Template error with dependent names

Question

In the following code, I get a compile error that I don't have if I remove the templates:

template<int DIM> class myClass
{
public :
    enum Mode {aa,bb};
    myClass(){};
};

template<int DIM> class  myClass2
{
    myClass2(){};
    void myfunc(myClass::Mode m);
};

template<int DIM>
void myClass2<DIM>::myfunc(myClass<DIM>::Mode m)
{
}

test.cpp(19): warning C4346: 'myClass::Mode': dependent name is not a type
prefix with 'typename' to indicate a type
test.cpp(19): error C2146: syntax error: missing ')' before identifier 'm'

If I remove the like:

template<int DIM>
void myClass2<DIM>::myfunc(myClass::Mode m)

I get:

test.cpp(19): error C2955: 'myClass': use of class template requires template argument list

And if I put the definition of myfunc directly in the declaration of the class (which I would like to avoid), it works.

What should I do and why does this happen?

Thanks

Answer 1

I believe that you have two problems in your code. The first is in this declaration in myClass2 :

void myfunc(myClass::Mode m);

Because myClass is a template, you need to specify what the template parameter is. I assume that you probably meant to write

void myfunc(myClass<DIM>::Mode m);

However, due to a weird idiosyncrasy in C++, you would write this as

void myfunc(typename myClass<DIM>::Mode m);

The typename keyword here tells C++ that Mode is the name of a type nested inside of the class myClass<DIM> .

Similarly, later in the code, the code

template<int DIM>
void myClass2<DIM>::myfunc(myClass<DIM>::Mode m)
{
}

should read

template<int DIM>
void myClass2<DIM>::myfunc(typename myClass<DIM>::Mode m)
{
}

to tell the compiler that Mode is the name of a type.

Hope this helps!

Answer 2

Here you go...

template<int DIM> class myClass
{
public :
    enum Mode {aa,bb};
    myClass(){};
};

template<int DIM> class  myClass2
{
    myClass2(){};

    // you need to pass the template parameter to myClass
    // "typename" needs to be present when using types from templated classes
    // from within a templated class/function.
    void myfunc(typename myClass<DIM>::Mode m);
};

template<int DIM>
void myClass2<DIM>::myfunc(typename myClass<DIM>::Mode m)
{
}