Question

Found this answer here:
sizeof a struct member

Copied it completely into my code, but my compiler objects that I have a pointer to an incomplete class type.

uint8_t clStructCount = sizeof(((struct ALMConfStr *) 0)->IntelRecsPerPg);

What am I doing wrong? I want to set clStructCount equal to the value of IntelRecsPerPg at runtime; I thought this was the trick to do so.

Thanks!

Answer 1

The definition of your structure needs to be visible at the point where the compiler encounters your sizeof code.

So, this translation unit should work:

struct ALMConfStr {
  int IntelRecsPerPg[MagicNumber];
};
// ...
uint8_t clStructCount = sizeof(((struct ALMConfStr *) 0)->IntelRecsPerPg);

whether it’s all in one file, or the struct is in a header #included before your sizeof code.

However, this:

struct ALMConfStr;
// ...
uint8_t clStructCount = sizeof(((struct ALMConfStr *) 0)->IntelRecsPerPg);

won’t work, because the compiler doesn’t know what a struct ALMConfStr consists of, or what an IntelRecsPerPg might be in that context.