Why does the child class not see the parent's typedef struct in C++?

Question

I ran across some C++ code recently that typedefed a struct in a parent class. However, it seemed to be unavailable in the child class, even tho it was not marked private (it was protected). I've created a minimal working example (below) below that demonstrates this failure. I have turned everything public and it still fails. The error that is given is (using g++):

B.h:8: error: expected ',' or '...' before '&' token
B.h.8: error: ISO C++ forbids declartion of 'Datum' with no type

A.h (compiles)

template<typename S, typename T> class A {
public:
    typedef struct {
        S x;
        T y;
    } Datum;
};

B.h (does not compile)

#include "A.h"
template<typename Q> class B : public A<Q, Q> {
public:
    void output(const Datum& dat);
};

B.h (compiles)

#include "A.h"
template<typename Q> class B : public A<Q, Q> {
public:
    typedef struct {
        Q x;
        Q y;
    } Datum;
    void output(const Datum& dat);
};

Why does the first version of B.h not compile? Is the second one a safe alternative? Is there a better (more concise or idiomatic) way to handle this?

Answer 1

This

typedef struct {
    S x;
    T y;
}

is ill-formed. typedef needs a type and an "alias" to name that type. What you probably need is

template<typename S, typename T> 
class A {
 public:
    struct Datum { S x; T y; };
};

without the typedef, which is not necessary at all here. Then, you need to qualify the Datum name properly, as typename A<Q,Q>::Datum:

#include "A.h"
template<typename Q> 
class B : public A<Q, Q> {
 public:
    void output(const typename A<Q,Q>::Datum& dat);
};

Answer 2

You need to say typename A<Q,Q>::Datum inside B. Since the base is a template, its names are dependent names and not visible during the first phase, and you need typename to specify that the name names a type (and not a value or a template).

You should also lose the typedef. C++ works differently from C, and you should say:

struct Datum { S x; T y };