>  
>  
>  
>  
> Ioannis Vranos
> 
> * Programming pages: http://www.noicys.freeurl.com
> <http://www.noicys.freeurl.com/> 
> * Alternative URL 1: http://run.to/noicys
> * Alternative URL 2: http://www.noicys.cjb.net
> <http://www.noicys.cjb.net/>
> 
> 
>  
>  
>  
>  
> 
> -----Original Message-----
> From: Shane Bryldt [mailto:ast...@te...] 
> Sent: Friday, June 20, 2003 11:04 AM
> To: Ioannis Vranos
> Cc: C++
> Subject: Re: [Dev-C++] Optimized memory allocation
> 
> 
> Your idea suggest a significant performance boost, if I was referencing
> the
> memory with accessing optimization.  The issue here, is accessing, it's
> allocation performance.
> As I believe you meant,
> "new class_type[count]"
> is equivilent to,
> "malloc(count * sizeof(class_type))"
> This is the fastest method I can determine for allocation, however the
> issue
> is that I want to allocate this memory to be used and delete'd not as an
> array, but perhaps individually.  What I mean is, currently the memory
> is
> allocated with many calls to "new", whereas one call to new, allocating
> all
> the memory at once would be faster.  However, I cannot take a pointer to
> a
> specific element of the array and only delete that memory when it's no
> longer needed, I would require a management system of the array.
>  
> Herein lies the problem, the array must be allocated and deleted
> entirely.
> Multiple calls to "new" is required to ensure each memory is allocated
> and
> internally tracked separately.  I do not want one large chunk of memory
> accessed as an array, but rather simply to preallocate the required
> memory
> all at once for a specific number of objects.
>  
> The actual situation, involves as I said before, loading 48k records
> from a
> database, and for each, creating an object.  I do not want this in an
> array,
> each object is later stored in an STL map and indexed according to a
> unique
> value.  I just want to preallocate, calling constructors correctly, 48k
> objects, and then use one at a time into the map, and may later be
> individually deleted out of the map by expiration of individual records.
>  
> Your suggestion to use a vector does not imply allocating with new, as
> is
> needed for each to be allocated correctly and constructed properly. 
> There
> may not be a better solution than calling new 48k times, unfortunately.
>  
> I thought perhaps at a lower level you could preallocate a whole chunk
> of
> memory, then with another special call, construct an object in
> preallocated
> memory.  This would speed up as I would just iterate through the
> construction of each object after allocating for 48k objects, instead of
> calling new 48k times separately allocating each time.
>  
> With using any sort of memory manager to manage an array of preallocated
> memory would probably negate the speed increase.
>  
>     -Shane
> p.s. I am really nitpicking here, the entire process takes about 30
> seconds
> on average, running on the same server as the database, on low end home
> hardware, 350mhz, 256megs of ram, and a standard 5400rpm IDE drive. 
> Nothing
> special, if there are no lower level calls as I described above, then I
> can
> live with the loading time.
> 
> ----- Original Message ----- 
> From: Ioannis Vranos <mailto:iv...@em...>  
> To: 'C++' <mailto:dev...@li...>  
> Sent: Thursday, June 19, 2003 8:47 PM
> Subject: RE: [Dev-C++] Optimized memory allocation
> 
> 
> new whatever[SIZE] is equivalent to malloc(SIZE*sizeof(whatever)), that
> is
> it should boe slower.
>  
> But better use an std::vector:
>  
>  
> #include <vector>
>  
> // ...
>  
> using std::vector;
>  
> // ...
>  
> vector<class_type> array(48);
>  
> // ...
> array[0]= ...
>  
>  
>  
>  
>  
> Ioannis Vranos
> 
> * Programming pages:  <http://www.noicys.freeurl.com>
> http://www.noicys.freeurl.com
> * Alternative URL 1:  <http://run.to/noicys> http://run.to/noicys
> * Alternative URL 2:  <http://www.noicys.cjb.net/>
> http://www.noicys.cjb.net
> 
>  
>  
>  
> 
> -----Original Message-----
> From: dev...@li...
> [mailto:dev...@li...] On Behalf Of Shane
> Bryldt
> Sent: Friday, June 20, 2003 3:39 AM
> To: C++
> Subject: [Dev-C++] Optimized memory allocation
> 
> 
> Hello,
>  
>     I have recently been attempting to find ways to speed up the process
> of
> 
=== message truncated ===


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