Question

Why is it that in the code below template<typename T> struct Child : public Parent requires a definition, whereas template<typename T> struct Orphan does not require one (but the presence of one does not hurt)?

#include <iostream>

struct Parent {};

// A definition is necessary.
template<typename T>
struct Child : public Parent
{};

template<>
struct Child<int> : public Parent
{
    Child() {
        std::cout << "Child<int>::Child: full specialization\n";
    }
};

// No definition is necessary (but the presence of one doesn't hurt).
template<typename T>
struct Orphan;

template<>
struct Orphan<int>
{
    Orphan() {
        std::cout << "Orphan<int>::Orphan: full specialization\n";
    }
};

int main()
{
    Orphan<int> orphan;
    Child<int> child;
}

Answer 1

template<typename T>
struct Orphan;

This is a perfectly normal forward declaration.

I suppose the language simply forbids forward declarations containing declaration of the parent. Such a syntax doesn’t exist.

So it doesn’t have much to do with templates. You can’t do the following either

class Base {};

class Derived: public Base;  //forward declaration of Derived

And in your case, you could forward declare Child as usual

template <class T>
class Child;

Now the only question is: when we are trying to create an instance of the class, do we still only have a forward declaration or do we have a complete type.