Re: Class hierarchies and concepts

ethouris <>
Mon, 19 Dec 2011 12:13:10 -0800 (PST)
On Dec 15, 12:22 am, kelvSYC <> wrote:

Suppose I have a class hierarchy, whose root class is Foo. Is there a
way to formally prescribe that subclasses of Foo has to export a
type? That is:

class Foo { // abstract base class
   // Subclasses must export a type BarType, possibly with a
consistent interface - none exists for Foo itself


struct Foo1 : public Foo {
   typedef ... BarType;


struct Foo2 : public Foo {
   struct BarType { ... };


// So Foo1::BarType and FooType::BarType are valid, and Foo::BarType
could be any of these two

If such a thing exists, is it possible to declare an abstract method
in Foo that returns BarType (where subclasses return their own

And how would you call such an abstract method?

Foo* f = obtainFoo1or2();

auto ret = f->someAbstractMethod();

then 'ret' will be of what type?

If not, is there a good workaround?

Use templates. You can use your "deriving types" as
types used to specialize a template and you can then
refer to some type defined inside it. For example:

template <class InFoo>
struct Foo
    typename InFoo::BarType someAbstractMethod() ...


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