Clash Royale CLAN TAG

restrict constructor access in variadic template class

I want to create a statically checked builder. Something along these lines:

class HasDough;
class HasCheese;

template <typename ... CurrentTypes>
class Pizza {
public:
friend Pizza<> startPizzaMaking();

Pizza<HasDough, CurrentTypes ...> addDough(){
return Pizza<HasDough, CurrentTypes ...>();
}

Pizza<HasCheese, CurrentTypes ...> addCheese(){
return Pizza<HasCheese, CurrentTypes ...>();
}

public: // I want to change to private
Pizza() = default;
};


Pizza<> startPizzaMaking() {
return Pizza<>();
}


use it like this:

int main(){
auto pizza = startPizzaMaking()
.addDough()
.addCheese();

return 0;
}


Now I can go along and check the current types with a disjunction. That way I can make sure everything is added.

My Problem is:
Each step along the way I create a new builder instance and want to pass my current state. For that the constructor should be private and I would use the friend function to obtain a new builder.

How can I restrict access to the constructor to force others to use the factory but internally I can still use it?

[update]

simply making the constructor private doesn't work. The code will expand to something like this:

Pizza<HasDough>::addCheese() {
return Pizza<HasCheese, HasDough>(); // fine with public constructor, not so with private
}


Pizza<>().addDough().addCheese()

Pizza

template <typename ...> friend class Pizza;

1 Answer
1

Different class template specialisations are completely separate classes and have no access to internals of each other. Fortunately there are always friends to save your day.

template <typename ... CurrentTypes>
class Pizza {
public:
friend Pizza<> startPizzaMaking();

Pizza<HasDough, CurrentTypes ...> addDough(){
return Pizza<HasDough, CurrentTypes ...>();
}

Pizza<HasCheese, CurrentTypes ...> addCheese(){
return Pizza<HasCheese, CurrentTypes ...>();
}

private: // OK
Pizza() = default;

friend Pizza<> startPizzaMaking();
// make every specialisation a friend of every other <=== this here trick
template <typename ...> friend class Pizza;
};

Pizza<> startPizzaMaking() {
return Pizza<>();
}


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