How to invoke the constructor of a class in C++ template?

Question

Consider a C++ template class (mixin class) that inherits from the same class declared on the template, ie,

template <class Model>
class Mixin : public Model {
  ...
}

The motivation is to reuse this Mixin to extend methods for different Model classes. For example, for a model class ModelOne ,

class ModelOne {
public: 
    ModelOne (int a, int b) { ... }
}

is 'dressed' up by the Mixin class and further extended as

class Realization : public Mixin<ModelOne> {
...
}

How do I explicitly invoke - in Realization class - the constructor of ModelOne class? Different model class may have different constructor signature.

Answer 1

Realization derives from Mixin<ModelOne> , so it needs to invoke the constructor for Mixin<ModelOne> , not for ModelOne directly. The constructor for Mixin can then invoke the constructor for its Model . But if Mixin doesn't know which parameters the Model constructor takes, then Mixin will have to use variadic template parameters to pass through all specified argument values, eg:

template<typename... T>
struct Mixin : T...
{
    Mixin() = delete;
    Mixin(Mixin const &) = delete;
    Mixin(Mixin &&) = delete;

    template<typename... U>
    Mixin(U&&... v) : T(std::forward<U>(v))... { }
};

class ModelOne
{
public:
    ModelOne (int a, int b) { ... }
};

class Realization : public Mixin<ModelOne>
{
...
public:
    Realization() : Mixin<ModelOne>(1, 2) {} 
}

Also see: Constructing a mixin based on variadic templates by forwarding constructor parameters