Question

I was recently reading this article on structs and classes in D, and at one point the author comments that

…this is a perfect candidate for a struct. The reason is that it contains only one member, a pointer to an ALLEGRO_CONFIG. This means I can pass it around by value without care, as it’s only the size of a pointer.

This got me thinking; is that really the case? I can think of a few situations in which believing you’re passing a struct around “for free” could have some hidden gotchas.

Consider the following code:

struct S
{
    int* pointer;
}

void doStuff(S ptrStruct)
{
    // Some code here
}

int n = 123;
auto s = S(&n);
doStuff(s);

When s is passed to doStuff(), is a single pointer (wrapped in a struct) really all that’s being passed to the function? Off the top of my head, it seems that any pointers to member functions would also be passed, as well as the struct’s type information.

This wouldn’t be an issue with classes, of course, since they’re always reference types, but a struct’s pass by value semantics suggests to me that any extra “hidden” data such as described above would be passed to the function along with the struct’s pointer to int. This could lead to a programmer thinking that they’re passing around an (assuming a 64-bit machine) 8-byte pointer, when they’re actually passing around an 8-byte pointer, plus several other 8-byte pointers to functions, plus however many bytes an object’s typeinfo is. The unwary programmer is then allocating far more data on the stack than was intended.

Am I chasing shadows here, or is this a valid concern when passing a struct with a single reference, and thinking that you’re getting a struct that is a pseudo reference type? Is there some mechanism in D that prevents this from being the case?

Answer 1

I think this question can be generalized to wrapping native types. E.g. you could make a SafeInt type which wraps and acts like an int, but throws on any integer overflow conditions.

There are two issues here:

1. Compilers may not optimize your code as well as with a native type.

   For example, if you’re wrapping an int, you’ll likely implement overloaded arithmetic operators. A sufficiently-smart compiler will inline those methods, and the resulting code will be no different than that as with an int. In your example, a dumb compiler might be compiling a dereference in some clumsy way (e.g. get the address of the struct’s start, add the offset of the pointer field (which is 0), then dereference that).

   Additionally, when calling a function, the compiler may decide to pass the struct in some other way (due to e.g. poor optimization, or an ABI restriction). This could happen e.g. if the compiler doesn’t pay attention to the struct’s size, and treats all structs in the same way.

2. struct types in D may indeed have a hidden member, if you declare it in a function.

   For example, the following code works:

   import std.stdio;
   
   void main()
   {
       string str = "I am on the stack of main()";
   
       struct S
       {
           string toString() const { return str; }
       }
   
       S s;
       writeln(s);
   }
   

   It works because S saves a hidden pointer to main()’s stack frame. You can force a struct to not have any hidden pointers by prefixing static to the declaration (e.g. static struct S).