模板和嵌套类型查找

Question

I have a hierarchy of template classes implementing a CRTP like pattern. I don't understand why the name lookup fails in the lines marked with errors, and succeeds in the line marked "no errors here".

class CPublishedTypes
{
    public:
    typedef int published_t;
};

template<class Derived, class PublishedTypes> class Cbase: public PublishedTypes
{
    public:
        Cbase():ibase_(42){}
    private:
        published_t ibase_; //error: 'published_t' does not name a type       
};

template<class Derived> class Cmiddle : public Cbase<Derived, CPublishedTypes>
{    
    public:
        Cmiddle():imiddle_(42){}
    private:
        published_t imiddle_; //error: 'published_t' does not name a type    
};

class Cderived : public Cmiddle<Cderived>
{
    public:
        Cderived():iderived_(42){}
    private:
        published_t iderived_; // No errors here
};

int main(int argc, char *argv[])
{
    Cderived derived;
    return 0;
}

Using 'typename' doesn't help.

The C++ FAQ describes a similar but not identical issue here: http://www.parashift.com/c++-faq/nondependent-name-lookup-types.html

Cbase inherits publicly from PublishedTypes, which is a template parameter , but it's not a template class . So I don't see why CPublishedTypes::published_t is inaccessible.

Cderived inherits from Cmiddle which in turn inherits from Cbase. So I don't see why I don't need a typename to access CPublishedTypes::published_t here.

What am I missing in the name lookup rules?

Answer 1

typename PublishedTypes::published_t

and

typename Cbase<Derived, CPublishedTypes>::published_t
// or simpler
typename Cmiddle::published_t

respectively, will work.

They're types that are depending on a template parameter (and can't be lookup up until the template gets instantiated) and thus need to be both qualified and prepended by typename .

Answer 2

Types dependent on the template parameter need to be prefixed with typename and, in case of template base classes, the name of the parent class. In the last example, however, the class is not a template and this is thus not necessary as type lookup for Cderived is not delayed until template instantiation, so the regular lookup rules apply.