Is it safe to pass a pointer to a method as a member of a record?

Question

Hi quick question. I want to implement a function in a dll that accepts a record as a parameter and this record as a few fields that hold pointers to callback routines. Would this be safe?

Answer 1

I don't see why not. I think all the usual issues with procedure/method pointers apply, so the object needs to exist if it's a method pointer.

Answer 2

It is safe but there are two issues that you should be aware about :

Records that declared as local variables are stored in the stack and they go away when the function returns. You should consider to allocate/dispose them on the heap with new/dispose functions.

If the DLL will be used by a program developed in other than delphi (or maybe even different versions of delpi), you have to use packed records.

Answer 3

Yes, it's perfectly safe to have a pointer to a record that holds other pointers.

Your title mentions methods, though. DLLs rarely request method pointers because method pointers only exist in Delphi and C++ Builder. DLLs written expecting other tools to be able to use them will request ordinary function pointers, so please beware that method pointers are not compatible with pointers to standalone subroutines. The compiler will usually balk if you try to mix them, but type-casting can quell that error message. As a rule of thumb, if you're type-casting a function pointer or method pointer, you're doing something wrong. If your type declarations and your function declarations are correct, you won't need to type-cast.

Likewise, if you're using the @ operator to create a function pointer or method pointer, you're probably doing it wrong. In most cases, the compiler can detect and assign compatible code pointers automatically, without you telling it that there's a pointer. Using @ may suppress some of Delphi's type checking.