Re: Templates HowTo?

"Alf P. Steinbach" <>
Thu, 21 Feb 2008 16:15:08 +0100

On 21 Feb, 09:20, "Alf P. Steinbach" <> wrote:


Now this seems from my reading to be the sort of thing that templates
are probably good at, but I can't for the life of me see how to do it,
without first creating a separate class for each of AAA, BBB, and CCC.

Each of group AAA, BBB and CCC essentially constitute a functor object.

Hi again. I'm going to try my luck one more time and ask for some
more advice on this!

Alf's suggestions upthread all worked swimmingly, but I'm left with
one thing that I can't quite work out.

The actual template is intended as a wrapper for the ciphers in
LibTomCrypt[1], and one of the things I need to do is find out the
'index' of a particular cipher by passing the cipher name as a string
to a helper function. Here's what I've got so far with Alf's help:

//--------------------- ltc.h START------------------------
#include "tomcrypt.h"

/* Our abstract base class */
class AbstractCipher
  virtual ~AbstractCipher() {}
  virtual int setup (const uint8_t* key, int keylen) = 0;
  virtual int ecb_encrypt (const uint8_t *ptext, uint8_t* ctext) = 0;
  virtual int ecb_decrypt (const uint8_t *ctext, uint8_t* ptext) = 0;
  virtual int test () = 0;
  virtual int keysize (int* keysize) = 0;
  virtual void done () = 0;
  virtual int index () = 0;

/* Our functors */
struct CFunctionTypes
  typedef int (*Setup)
    (const uint_8t* key, int keylen, int rounds, symmetric_key* skey);
  typedef int (*ECBencrypt)
    (const uint8_t *ptext, uint8_t* ctext, symmetric_key* skey);
  typedef int (*ECBdecrypt)
    (const uint8_t *ctext, uint8_t* ptext, symmetric_key* skey);
  typedef int (*Test)
  typedef int (*Keysize)
    (int* keysize);
  typedef void (*Done)
    (symmetric_key* skey);

/* Now specialise the functors for each cipher */
struct AESCipher:CFunctionTypes
  static Setup const setup;
  static ECBencrypt const ecb_encrypt;
  static ECBdecrypt const ecb_decrypt;
  static Test const test;
  static Keysize const keysize;
  static Done const done;
  AESCipher() { register_cipher(&aes_desc); index =
  ~AESCipher() { unregister_cipher(&aes_desc); }

  int index;

CFunctionTypes::Setup const AESCipher::setup = &aes_setup;
CFunctionTypes::ECBencrypt const AESCipher::ecb_encrypt =
CFunctionTypes::ECBdecrypt const AESCipher::ecb_decrypt =
CFunctionTypes::Test const AESCipher::test = &aes_test;
CFunctionTypes::Keysize const AESCipher::keysize =
CFunctionTypes::Done const AESCipher::done = &aes_done;

struct TwofishCipher:CFunctionTypes
  static Setup const setup;
  static ECBencrypt const ecb_encrypt;
  static ECBdecrypt const ecb_decrypt;
  static Test const test;
  static Keysize const keysize;
  static Done const done;
  TwofishCipher() { register_cipher(&twofish_desc); index =
  ~TwofishCipher() { unregister_cipher(&twofish_desc); }

  int index;

CFunctionTypes::Setup const TwofishCipher::setup =
CFunctionTypes::ECBencrypt const TwofishCipher::ecb_encrypt =
CFunctionTypes::ECBdecrypt const TwofishCipher::ecb_decrypt =
CFunctionTypes::Test const TwofishCipher::test =
CFunctionTypes::Keysize const TwofishCipher::keysize =
CFunctionTypes::Done const TwofishCipher::done =

template < class CFunctions > class Cipher : public AbstractCipher
  Cipher () : skey() {}

  virtual int setup (const uint_8t* key, int keylen)
  { return CFunctions::setup (key, keylen, 0, &skey); }

  virtual int ecb_encrypt (const uint8_t *ptext, uint8_t* ctext)
  { return CFunctions::ecb_encrypt (ptext, ctext, &skey); }

  virtual int ecb_decrypt (const uint8_t *ctext, uint8_t* ptext)
  { return CFunctions::ecb_decrypt (ctext, ptext, &skey); }

  virtual int test ()
  { return CFunctions::test(); }

  virtual int keysize (int* keysize)
  { return CFunctions::keysize(keysize); }

  virtual int index()
  { return CFunctions::index; }

  virtual ~Cipher ()
  { CFunctions::done (&skey); }

    symmetric_key skey;

typedef Cipher < AESCipher > AES;
typedef Cipher < TwofishCipher > Twofish;

//--------------------- ltc.h END------------------------
//--------------------- BEGIN-----------------------
#include <iostream>
#include "ltc.h"

int main()
  AES aes;
  std::cerr << "AES.index() = " << aes.index();
  return 0;

//--------------------- END-------------------------

Now you can probably see what I'm trying to do with this 'index' call;
it has to return the result of find_cipher("ciphername"). The problem
is that at compile time, I get the following error:

ltc.h: In member function 'int Cipher<CFunctions>::index() [with
CFunctions = AESCipher]': instantiated from here
ltc.h:40: error: object missing in reference to 'AESCipher::index'
ltc.h:93: error: from this location

I can't see what it is that's missing. If someone can clarify, or
even better, show me a better way to do what I'm trying to do, I'd be
grateful. Meanwhile, I'm learning a _lot_ doing this stuff!

Well, AESCipher::index is non-static data member, that is, it only
exists in instances of AESCipher, one per instance.

To access it you'd need an AESCipher instance, but AESCipher is
currently not designed for instantiation: it's only designed as a
compile time carrier of addresses (by the way, I forgot, those function
types should probably all be 'extern "C"', but that's just a nit).

What you need seems to be singletons, and for singletons the usual way,
when you don't have other requirements, is the "Meyer's singleton"

   class SingletonType
       SingletonType( SingletonType const& );
       SingletonType& operator=( SingletonType const& );

       static SingletonType& instance()
           static SingletonType theInstance; // Add constructor args
           return theInstance;

It might also be a good idea to put all initialization calls in
constructors, and all cleanup calls in destructors. That is, no "setup"
or "cleanup" functions visible to client code at the C++ level. Of
course there may be some requirement I don't know or understand, but
usually, constructors and destructors are best for initialization and
cleanup: it automates the calls, and makes for better exception safety.

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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