From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Kenneth Porter" <shiva@well.com>
To: "c++-embedded@cygnus.com" <c++-embedded@cygnus.com>
Subject: Re: biggest deterrant to using C++?
Date: Mon, 24 Aug 1998 18:39:00 -0000
Message-id: <199808250049.UAA22357@mail1.ispnews.com>
X-SW-Source: 1998/msg00027.html

On Mon, 24 Aug 1998 03:37:50 +0200, Michael Bruck wrote:

>const    atom_bomb    my_bomb(50, 100);
>
>I would like to have my_bomb somewhere in the ROM. And the example above
>should not generate any code as long as I don't call the constructors in 
>another context. What happens is, that the compiler always generates 
>a function for each source file that calls all the constructors and 
>then generates a section that contains the pointer to that function.
>These sections from all files are then linked together into a table of
>function pointers and the init-functions of all global objects are 
>called at program startup through this table. This is ok if my 
>constructors are doing something other than just copying around values.
>This would cause in most situations just some overhead, 
>but if you want to use ROM it's impossible to use (virtual) classes.

I doubt we'll see compilers soon that are clever enough to statically
init an object with a vpointer and only scalar initializers in the
ctor. 'Twould be nice, though!

I recall looking up this question a few weeks ago (can't remember
where) and I recall reading that the proper way to do this was to put
things with static data in ROM'd POD structs and including an instance
or reference to the struct in a small RAM-based class object. To hide
the POD internal structure, declare it within the class:

class Thing {
    struct ThingROMData {
        const int weight;
    };
    const ThingROMData& data;
    Thing(const ThingROMData& _data) : data(_data) {};
public:
    static void InitThings(); // calls special ctor
    virtual bool get();
};

static const Thing::ThingROMData thing1data = { 100 }; // this gets
ROM'd

The actual machinery of Thing initialization will be
application-specific, but this gives the basic idea of how to structure
the ROM part of the data.

In your example, since you want to parameterize the ctor, I'd suggest
using a macro to perform the parameterization (since macros are
expanded at compile time). I was going to suggest a template, but I
can't see how to use a template to instantiate a class instead of
creating a new type.


Kenneth Porter
Systems Engineer
Kensington Laboratories, Inc.
750 National Court
Richmond, CA 94804-2008
Voice: 510-620-0235
FAX: 510-233-5544
mailto:kenneth_porter@kensingtonlabs.com
http://www.kensingtonlabs.com