Question

I have these two functors:

template<typename T>
struct identity {
    const T &operator()(const T &x) const {
        return x;
    }
};

template<typename KeyFunction>
class key_helper {

public:
    key_helper(const KeyFunction& get_key_) : get_key(get_key_) { }

    template<typename T, typename K>
    const K operator()(const T& x, const int& n) {
        return get_key(x);
    }

private:
    KeyFunction get_key;
};

However, if i use the second functor in a templated function. I got errors:

template<typename T, typename K>
void test(T item, K key) {

    identity<T> id;
    key_helper<identity<T> > k(id);

    K key2 = k(item, 2); // compiler cannot deduce type of K
    key2 = k.operator()<T, K>(item, 2); // expected primary-expression before ',' token
}

How can i call the functor’s operator() from the test function?

Answer 1

It can’t deduce the return type of operator(), K, in any way, so you need to explicitly specify the template arguments. The reason your second attempt doesn’t work is because you need to include the template keyword:

K key2 = k.template operator()<T,K>(item, 2);