Answer to C Programming Puzzle

Well yes it compiles and run with the output "d.d = 200". the main function parameter was mistyped but it compiled anyway and ran. Only if you'd used the argv parameters would you have had problems but that was a distraction from the real quirk!

With C (but not C++) struct and enum definitions is that allow the use of tags. These are optional names that give a type name to the struct and are in effect like typedefs. You could declare the struct like this

struct {
  int d;
} d;


which just declares the variable d in place with a int field d. It has no type as such. You wouldn't then need the line

struct d d;


So why would do you need tags? Linked Lists. This isn't particularly useful but it compiles!

struct d2 {
  struct d2 * pd;
};


If a struct has a tag (ie has a named type) then you can declare a pointer to it within the struct.

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