template class class indirect_fun public explicit indirect_fun const f

template<class T> class indirect_fun {
public:
        explicit indirect_fun(const T& fun): fun_(fun) {}
        
        indirect_fun() = default;
        indirect_fun(const indirect_fun<T>&) = default;
        indirect_fun(indirect_fun<T>&&) = default;
        
        indirect_fun<T>& operator=(const indirect_fun<T>&) = default;
        indirect_fun<T>& operator=(indirect_fun<T>&&) = default;

        template<class... Args> auto operator() (Args&&... args) -> decltype(this->fun_((*args)...)) {
                return fun_((*args)...);
        }
        template<class... Args> auto operator() (Args&&... args) const -> decltype(this->fun_((*args)...)) {
                return fun_((*args)...);
        }
private:
        T fun_;
};