Question

Say I have a template declaration like this:

template <class A, class B, class C = A (&)(B)>

How would I make it so that I could have a variable amount of objects of type C? Doing class C ...c = x won’t work because variadic template arguments can’t have default values. So this is what I’ve tried:

template <typename T>
struct helper;

template <typename F, typename B>
struct helper<F(B)> {
    typedef F (&type)(B);
};

template <class F, class B, typename helper<F(B)>::type ... C>
void f(C ...c) { // error

}

But up to the last part I get error messages. I don’t think I’m doing this right. What am I doing wrong here?

Answer 1

template <typename T>
struct helper;

template <typename F, typename B>
struct helper<F(B)> {
    typedef F (*type)(B);
};

template<class F, class B>
void f()
{
}

template <class F, class B, typename... C>
void f(typename helper<F(B)>::type x, C... c)
{
    std::cout << x(B(10)) << '\n';
    f<F,B>(c...);
}

int identity(int i) { return i; }
int half(int i) { return i/2; }
int square(int i) { return i * i; }
int cube(int i) { return i * i * i; }

int main()
{
    f<int,int>(identity,half,square,cube);
}

Here’s a modified version that can deduce types:

template<class F, class B>
void f(F(*x)(B))
{
    x(B());
}

template <class F, class B, typename... C>
void f(F(*x)(B), C... c)
{
    f(x);
    f<F,B>(c...);
}

int identity(int i) { return i; }
int half(int i) { return i/2; }
int square(int i) { return i * i; }
int cube(int i) { return i * i * i; }
int string_to_int(std::string) { return 42; }

int main()
{
    f(identity,half,square,cube);
    // f(identity,half,string_to_int);
}