Question

Suppose I have this struct (which incidentally contain bit-fields, but you shouldn’t care):

struct Element {
    unsigned int a1 : 1;
    unsigned int a2 : 1;
    ...
    unsigned int an : 1;
};

and I want to access the i’th member in a convenient way. Let’s examine a retrieval solution.
I came up with this function:

int getval(struct Element *ep, int n)
{
    int val;
    switch(n) { 
         case 1: val = ep->a1; break;
         case 2: val = ep->a2; break;
         ...
         case n: val = ep->an; break;
    }
    return val;
}

But I suspect that there is a much simpler solution. Something like array accessing style, maybe.

I tried to do something like that:

 #define getval(s,n)   s.a##n

But expectedly it doesn’t work.
Is there a nicer solution?

Answer 1

Unless you have specific knowledge of the underlying structure of the struct, there is no way to implement such a method in C. There are all sorts of problems that will get in the way including

• Members of different sizes
• Packing issues
• Alignment issues
• Tricks like bitfields will be problematic

You’re best off implementing a method by hand for your struct which has a deep understanding of the internal members of the structure.