Re: casting from void*
On 9/11/2012 7:15 AM, Cristiano wrote:
I have a structure like this (the actual structure is much bigger):
struct Generic {
char ID[6];
std::vector <obj1_info> info1;
std::vector <obj2_info> info2;
std::vector <obj3_info> info3;
std::vector <obj4_info> info4;
};
I use it to store 4 types of objects.
Each object needs its own std::vector to store informations related to
that object.
To avoid wasting of space I though to use a void * instead:
struct Generic {
char ID[6];
void *ptr;
};
Not a good idea, I know.
First question: does anybody have a good idea?
Use a union, that's what they are for.
struct Generic {
char ID[6];
union {
std::vector<obj1_info> info1;
std::vector<obj2_info> info2;
...
} u;
};
That way all of your 'infoN' objects will share the memory, and since
they can never co-exist (IOW, it's exclusive use of memory) and you can
ensure that what you put into the 'u' is what you get out (IOW, never
try to write 'u.info1' and then read 'u.info2'), you're going to be OK.
I allocate the right std::vector using:
ptr= new std::vector<obj1_info>;
obj1_info x; x.var= 0;
((std::vector<obj1_info>*)ptr)->push_back(x);
but when I try to read x (using some horrible casting) I get errors
(violation access, wrong result).
How should I cast the ptr pointer?
Don't.
With a union you will need to make sure to clear the currently allocated
vector just before you put the other one in (if ever), but that's the
only hassle.
Now, if you still don't want to use a union, I'd recommend writing
accessor methods for 'Generic'. Something like
struct Generic {
... // whatever data layout
template<class D> Generic(const D& primer) {
// set the ID somehow
std::vector<D> *pV = new std::vector<D>();
pV->push_back(primer);
ptr = pV;
}
};
and use it similarly to
obj1_info x; // and whatever else
..
Generic gen(x); // will invoke the right constructor
You will also need accessors for the elements of those vectors that will
check the ID, but for those you will need to supply the ID. Do you have
the interface in mind?
As you probably already understand, I *strongly* recommend *against*
keeping your 'Generic' type the "dumb data". Give it some intelligence
by supplying proper member functions for accessing the data. You will
thank yourself later.
Good luck!
V
--
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