Can't use struct in derived templated class?

Question

Ok, maybe not the best title, but here's the deal:

I have a templated interface:

template<typename T>
class MyInterface
{
public:
    struct MyStruct 
    {
        T value;
    };

    virtual void doThis(MyStruct* aPtr) = 0;
};

and an implementation:

template <typename T>
class MyImpl : public MyInterface<T>
{
public:
    void doThis(MyStruct* aPtr)
    {
    } // doThis
};

However, the compiler complains:

In file included from MyTest.cpp:3:0:
MyImpl.h:7:17: error: ‘MyStruct’ has not been declared
     void doThis(MyStruct* aPtr)

Why is that?

Answer 1

The following compiled for me:

template<typename T>
class MyInterface
{
public:
    struct MyStruct
    {
        T value;
    };

    virtual void doThis(MyStruct* aPtr) = 0;
};

template <typename T>
class MyImpl : public MyInterface<T>
{
public:
    void doThis(typename MyInterface<T>::MyStruct* aPtr)
    {
    }
};


int main() {
  MyImpl<int> t;

}

The main change is that you need to qualify that the MyStruct was defined within MyInterface<T> .

Since the compiler cannot determine what kind of identifier is the templated subtype, you must help it using the typename keyword. (See When is the "typename" keyword necessary? for more details on typename)