In C++ it's possible to declare type aliases that are members of a class or struct:

struct Foo
{// internal type aliastypedef int DataType;// ...
};

Is there any way to do the same thing in Cython? I've tried the most obvious approach:

cdef struct Foo:ctypedef int DataType

But this doesn't work:

Error compiling Cython file:
------------------------------------------------------------
...
# distutils: language=c++cdef struct Foo:ctypedef int DataType^
------------------------------------------------------------internal_typedefs_example.pyx:4:4: Expected an identifier, found 'ctypedef'

Is this just a fundamental limitation of Cython (I'm using v0.21.2), or is there a workaround?

Why bother with internal typedefs? There are several general reasons - this previous SO question covers a few of them.

The specific case I'm interested in is wrapping a set of templated C++ classes that look something like this:

struct FooDataset
{typedef int DataType;typedef float ReturnType;// methods, other important stuff
};struct BarDataset
{typedef long DataType;typedef double ReturnType;// methods, other important stuff
};template <class Dataset>
class DataProcessor{DataProcessor(Dataset& input_data);typedef typename Dataset::DataType T;typedef typename Dataset::ReturnType R;T getDataItem();R computeSomething(); /* etc. */// do some other stuff that might involve T and/or R
};

Having typedef(s) internal to the struct gives me better encapsulation since I only need to pass a single template parameter (the Dataset class) rather than individually specifying the Dataset plus T, R, ... specific to that Dataset type.

I realise that it's not too difficult to find workarounds for this case - I'm mostly just interested in getting a definitive answer as to whether or not internal typedefs are currently possible in Cython.

As far as I know this is not currently supported in Cython. But can't you just define it outside of the struct?

Cython is not currently designed as a replacement for C++, rather a way to speed up hot spots in python code. If you need something more involved just write it in C++ and expose python bindings.