C++: Iterating through all of an object's members?

Question

Suppose I have an object with many members:

class Example {
    AnotherClass member1;
    AnotherClass member2;
    YetAnotherClass member3;
    ...
};

Is there a short/concise way to do something like:

foreach(member m: myExample)
    m.sharedMethod();

Instead of accessing each one individually?

I think I could put them in a vector and use a shared_ptr for the same effect, I was just wondering if say, Boost or some other popular library doesn't have something to do this automatically.

Answer 1

C++ does not support class introspection, so you cannot iterate over all of the members in a class like that - not without having a (manually written) function that iterates over all members for you anyway.

You could, in principle, add a template member like so:

template<typename Functor>
void doAllMembers(Functor &f) {
  f(member1);
  f(member2);
  f(member3);
}

That said, I would regard this as a broken design; you've gone and exposed all of your internal members publicly. What happens if you add one later? Or change the semantics of one? Make one a cached value that's sometimes out of date? etc. Moreover, what happens if you have members which don't all inherit from the same types?

Step back and reconsider your design.

Answer 2

There are several solutions to this issue, contrary to what the naysayers blabber, but no built-in way.

C++ support a limited kind of introspection, at compile-time: you can check the template parameters.

Using either Boost.Tuple or Boost.Fusion (for its map), you can indeed achieve what you wish. In Boost.Fusion you even have BOOST_FUSION_ADAPT_STRUCT to transform a basic structure into a Fusion Sequence (and thus iterate it).

It requires quite a bit of template meta-programming though.