CArray help
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From: RB on 7 May 2010 09:33 Hello, I am struggling to learn the concept of template language and the usage of CArray. I have found the CArray declaration in the afxtempl.h file and I have managed to compile with no errors the apparent construction of CArray(MyStructType); But I don't really understand what next to do with it. I am having a mental block with understanding the template declaration of template<class TYPE, class ARG_TYPE> What exactly is this saying ? The constructor is just CArray( ); My compiler docs don't seem to have much on CArray except a listing of it's class members. I need to find a good (for dummies) example with comments on the usage of CArray with a struct as the data type. I feel like if I could just see an example of what it going on with it, and what incremental steps are necessary to set it up, I could start to get the concept.
From: Seetharam on 7 May 2010 09:55 Here is a sample from MSDN: http://msdn.microsoft.com/en-us/library/dh1a0227(v=VS.80).aspx Use google :-) -Seetharam
From: Hector Santos on 7 May 2010 10:14 Seetharam wrote: > Here is a sample from MSDN: > > http://msdn.microsoft.com/en-us/library/dh1a0227(v=VS.80).aspx > > Use google :-) > > -Seetharam You see, maybe if we started to say: Use Bing Or Bing it! then maybe Microsoft would change its tune about dropping the microsoft.* newsgroups. :) -- HLS
From: Giovanni Dicanio on 7 May 2010 10:37 "RB" <NoMail(a)NoSpam> wrote: > Hello, I am struggling to learn the concept of template language > and the usage of CArray. I have found the CArray declaration in > the afxtempl.h file and I have managed to compile with no errors > the apparent construction of May I suggest you to just go with STL std::vector? It is more powerful than CArray. For example, CArray does copies in a "naive" way using memcpy. This works correctly only for PODs (Plain Ol' Data) but not for more complex classes. Strange bugs may arise if you use CArray with non-POD classes. (I recall some of these bugs were discussed in this newsgroup and on some web forum.) Moreover, you can easily compose std::vector with other containers (e.g. you could simply build a vector<vector>). And vector has a better dynamically-growth policy than CArray (not bad if you store few items, but can make a difference for millions of items). CArray uses arithmetic growth, which has a very bad O(N^2) asympotic complexity; instead vector::push_back increases vector's capacity using a 1.5x factor, offering an amortized O(1) time. > I am having a mental block with understanding the template > declaration of > template<class TYPE, class ARG_TYPE> > What exactly is this saying ? The constructor is just CArray( ); Note that STL vector in its simplest form has only one template parameter, i.e. the type you store in the vector, e.g. vector<int> someIntegers; vector<MyStruct> collectionOfMyStructs; No need for the (useless and confusing, IMHO) ARG_TYPE thing. Using vector is very easy: #include <vector> // header file for std::vector // Creates an empty vector storing instances of MyClass std::vector<MyClass> data; // Add some stuff data.push_back( MyClass(...) ); data.push_back( MyClass(...) ); ... Use operator[] to retrieve vector elements (index is 0-based). And you can use resize() to change vector size. Other samples on using std::vector and description of its public methods can be easily found using your favourite search engine. HTH, Giovanni
From: Giovanni Dicanio on 7 May 2010 11:58
"Giovanni Dicanio" <giovanniDOTdicanio(a)REMOVEMEgmail.com> ha scritto nel messaggio news:#YtSPLf7KHA.1888(a)TK2MSFTNGP05.phx.gbl... > For example, CArray does copies in a "naive" way using memcpy. About the (improper) use of memcpy in CArray, it is clearly stated on MSDN documentation: CArray Class http://msdn.microsoft.com/en-us/library/4h2f09ct.aspx ---[begin]--- Most methods that resize a CArray object or add elements to it use memcpy_s to move elements. This is a problem because memcpy_s is not compatible with any objects that require the constructor to be called. If the items in the CArray are not compatible with memcpy_s, you must create a new CArray of the appropriate size. You must then use CArray::Copy and CArray::SetAt to populate the new array because those methods use an assignment operator instead of memcpy_s. ---[end]--- Giovanni |