Re: casting from void*
On 9/11/2012 9:53 AM, Cristiano wrote:
On 11/09/2012 14:59, Victor Bazarov wrote:
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;
};
I like that idea, but I get the compiler error that a union member
cannot have a copy ctor:
http://msdn.microsoft.com/en-us/library/bd149yt8%28v=vs.80%29.aspx
OK, I didn't think of that. Then perhaps unions are not such a great
idea, although you could still use a union if instead of straight
vectors, you use pointers to vectors.
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?
No, I'm still building the structures I need.
Instead of writing all that code, I guess it's better to waste some
memory, but I need to check what happens without the union.
Also, consider writing your 'Generic' actually generically (as a template):
template<class D> struct MyStuff {
char ID[6];
std::vector<D> obj_info;
};
Perhaps you need to come up with a clear interface to your class instead
of its inner workings first.
V
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