Question

In our code we have a big struct of mixed types, and want to store a duplicate (const) struct of default values for these.

When the user want to default a setting, it would be nice to be able to do this by just taking the address offset of the item within the struct, and then assign the value with the same offset in the “defaults” struct, a bit like this:

void *setting = &settings->thing; // Points to a setting
int offset = setting - &settings;
void *default = &defaults_struct + offset; // Points to the default value
*setting = *default; // Set setting to default value

The idea being that if settings->thing points to an int8, the int8 value is copied from defaults, but if settings->other_thing is an int32, the full 32-bits are copied over.

The question is, does this work with void pointers as I’ve described above? If not, is there a way of doing this? Am I missing a better way of achieving this?

Edit to clarify: We want to set a single value within the struct to the corresponding value within the “defaults” struct.

Answer 1

No, this doesn’t work. Pointers of type void * don’t have a size associated with them, i.e. it’s unknown what data type they point at. It follows that you can’t do the assignment like that, either.

You should probably just copy from the defaults directly, as suggested in a comment.

Also note that default is a reserved word in C, so you shouldn’t use it for a variable.

If the defaults are globally visible, you might define a macro to hide this:

#define INITIALIZE(s, f) s. f = default_struct. f

The user (which I assume is a developer) can now do:

struct settings s;

INITIALIZE(s, thing);

That said, I would (as a developer) prefer something like:

struct settings s = get_default_settings();

This is more clear, and less magic. Sure, it probably takes more time to copy all the fields, but settings are generally not performance-critical.

Note that the above function might just be:

struct settings get_default_settings(void)
{
  static const struct settings the_defaults = {/* the default values go here*/};

  return the_defaults;
}

Which neatly encapsulates the default values, and removes the global.