Operator sizeof (AKA Reading Berkeley’s FM, take II)

Last time I told you about an evil snipet I found on Oracle Berkeley DB’s manual:

  skey->size = sizeof((struct student_record *)pdata->data)->last_name;
And we concluded it’s trying to… well, dereference a number. And yet it compiles. What the hell is going on there?

The answer here is in the subtleties of the sizeof operator. That’s right, operator, not function. Plus is an operator. Less is an operator. * is a (unary) operator. sizeof is a unary operator too. The relevance of this is that operators can behave in more bizzare ways than functions do. In this case there’s a difference between this two lines:

  MyClass x;
  int a = sizeof(MyClass);
  int b = sizeof(x);

A very subtle difference. Can you spot it? a and b will have the exact same value, rest assured. The difference is in the operator itself: sizeof MUST have parenthesis when applied to a type name, yet parenthesis are optional when applied to an instance of a datatype, so this code is legal:

  MyClass x;
  int a = sizeof(MyClass);
  int b = sizeof x;

Oh, wait, the fun doesn’t stop there: sizeof also has bizarre precedence order, meaning it won’t get applied as you expect it. So, this is valid too:

  struct MyClass { int y; } x;
  int b = sizeof x->y;

Can you see where we are going? Knowing that sizeof will be applied last lets you write something like this too:

  void *ptr = ...
  int b = sizeof((X*)ptr)->y;

Which means nothing else than “store in b the size of member y in struct X. It should be easy to see why BDB’s example does compile, and why did I spend half an hour trying to understand the reason it compiled fine.

By using some more casts and a clever arangement of parenthesis you can come up with a great job security device.


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