CODEWITHSUNDEEP
c++templatestemplate-specialization
motam79
motam79

Reputation: 3824

What is this forward declaration needed in a variadic template class?

Here is a code for implementing decorator pattern in C++. It basically accepts a function, and returns a functor. This code works, but I would like to understand why we need the first line (i.e. forward declaration). If this line is removed, I get this compile error: ‘Decorator’ is not a class template

template <class> class Decorator;

template <class R, class... Args>
class Decorator<R(Args ...)>
{
    std::function<R(Args ...)> f_;
public:
    Decorator(std::function<R(Args ...)> f) : f_(f) {}
    R operator()(Args ... args)
    {
        auto res = f_(args...);
        std::cout << fmt::format("Calling the decorated function with res={}.\n", res);
        return res;
    }
};

Upvotes: 2

Views: 228

Answers (1)

songyuanyao
songyuanyao

Reputation: 172964

This is using partial template specialization. The 1st one is the declaration of the primary template, the 2nd one is the partial specialization, for which you have to declare the primary template first.

When you specify the template argument with function type, the specialization version is applied, and it would get the parameter type (i.e. Args...) and return type (i.e. R) of the specified function type.

Upvotes: 2

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