Re: C++ Practice

"Victor Bazarov" <>
Fri, 18 Apr 2008 12:16:48 -0400
mc wrote:

Hi all,

I've a long experience with C but am just getting up to speed with
C++, so please bear with me if that's obvious.

I've got a class that parses a data file and creates a vector of
AESKey, a vector of Package, GPS data and more. This data is not
sequential in the file so the parsing in the file is cumbersome and
out of my control (provided by customer).

Now, I've got another class that is build based of AESKey objects.
Currently, the constructor is as follows:
   CryptoEngine(int numKeys, AESKey * keys);

To make it more elegant -or so I think, I'm thinking of passing a
vector of AESKey to make it like this:
   CryptoEngine(std::vector<AESKey>& keys);

Unless the constructor changes the values, you might consider passing
the vector by a reference to const:

    CryptoEngine(std::vector<AESKey> const& keys);

and make your CryptoEngine actually store the reference to const
vector (and initialise in the constructor's initialiser list).

so that the CryptoEngine "owns" this vector (it's anyways the only
object using it). This would be best as when the object is deleted,
all the data it needs is deleted as well.

Without seeing more of your code it is hard to understand what you
mean by that statement.

The problem is that this vector of AESKey can be huge (100 of Mb),
don't ask me why, that's how our application is. :-( So when I'd
be copying the argument being passed to the CryptoEngine, the amount
of RAM needed for the argument is duplicated by the copy constructor
and there's basically a "memcpy". Is there a way to work this
around in a C++ way? The obvious would be to parse the file in the
CryptoEngine but it's a no-can-do as this file contains data (non
sequential) for mutiple types of objects.

Well, you can create an empty vector in your CryptoEngine and swap
it immediately with the vector passed in by a reference:

    class CryptoEngine {
        std::vector<AESKey> mKeys;
        CryptoEngine(std::vector<AESKey> & keys) {

That way your object takes possession of all the _contents_ of the
vector and clears the argument out (the data are essentially "moved"
from the argument to the member data).

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