c++: inherit from instantiated template class fail to compile: cannot find members - c++

Question

I've got the following code:
struct My {
int id;
int value;
};
template < class T >
class IsElement : public T {
protected:
IsElement<T>* the_next;
};
struct elem : public IsElement<My> {
elem(int i, int v) : id(i), value(v), the_next(nullptr) {
}
};
int main() {
return 0;
}
gcc 8.1 failed to compile it, and gives error:
<source>: In constructor 'elem::elem(int, int)':
<source>:13:26: error: class 'elem' does not have any field named 'id'
elem(int i, int v) : id(i), value(v), the_next(nullptr) {
^~
<source>:13:33: error: class 'elem' does not have any field named 'value'
elem(int i, int v) : id(i), value(v), the_next(nullptr) {
^~~~~
<source>:13:43: error: class 'elem' does not have any field named 'the_next'
elem(int i, int v) : id(i), value(v), the_next(nullptr) {
^~~~~~~~
Execution build compiler returned: 1
I think all members are available to be accessed, why can't find "field name"? How to fix it?

Answer 1

Even though you have not implemented it, the base class has a constructor, a default constructor. When the default constructor is called the variables are allocated and initialized. You cannot initialize base class variables in the initialize list, as the base class is not yet created. You can create a constructor in base class and call that in the initialize list.
struct My {
public:
int id;
int value;
};
template < class T >
class IsElement : public T {
public:
IsElement(IsElement<T> * p) : the_next(p) {}
protected:
IsElement<T>* the_next;
};
struct elem : public IsElement<My> {
elem(int i, int v) : IsElement(nullptr) {
id = i,
value = v;
}
};
int main() {
elem e1(0, 0);
return 0;
}
Another way without introducing constructor in template class:
struct My {
public:
int id;
int value;
};
template < class T >
class IsElement : public T {
public:
//IsElement(IsElement<T> * p) : the_next(p) {}
protected:
IsElement<T>* the_next;
};
struct elem : public IsElement<My> {
elem(int i, int v) {
id = i,
value = v;
the_next = nullptr;
}
};
int main() {
elem e1(0, 0);
return 0;
}
You can also call constructor explicitly in the code block of derived class constructor.
elem(int i, int v) :IsElement(nullptr) {
IsElement(nullptr);
the_next = nullptr;
id = i,
value = v;
}