How much functionality is "acceptable" for a C++ struct?

Question

My first post so please go easy on me!

I know that there's no real difference between structs and classes in C++, but a lot of people including me use a struct or class to show intent - structs for grouping "plain old data" and classes for encapsulated data that has meaningful operations.

Now, that's fine but at what point do you start to think that something isn't just a struct anymore and should become a class?

Things I think are reasonable for structs to have:

1. constructors with simple initialisation code only.
2. serialization code such as stream insertion / extraction operators.

Things I'm not so sure about, but would probably do:

1. comparison operators
2. Simple transformation functions - for example byteswapping all the members after receiving data from an external source.

I don't think structs should have:

1. dynamic memory allocation.
2. destructor.
3. complex member functions.

Where do the boundaries lie???

Also, is it reasonable to have class instances as members of a struct? e.g.

class C {private: int hiddenData; public: void DoSomething();};

struct S {int a; float b; C c; };

S s; s.c.DoSomething();

Remember, I'm not on about what you CAN do with C++, I'm interested in what you SHOULD do when designing good software.

Thoughts?

Answer 1

I think there are three major, coherent schools of thought:

1. People that don't care either way and use struct and class interchangeably.
2. People that use structs only to represent small POD.
3. People who use structs as records.

I can't make a conclusive argument for either of these strategies. I tend to follow path 2 but I also use structs for non-POD types when I see it fitting, especially for function objects (even if these may not fulfil POD requirements).

(Incidentally, the C++ FAQ lite has a pretty good definition of POD).

EDIT I didn't touch template metaprogramming techniques, such as using struct for placeholders (type tags) or to implement metafunctions. I guess there's absolutely no controversy in these cases: since they never contain methods (or even data), always use struct).

Answer 2

My personal preference is to only use structs if there's no methods at all. If I need to add a method for any reason, it's a class.