混合模板/非模板继承分类和成员继承

Question

Below is a simple class hierarchy with mix of templatized and non-templatized classes. Embedded in the comments of this example is a compilation error I'm trying to fix.

I've read a few other related threads here on SO and attempted a few forms of the "using" keyword to no avail.

I learned in these trials that: Parent::parent_ivar_ = 99; is viable syntax but don't know how to tell the compiler about the inherited ivar grandparent_ivar_ that I'd like to set/use in the Child class.

Thanks in advance!

class GrandParent {
 public:
  int grandparent_ivar_;
};

template <typename T>
class Parent : public GrandParent {
 public:
  int parent_ivar_;
};

template <typename T>
class Child : public Parent<T> {
 public:
  Child() {
    // The following stmt produces this compiler error using ubuntu g++:
    //   templateinheritance.cpp: In constructor ‘Child<T>::Child()’:
    //   templateinheritance.cpp:20:5: error: ‘grandparent_ivar_’ was
    //   not declared in this scope
    //        grandparent_ivar_ = 100;
    //  
    grandparent_ivar_ = 100;
  }
};

int main(int argc, char *argv[]) {
  Child<int> c;
}

Answer 1

There are actually two problems here.

The first problem is that, as it's been noted in another answer, Parent must publicly inherit from GrandParent .

But that's not the only issue. Even with that fix, this will fail to compile. until Child 's constructor is changed to:

Child() {
    this->grandparent_ivar_ = 100;
  }

This is a rather nuanced parsing issue that involves templates. Until the template is completely parsed, the compiler does not have enough information to know what the heck is grandparent_ivar_ , by itself. It's not declared in the template class. It's not some global variable that has been declared previously.

Until the template is parsed fully, the compiler really can't look at its superclasses, to see what's there. Maybe that's where the grandparent_ivar_ is, perhaps. Maybe not. Who knows.

This is a rough, basic, capsule summary. The nuts and the bolts of it is that, when declaring templates, you need to give the compiler a bit more slack, and be a bit more explicit. There are a couple of ways to do it, but the easiest way is to be more liberal, and explicitly say "this-> foo ", in order to use some foo that might eventually get pulled in from the template's superclass.

Answer 2

Your problem is that Parent privately inherits GrandParent , thus Child cannot see it.

If Child needs to access GrandParent then you will need to change the inheritance level, or provide a method on Parent (which Child can see) that can be used to change grandparent_ivar_