Unpack ts... to t0.a(), t0.b(), t1.a(), t1.b(), - c++

I want to call one variadic function from another variadic function in the following manner:
template <typename ...Ts>
void f(Ts const & ...) { /* ... */ }
template <typename ...Args>
void g(Args const & ...args)
{
// shall call f(arg0.a(), arg0.b(), arg1.a(), arg1.b(), ...)
}
I have done it the following way:
struct sentinel_t { };
template <typename Arg, typename ...Args>
void g_impl(Arg const & arg, Args const & ...args)
{
g_impl(args..., arg.a(), arg.b());
}
template <typename ...Ts>
void g_impl(sentinel_t , Ts const & ...ts)
{
f(ts...);
}
template <typename ...Args>
void g(Args const & ...args)
{
g_impl(args..., sentinel_t{});
}
Is there another/better way to implement this pattern?


template <class... Args> void g(Args const &... args) {
impl::apply([](auto const &... p) { f(p...); },
std::tuple_cat(std::forward_as_tuple(args.a(), args.b())...));
}
until std::apply will be standardized you can make your own pretty simple with c++14 (taken from the referenced paper):
namespace impl {
template <typename F, typename Tuple, size_t... I>
decltype(auto) apply_impl(F &&f, Tuple &&t, std::index_sequence<I...>) {
return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
}
template <typename F, typename Tuple> decltype(auto) apply(F &&f, Tuple &&t) {
using Indices =
std::make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>;
return apply_impl(std::forward<F>(f), std::forward<Tuple>(t), Indices{});
}
}


You may do
namespace detail
{
// dispatcher
template <typename T>
decltype(auto) call_a_b(std::integral_constant<std::size_t, 0u>, const T& arg) {return arg.a();}
// dispatcher
template <typename T>
decltype(auto) call_a_b(std::integral_constant<std::size_t, 1u>, const T& arg) {return arg.b();}
template <std::size_t... Is, typename Tuple>
void call_f_a_b(std::index_sequence<Is...>, const Tuple& tuple)
{
f(call_a_b(std::integral_constant<std::size_t, Is % 2>{}, std::get<Is / 2>(tuple))...);
}
}
template <typename...Ts>
void g(const Ts&... args)
{
return detail::call_f_a_b(std::make_index_sequence<2 * sizeof...(Ts)>{}, std::tie(args...));
// call f(arg0.a(), arg0.b(), arg1.a(), arg1.b(), ...)
}
Live Demo

Related

Combing two factory methods returning unique_ptr and shared_ptr into one in C++?

Lets assume I have two factory functions, one returning std::unique_ptr and the other returning std::shared_ptr:
template<class T, class... Args>
std::shared_ptr<T> createObjectS (Args... args)
{
// running some code
return std::make_shared<T>(args...);
}
template<class T, class... Args>
std::unique_ptr<T> createObjectU (Args... args)
{
// running some code
return std::make_unique<T>(args...);
}
Is it possible to combine these two functions into one using template meta programming?

You could use SFINAE, but then I don't really see the point to have this inside a function anymore. It's pretty redundant.
#include <memory>
#include <type_traits>
template <class T, class... Args>
typename std::enable_if<
std::is_same<T, std::shared_ptr<typename T::element_type>>::value, T>::type
createObject(Args&&... args) {
// running some code
return std::make_shared<typename T::element_type>(std::forward<Args>(args)...);
}
template <class T, class... Args>
typename std::enable_if<
std::is_same<T, std::unique_ptr<typename T::element_type>>::value, T>::type
createObject(Args&&... args) {
// running some code
return std::make_unique<typename T::element_type>(std::forward<Args>(args)...);
}
int main() {
auto s = createObject<std::shared_ptr<int>>(1);
auto u = createObject<std::unique_ptr<int>>(1);
}
A little bit more compact but essentially the same idea with a scoped enum
#include <memory>
enum class ptr_t { shared, unique };
template <ptr_t P, class T, class... Args>
typename std::enable_if<P == ptr_t::shared, std::shared_ptr<T>>::type
createObject(Args&&... args) {
// running some code
return std::make_shared<T>(std::forward<Args>(args)...);
}
template <ptr_t P, class T, class... Args>
typename std::enable_if<P == ptr_t::unique, std::unique_ptr<T>>::type
createObject(Args&&... args) {
// running some code
return std::make_unique<T>(std::forward<Args>(args)...);
}
int main() {
auto s = createObject<ptr_t::shared, int>(1);
auto u = createObject<ptr_t::unique, int>(1);
}
In C++17 you of course use if constexpr in both cases rather than SFINAE.
#include <memory>
enum class ptr_t { shared, unique };
template <ptr_t P, class T, class... Args>
decltype(auto) createObject(Args &&... args) {
// running some code
if constexpr (P == ptr_t::shared) {
return std::make_shared<T>(std::forward<Args>(args)...);
} else if (P == ptr_t::unique) {
return std::make_unique<T>(std::forward<Args>(args)...);
}
}

With specialization, you may do:
template <typename T> struct FactoryImpl;
template <typename T> struct FactoryImpl<std::unique_ptr<T>>
{
template <typename ... Ts>
auto operator ()(Ts&&... args) const
{
return std::make_unique<T>(std::forward<Ts>(args)...);
}
};
template <typename T> struct FactoryImpl<std::shared_ptr<T>>
{
template <typename ... Ts>
auto operator ()(Ts&&... args) const
{
return std::make_shared<T>(std::forward<Ts>(args)...);
}
};
template<class T, class... Ts>
auto createObjectS (Ts&&... args)
{
return FactoryImpl<T>{}(std::forward<Ts>(args)...);
}
with usage:
auto s = createObject<std::shared_ptr<MyObject>>(42);
auto u = createObject<std::unique_ptr<MyObject>>(42);

generic call of a std::function from QVariantList

I'm looking for a generic way to call a std::function with arguments from a QVariantList. This Version works but has the drawback the template parameters must be specified:
template <typename... T>
struct VariantFunc {
static void Invoke(std::function<void(T...)> f, const QVariantList& args)
{
InvokeHelper(f, args);
}
private:
template <typename T1>
static void InvokeHelper(std::function<void(T1)> f, const QVariantList& args)
{
f(args.at(0).value<T1>());
}
template <typename T1, typename T2>
static void InvokeHelper(std::function<void(T1, T2)> f, const QVariantList& args)
{
f(args.at(0).value<T1>(), args.at(1).value<T2>());
}
template <typename T1, typename T2, typename T3>
static void InvokeHelper(std::function<void(T1, T2, T3)> f, const QVariantList& args)
{
f(args.at(0).value<T1>(), args.at(1).value<T2>(), args.at(2).value<T3>());
}
};
auto args = QVariantList() << 100 << QString("hello") << QJsonValue(1234);
auto f = [](int i, QString s, QJsonValue j) { qDebug() << i << s << j; };
VariantFunc<int, QString, QJsonValue>::Invoke(f, args);
I'd like to have such an implementation:
struct VariantFunc2 {
template<typename Func>
static void Invoke(Func f, const QVariantList& args)
{
// ???
}
};
auto args = QVariantList() << 100 << QString("hello") << QJsonValue(1234);
auto f = [](int i, QString s, QJsonValue j) { qDebug() << i << s << j; };
VariantFunc2::Invoke(f, args);
Is there a way to do this in C++11?
Obviously the deduction guides for std::function provides a solution, but not with C++11.

Here is a C++14 implementation. It does not use std::function, to avoid the overhead associated with said class. Instead it uses perfect forwarding of the callable. Additionally, the Invoke function returns the result of invoking the callable.
template <typename F>
struct function_traits;
template <typename R, class... Args>
struct function_traits<R(*)(Args...)> : public function_traits<R(Args...)> {};
template <typename R, class... Args>
struct function_traits<R(Args...)> {};
template <typename C, class R, class... Args>
struct function_traits<R(C::*)(Args...) const> : public function_traits<R(C&, Args...)>
{
using args = std::tuple<Args...>;
};
template <typename F, typename Args, std::size_t ... I>
auto Invoke_impl(F&& f, const QVariantList& args, std::index_sequence<I ...>)
{
return f(args.at(I).value<std::remove_const_t<std::remove_reference_t<std::tuple_element_t<I, Args>>>>() ...);
}
template <typename Func>
auto Invoke(Func&& f, const QVariantList& args)
{
using F = typename std::remove_reference<Func>::type;
using Args = typename function_traits<decltype(&F::operator())>::args;
using Indices = std::make_index_sequence<std::tuple_size<Args>::value>;
return Invoke_impl<Func, Args>(std::forward<Func>(f), args, Indices{});
}
int main()
{
QString msg = "From Lambda";
auto f = [msg](int i, const QString& s, const QJsonValue& j)
{
qDebug() << msg << i << s << j;
return 5;
};
auto args = QVariantList() << 11 << "hello" << QJsonValue(123.456);
qDebug() << Invoke(f, args);
}

Wrapping it in a class just makes this harder. You want the pack T... to be a template parameter of Invoke
namespace detail {
template <typename... T, std::size_t... I>
void invoke_helper(std::function<void(T...)> f, const QVariantList& args, std::index_sequence<I...>)
{
f(args.at(I).value<T>()...);
}
template <typename... T>
void deduce_invoke(std::function<void(T...)> f, const QVariantList& args)
{
std::index_sequence_for<T...> idxs;
invoke_helper<T...>(std::move(f), args, idxs);
}
}
template <typename Func>
void Invoke(Func&& f, const QVariantList& args)
{
detail::deduce_invoke(std::function{f}, args);
}
This will deduce T... for unambiguous cases.
This shares more similarity with std::apply than with std::invoke, so I think Apply would be a better name.

The Key is the correct type deduction from functor.
namespace FunctorHelper {
template <typename R, typename... Args>
struct VariantFunc {
static R invoke(const std::function<R(Args...)>& f, const QVariantList& args)
{
std::index_sequence_for<Args...> idxs;
return invoke_helper(f, args, idxs);
}
private:
template <std::size_t... I>
static R invoke_helper(const std::function<R(Args...)>& f, const QVariantList& args, std::index_sequence<I...>)
{
return f(args.at(I).value<std::remove_const<std::remove_reference<Args>::type>::type>()...);
}
};
template <typename F>
struct function_traits;
// function pointer
template <typename R, class... Args>
struct function_traits<R(*)(Args...)> : public function_traits<R(Args...)>
{};
template <typename R, class... Args>
struct function_traits<R(Args...)>
{};
// const member function pointer
template <typename C, class R, class... Args>
struct function_traits<R(C::*)(Args...) const> : public function_traits<R(C&, Args...)>
{
static constexpr std::size_t arity = sizeof...(Args);
static R deduce_invoke(const std::function<R(Args...)>& f, const QVariantList& args)
{
return VariantFunc<R, Args...>::invoke(f, args);
}
};
} // namespace
template <typename Func>
void Invoke(Func f, const QVariantList& args)
{
using call_type = FunctorHelper::function_traits<decltype(&Func::operator())>;
call_type::deduce_invoke(f, args);
}
void main()
{
QString msg = "From Lambda";
auto f = [msg](int i, const QString& s, const QJsonValue& j)
{
qDebug() << msg << i << s << j;
};
auto args = QVariantList() << 11 << "hello" << QJsonValue(123.456);
Invoke(f, args);
// "From Lambda" 11 "hello" QJsonValue(double, 123.456)
}

Passing vector items to function-object with unknown number of parameters

Here is a simplified version of what I am trying to achieve:
template <class Func, class Params>
void foo(Func f, Params p) {
f(p[0], p[1], ...) // <-- this is the problem. How to do this?
}
...
foo([](int a, int b){ cout<<(a+b); }, std::vector<int>{1,2});
foo([](char a){ cout<<a; }, std::vector<char>{'a'});
I hope the problem is clear.
EDIT:
The above example did not convey the problem well. I have a vector, populated at some earlier stage, of the parameters. I want a function that will accept a function-object and call it with the parameters from the vector. I can assume that the vector size is equal to the number of parameters.
Hopefully better example:
class C {
std::vector<int> v;
public:
void add_param(int);
... // other functions that manipulate the vector in various ways
template<class Func>
void run(Func f) {
f(v[0], etc...); // <-- problem
}
};

You can use variardic templates:
template <class Func, class... Params>
void foo(Func f, Params... p) {
f(p...);
}
foo([](int a, int b){ cout<<(a+b); }, 1, 2);
foo([](char a){ cout<<a; }, 'a');

You may use something like:
// Minimal traits to have information about function
template <typename Func> struct function_traits;
template <typename Ret, typename ... Ts>
struct function_traits<Ret (Ts...)>
{
constexpr static auto arity = sizeof...(Ts);
};
template <typename Ret, typename ... Ts>
struct function_traits<Ret (*)(Ts...)> : function_traits<Ret(Ts...)> {};
template <typename C, typename Ret, typename ... Ts>
struct function_traits<Ret (C::*)(Ts...) const> : function_traits<Ret(Ts...)> {};
template <typename C>
struct function_traits : function_traits<decltype(&C::operator())> {};
namespace detail
{
template <typename F, typename Vec, std::size_t ... Is>
void call(const F& f, Vec&& v, std::index_sequence<Is...>)
{
f(v[Is]...);
}
}
template <class Func, class Vec>
void foo(const Func& f, Vec&& v) {
detail::call(f,
std::forward<Vec>(v),
std::make_index_sequence<function_traits<Func>::arity>());
}
Demo

Why template instantiations go on forever here?

In the following code, I want to replace
template <typename T, typename... Args>
auto check (rank<1,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<2, Ts...>{}, args...))> {
return check(rank<2, Ts...>{}, args...); // Since rank<1,T> derives immediately from rank<2, Ts...>.
}
template <typename T, typename... Args>
auto check (rank<2,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<3, Ts...>{}, args...))> {
return check(rank<3, Ts...>{}, args...); // Since rank<2,T> derives immediately from rank<3, Ts...>.
}
// etc... until rank<9>.
with the simple
template <std::size_t N, typename T, typename... Args>
auto check (rank<N,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
return check(rank<N+1, Ts...>{}, args...); // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
}
template <typename T, typename... Args>
auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }
But when I do, the template instantiations go on forever despite the terminating check(rank<10, T>, Args... args) function. Here is the full code using the long version above. Sorry for not minimizing the problem because I don't think I can minimize it and show what the problem is. Jumping to main() will show you the simple task I'm after, but I want to solve it using sequence ranking and overload resolution.
#include <iostream>
#include <type_traits>
#include <tuple>
template <typename T, typename... Args>
constexpr auto has_argument_type_impl(int)
-> decltype(std::is_same<typename T::argument_type, std::tuple<Args...>>{}); // Checking both that T::argument_type exists and that it is the same as std::tuple<Args...>.
template <typename T, typename... Args>
constexpr auto has_argument_type_impl(long) -> std::false_type;
template <typename T, typename... Args>
constexpr bool has_argument_type() { return decltype(has_argument_type_impl<T, Args...>(0))::value; }
template <typename T, std::size_t N, typename... Args>
constexpr auto has_argument_type_n_impl(int)
-> decltype(std::is_same<typename T::template argument_type<N>, std::tuple<Args...>>{}); // Checking both that T::argument_type<N> exists and that it is the same as std::tuple<Args...>.
template <typename T, std::size_t N, typename... Args>
constexpr auto has_argument_type_n_impl(long) -> std::false_type;
template <typename T, std::size_t N, typename... Args>
constexpr bool has_argument_type_n() { return decltype(has_argument_type_n_impl<T, N, Args...>(0))::value; }
template <typename... Ts>
class Factory {
template <std::size_t, typename...> struct rank;
template <std::size_t N, typename First, typename... Rest>
struct rank<N, First, Rest...> : rank<N, Rest...> {};
template <std::size_t N, typename T> struct rank<N,T> : rank<N+1, Ts...> {};
template <typename T> struct rank<10, T> {}; // Need to end the instantiations somewhere.
public:
template <typename... Args>
decltype(auto) create (Args... args) const {
return check(rank<0, Ts...>{}, args...);
}
private:
template <typename T, typename... Rest, typename... Args>
auto check (rank<0, T, Rest...>, Args... args) const
-> std::enable_if_t<has_argument_type<T, Args...>(), decltype(T(args...))> {
return T(args...);
}
template <typename T, typename... Rest, typename... Args>
auto check (rank<0, T, Rest...>, Args... args) const
-> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<0, Rest...>{}, args...))> {
return check(rank<0, Rest...>{}, args...);
}
template <typename T, typename... Args>
auto check (rank<0,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T, Args...>(), decltype(check(rank<1, Ts...>{}, args...))> {
return check(rank<1, Ts...>{}, args...); // Since rank<0,T> derives immediately from rank<1, Ts...>.
}
template <std::size_t N, typename T, typename... Rest, typename... Args>
auto check (rank<N, T, Rest...>, Args... args) const
-> std::enable_if_t<has_argument_type_n<T, N-1, Args...>(), decltype(T(args...))> {
return T(args...);
}
template <std::size_t N, typename T, typename... Rest, typename... Args>
auto check (rank<N, T, Rest...>, Args... args) const
-> std::enable_if_t<!has_argument_type_n<T, N-1, Args...>(), decltype(check(rank<N, Rest...>{}, args...))> {
return check(rank<N, Rest...>{}, args...);
}
// I want to use the following instead of what's below it.
// template <std::size_t N, typename T, typename... Args>
// auto check (rank<N,T>, Args... args) const
// -> std::enable_if_t<!has_argument_type_n<T, N-1, Args...>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
// return check(rank<N+1, Ts...>{}, args...); // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
// }
//
// template <typename T, typename... Args>
// auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }
template <typename T, typename... Args>
auto check (rank<1,T>, Args... args) const
-> std::enable_if_t<!has_argument_type_n<T, 0, Args...>(), decltype(check(rank<2, Ts...>{}, args...))> {
return check(rank<2, Ts...>{}, args...); // Since rank<1,T> derives immediately from rank<2, Ts...>.
}
template <typename T, typename... Args>
auto check (rank<2,T>, Args... args) const
-> std::enable_if_t<!has_argument_type_n<T, 1, Args...>(), decltype(check(rank<3, Ts...>{}, args...))> {
return check(rank<3, Ts...>{}, args...); // Since rank<2,T> derives immediately from rank<3, Ts...>.
}
// etc... until rank<9>.
};
// Testing
struct Object {
template <std::size_t, typename = void> struct ArgumentType;
template <typename T> struct ArgumentType<0,T> { using type = std::tuple<int, bool, char, double>; };
template <typename T> struct ArgumentType<1,T> { using type = std::tuple<bool, char, double>; };
template <std::size_t N> using argument_type = typename ArgumentType<N>::type;
Object (int, bool, char, double) { print(); }
Object (bool, char, double) { print(); }
void print() const { std::cout << "Object\n"; }
};
struct Thing {
template <std::size_t, typename = void> struct ArgumentType;
template <typename T> struct ArgumentType<0,T> { using type = std::tuple<int, int, char>; };
template <typename T> struct ArgumentType<1,T> { using type = std::tuple<int, char>; };
template <typename T> struct ArgumentType<2,T> { using type = std::tuple<char>; };
template <std::size_t N> using argument_type = typename ArgumentType<N>::type;
Thing (int, int, char) { print(); }
Thing (int, char) { print(); }
Thing (char) { print(); }
void print() const { std::cout << "Thing\n"; }
};
struct Blob {
using argument_type = std::tuple<int, double>;
Blob (int, double) { print(); }
void print() const { std::cout << "Blob\n"; }
};
struct Widget {
using argument_type = std::tuple<int>;
Widget (double, double, int, double) { print(); }
Widget (int) { print(); }
void print() const { std::cout << "Widget\n"; }
};
int main() {
Factory<Blob, Object, Thing, Widget>().create(4,3.5); // Blob
Factory<Object, Blob, Widget, Thing>().create(2); // Widget
Factory<Object, Thing, Blob, Widget>().create(5); // Widget
Factory<Blob, Object, Thing, Widget>().create(4,true,'a',7.5); // Object
Factory<Blob, Thing, Object, Widget>().create(true,'a',7.5); // Object
Factory<Blob, Object, Thing, Widget>().create('a'); // Thing
}
I know that there are other ways of accomplishing this, but I'm trying to understand sequence ranking better, and would like to know why I cannot use the commented-out section. How to avoid the repetitive code that I need to put (to rank<9>, or even higher rank) that is currently making this code work? Thanks for your patience.
Note: I actually must NOT enter the repetitive part of the code manually as I currently have. Because the highest N value for rank<N, Ts...> used in the check overloads will be determined during compile-time as the highest N value such that a argument_type<N> member type exists among all the Ts.... Thus I HAVE to use the generic part that I commented out, and the rank<10,T> I'm using will have to have the 10 replaced by that specific N value. Thus, this is not just a matter of convenience. I have to solve this problem to continue developing the program.
Edit: Here is a more minimal example, showing the same problem:
#include <iostream>
#include <type_traits>
#include <tuple>
template <typename T>
constexpr auto has_argument_type_impl(int)
-> decltype(typename T::argument_type{}, std::true_type{});
template <typename T>
constexpr auto has_argument_type_impl(long) -> std::false_type;
template <typename T>
constexpr bool has_argument_type() { return decltype(has_argument_type_impl<T>(0))::value; }
template <typename... Ts>
class Factory {
template <std::size_t, typename...> struct rank;
template <std::size_t N, typename First, typename... Rest>
struct rank<N, First, Rest...> : rank<N, Rest...> {};
template <std::size_t N, typename T> struct rank<N,T> : rank<N+1, Ts...> {};
template <typename T> struct rank<10, T> {}; // Need to end the instantiations somewhere.
public:
template <typename... Args>
decltype(auto) create (Args... args) const {
return check(rank<0, Ts...>{}, args...);
}
private:
template <std::size_t N, typename T, typename... Rest, typename... Args>
auto check (rank<N, T, Rest...>, Args... args) const
-> std::enable_if_t<has_argument_type<T>(), decltype(T(args...))> {
return T(args...);
}
template <std::size_t N, typename T, typename... Rest, typename... Args>
auto check (rank<N, T, Rest...>, Args... args) const
-> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<N, Rest...>{}, args...))> {
return check(rank<N, Rest...>{}, args...);
}
template <typename T, typename... Args>
auto check (rank<0,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<1, Ts...>{}, args...))> {
return check(rank<1, Ts...>{}, args...); // Since rank<0,T> derives immediately from rank<1, Ts...>.
}
// I want to use the following instead of what's below it.
// template <std::size_t N, typename T, typename... Args>
// auto check (rank<N,T>, Args... args) const
// -> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<N+1, Ts...>{}, args...))> {
// return check(rank<N+1, Ts...>{}, args...); // Since rank<N,T> derives immediately from rank<N+1, Ts...>.
// }
//
// template <typename T, typename... Args>
// auto check (rank<10, T>, Args... args) const { std::cout << "Nothing found.\n"; }
template <typename T, typename... Args>
auto check (rank<1,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<2, Ts...>{}, args...))> {
return check(rank<2, Ts...>{}, args...); // Since rank<1,T> derives immediately from rank<2, Ts...>.
}
template <typename T, typename... Args>
auto check (rank<2,T>, Args... args) const
-> std::enable_if_t<!has_argument_type<T>(), decltype(check(rank<3, Ts...>{}, args...))> {
return check(rank<3, Ts...>{}, args...); // Since rank<2,T> derives immediately from rank<3, Ts...>.
}
// etc... until rank<9>.
};
// Testing
struct Object {};
struct Thing {};
struct Blob {
using argument_type = std::tuple<int, double>;
Blob (int, double) { std::cout << "Blob\n"; }
};
int main() {
Factory<Object, Thing, Blob>().create(4,3.5); // Blob
}

Partial ordering does not kick in until very late in the overload resolution process.
Ignoring all the ping-ponging amongst your various check overloads, eventually you end up with
template <std::size_t N, typename T, typename... Args>
auto check (rank<N,T>, Args... args) const
-> std::enable_if_t<!has_argument_type_n<T, N, Args...>(),
decltype(check(rank<N+1, Ts...>{}, args...))>;
template <typename T, typename... Args>
auto check (rank<10, T>, Args... args) const;
with a rank<10, something I frankly don't care about>. Deduction and substitution will be performed for both overloads; and as part of substitution into the return type of the first signature, you'll instantiate rank<11, Ts...>, which in turn bypasses the terminating specialization of rank, resulting in an infinite chain of template instantiations. You don't even get to the point where the partial ordering tiebreaker chooses the second overload.
Just constrain the first overload to N < 10. It will need to lexically precede the return type (so that when N >= 10 the compiler doesn't attempt to substitute into it), so put it in a default template argument.

Passing many functions and storing all their results in a tuple

Consider this output:
int foo (int, char) {std::cout << "foo\n"; return 0;}
double bar (bool, double, long ) {std::cout << "bar\n"; return 3.5;}
bool baz (char, short, float) {std::cout << "baz\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
multiFunction<2,3,3> (tuple, foo, bar, baz); // foo bar baz
}
So multiFunction<2,3,3> takes the first 2 elements of tuple and passes them to foo, the next 3 elements of tuple and passes them to bar, etc... I got this working (except when the functions have overloads, which is a separate problem). But the return values of each function called are lost. I want those return values stored somewhere, something like
std::tuple<int, double, bool> result = multiFunction<2,3,3> (tuple, foo, bar, baz);
But I don't know how to implement that. For those who want to help get this done, here is my (updated) working code so far, which stores the outputs into a stringstream only. Not easy to get all the values back, especially if the objects saved in the stream are complex classes.
#include <iostream>
#include <tuple>
#include <utility>
#include <sstream>
template <std::size_t N, typename Tuple>
struct TupleHead {
static auto get (const Tuple& tuple) { // The subtuple from the first N components of tuple.
return std::tuple_cat (TupleHead<N-1, Tuple>::get(tuple), std::make_tuple(std::get<N-1>(tuple)));
}
};
template <typename Tuple>
struct TupleHead<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <std::size_t N, typename Tuple>
struct TupleTail {
static auto get (const Tuple& tuple) { // The subtuple from the last N components of tuple.
return std::tuple_cat (std::make_tuple(std::get<std::tuple_size<Tuple>::value - N>(tuple)), TupleTail<N-1, Tuple>::get(tuple));
}
};
template <typename Tuple>
struct TupleTail<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <typename Tuple, typename F, std::size_t... Is>
auto functionOnTupleHelper (const Tuple& tuple, F f, const std::index_sequence<Is...>&) {
return f(std::get<Is>(tuple)...);
}
template <typename Tuple, typename F>
auto functionOnTuple (const Tuple& tuple, F f) {
return functionOnTupleHelper (tuple, f, std::make_index_sequence<std::tuple_size<Tuple>::value>{});
}
template <typename Tuple, typename... Functions> struct MultiFunction;
template <typename Tuple, typename F, typename... Fs>
struct MultiFunction<Tuple, F, Fs...> {
template <std::size_t I, std::size_t... Is>
static inline auto execute (const Tuple& tuple, std::ostringstream& oss, const std::index_sequence<I, Is...>&, F f, Fs... fs) {
const auto headTuple = TupleHead<I, Tuple>::get(tuple);
const auto tailTuple = TupleTail<std::tuple_size<Tuple>::value - I, Tuple>::get(tuple);
// functionOnTuple (headTuple, f); // Always works, though return type is lost.
oss << std::boolalpha << functionOnTuple (headTuple, f) << '\n'; // What about return types that are void???
return MultiFunction<std::remove_const_t<decltype(tailTuple)>, Fs...>::execute (tailTuple, oss, std::index_sequence<Is...>{}, fs...);
}
};
template <>
struct MultiFunction<std::tuple<>> {
static auto execute (const std::tuple<>&, std::ostringstream& oss, std::index_sequence<>) { // End of recursion.
std::cout << std::boolalpha << oss.str();
// Convert 'oss' into the desired tuple? But how?
return std::tuple<int, double, bool>(); // This line is just to make the test compile.
}
};
template <std::size_t... Is, typename Tuple, typename... Fs>
auto multiFunction (const Tuple& tuple, Fs... fs) {
std::ostringstream oss;
return MultiFunction<Tuple, Fs...>::execute (tuple, oss, std::index_sequence<Is...>{}, fs...);
}
// Testing
template <typename T> int foo (int, char) {std::cout << "foo<T>\n"; return 0;}
double bar (bool, double, long ) {std::cout << "bar\n"; return 3.5;}
template <int...> bool baz (char, short, float) {std::cout << "baz<int...>\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
std::tuple<int, double, bool> result = multiFunction<2,3,3> (tuple, foo<bool>, bar, baz<2,5,1>); // foo<T> bar baz<int...>
}

Here's an approach where the number of arguments is deduced greedily:
#include <tuple>
namespace detail {
using namespace std;
template <size_t, size_t... Is, typename Arg>
constexpr auto call(index_sequence<Is...>, Arg&&) {return tuple<>{};}
template <size_t offset, size_t... Is, typename ArgT, typename... Fs>
constexpr auto call(index_sequence<Is...>, ArgT&&, Fs&&...);
template <size_t offset, size_t... Is,
typename ArgT, typename F, typename... Fs,
typename=decltype(declval<F>()(get<offset+Is>(declval<ArgT>())...))>
constexpr auto call(index_sequence<Is...>, ArgT&& argt, F&& f, Fs&&... fs) {
return tuple_cat(make_tuple(f(get<offset+I>(forward<ArgT>(argt))...)),
call<offset+sizeof...(Is)>(index_sequence<>{},
forward<ArgT>(argt),
forward<Fs>(fs)...));}
template <size_t offset, size_t... Is, typename ArgT, typename... Fs>
constexpr auto call(index_sequence<Is...>, ArgT&& argt, Fs&&... fs) {
return call<offset>(index_sequence<Is..., sizeof...(Is)>{},
forward<ArgT>(argt), forward<Fs>(fs)...);}
}
template <typename ArgT, typename... Fs>
constexpr auto multifunction(ArgT&& argt, Fs&&... fs) {
return detail::call<0>(std::index_sequence<>{},
std::forward<ArgT>(argt), std::forward<Fs>(fs)...);}
Demo. However, the above has quadratic time complexity in the number of return values, because tuple_cat is called recursively. Instead, we can use a slightly modified version of call to obtain the indices for each call - the actual tuple is then obtained directly:
#include <tuple>
namespace detail {
using namespace std;
template <size_t, size_t... Is, typename Arg>
constexpr auto indices(index_sequence<Is...>, Arg&&) {return tuple<>{};}
template <size_t offset, size_t... Is, typename ArgT, typename... Fs>
constexpr auto indices(index_sequence<Is...>, ArgT&&, Fs&&...);
template <size_t offset, size_t... Is, typename ArgT, typename F, class... Fs,
typename=decltype(declval<F>()(get<offset+Is>(declval<ArgT>())...))>
constexpr auto indices(index_sequence<Is...>, ArgT&& argt, F&& f, Fs&&... fs){
return tuple_cat(make_tuple(index_sequence<offset+Is...>{}),
indices<offset+sizeof...(Is)>(index_sequence<>{},
forward<ArgT>(argt),
forward<Fs>(fs)...));}
template <size_t offset, size_t... Is, typename ArgT, typename... Fs>
constexpr auto indices(index_sequence<Is...>, ArgT&& argt, Fs&&... fs) {
return indices<offset>(index_sequence<Is..., sizeof...(Is)>{},
forward<ArgT>(argt), forward<Fs>(fs)...);}
template <typename Arg, typename F, size_t... Is>
constexpr auto apply(Arg&& a, F&& f, index_sequence<Is...>) {
return f(get<Is>(a)...);}
template <typename ITuple, typename Args, size_t... Is, typename... Fs>
constexpr auto apply_all(Args&& args, index_sequence<Is...>, Fs&&... fs) {
return make_tuple(apply(forward<Args>(args), forward<Fs>(fs),
tuple_element_t<Is, ITuple>{})...);
}
}
template <typename ArgT, typename... Fs>
constexpr auto multifunction(ArgT&& argt, Fs&&... fs) {
return detail::apply_all<decltype(detail::indices<0>(std::index_sequence<>{},
std::forward<ArgT>(argt),
std::forward<Fs>(fs)...))>
(std::forward<ArgT>(argt), std::index_sequence_for<Fs...>{},
std::forward<Fs>(fs)...);}
Demo 2.

Building from the ground up and ignoring perfect forwarding so that I have to type less. We need a couple helpers. First, we need a partial version of apply that takes which indices from the tuple we want to apply to the function:
<class Tuple, class F, size_t... Is>
auto partial_apply(Tuple tuple, F f, std::index_sequence<Is...>) {
return f(get<Is>(tuple)...);
}
Then, we need to call that function for each subset of the tuple. Let's say we have all of our functions and indexes wrapped in a tuple already:
template <class Tuple, class FsTuple, class IsTuple, size_t... Is>
auto multi_apply(Tuple tuple, FsTuple fs, IsTuple indexes, std::index_sequence<Is...>) {
return std::make_tuple(
partial_apply(tuple,
std::get<Is>(fs),
std::get<Is>(indexes)
)...
);
}
So in this case, we'd want to end up calling multi_apply(tuple, <foo,bar,baz>, <<0,1>,<2,3,4>,<5,6,7>>, <0, 1, 2>).
All we need know is to build the indexes part. We're starting with <2,3,3>. We need to get the partial sums (<0,2,5>) and add that to the index sequences <<0,1>,<0,1,2>,<0,1,2>>. So we can write a partial sum function:
template <size_t I>
using size_t_ = std::integral_constant<size_t, I>;
template <class R, size_t N>
R partial_sum_(std::index_sequence<>, R, size_t_<N> ) {
return R{};
}
template <size_t I, size_t... Is, size_t... Js, size_t S>
auto partial_sum_(std::index_sequence<I, Is...>,
std::index_sequence<Js...>, size_t_<S> )
{
return partial_sum_(std::index_sequence<Is...>{},
std::index_sequence<Js..., S>{}, size_t_<S+I>{} );
}
template <size_t... Is>
auto partial_sum_(std::index_sequence<Is...> is)
{
return partial_sum_(is, std::index_sequence<>{}, size_t_<0>{} );
};
Which we can use to generate all of our indexes as a tuple:
template <size_t... Is, size_t N>
auto increment(std::index_sequence<Is...>, size_t_<N> )
{
return std::index_sequence<Is+N...>{};
}
template <class... Seqs, size_t... Ns>
auto make_all_indexes(std::index_sequence<Ns...>, Seqs... seqs)
{
return std::make_tuple(increment(seqs, size_t_<Ns>{})...);
}
Like so:
template <size_t... Is, class Tuple, class... Fs>
auto multiFunction(Tuple tuple, Fs... fs)
{
static_assert(sizeof...(Is) == sizeof...(Fs));
return multi_apply(tuple,
std::make_tuple(fs...),
make_all_indexes(
partial_sum_(std::index_sequence<Is...>{}),
std::make_index_sequence<Is>{}...
),
std::make_index_sequence<sizeof...(Is)>{}
);
}
If you want to handle void returns, then just make partial_apply return a tuple of a single element (or an empty tuple) and change the make_tuple() usage in multi_apply to tuple_cat().

Here's yet another impl:
template<std::size_t N>
constexpr Array<std::size_t, N> scan(std::size_t const (&a)[N])
{
Array<std::size_t, N> b{};
for (int i = 0; i != N - 1; ++i)
b[i + 1] = a[i] + b[i];
return b;
}
template<std::size_t O, std::size_t... N, class F, class Tuple>
inline decltype(auto) eval_from(std::index_sequence<N...>, F f, Tuple&& t)
{
return f(std::get<N + O>(std::forward<Tuple>(t))...);
}
template<std::size_t... O, std::size_t... N, class Tuple, class... F>
inline auto multi_function_impl1(std::index_sequence<O...>, std::index_sequence<N...>, Tuple&& t, F... f)
{
return pack(eval_from<O>(std::make_index_sequence<N>(), f, std::forward<Tuple>(t))...);
}
template<std::size_t... I, std::size_t... N, class Tuple, class... F>
inline auto multi_function_impl0(std::index_sequence<I...>, std::index_sequence<N...>, Tuple&& t, F... f)
{
constexpr std::size_t ns[] = {N...};
constexpr auto offsets = scan(ns);
return multi_function_impl1(std::index_sequence<offsets[I]...>(), std::index_sequence<N...>(), std::forward<Tuple>(t), f...);
}
template<std::size_t... N, class Tuple, class... F>
auto multi_function(Tuple&& t, F... f)
{
return multi_function_impl0(std::make_index_sequence<sizeof...(N)>(), std::index_sequence<N...>(), std::forward<Tuple>(t), f...);
}
where pack and Array are similar to std::make_tuple and std::array respectively, but to overcome some problems:
std::make_tuple decays it args, so references are lost
std::array cannot have its elems written in constexpr in c++14
DEMO

Here's my solution after following T.C.'s advice, adding to my previous (albeit inefficient) solution:
#include <iostream>
#include <tuple>
#include <utility>
struct NoReturnValue {
friend std::ostream& operator<< (std::ostream& os, const NoReturnValue&) {
return os << "[no value returned]";
}
};
template <std::size_t N, typename Tuple>
struct TupleHead {
static auto get (const Tuple& tuple) { // The subtuple from the first N components of tuple.
return std::tuple_cat (TupleHead<N-1, Tuple>::get(tuple), std::make_tuple(std::get<N-1>(tuple)));
}
};
template <typename Tuple>
struct TupleHead<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <std::size_t N, typename Tuple>
struct TupleTail {
static auto get (const Tuple& tuple) { // The subtuple from the last N components of tuple.
return std::tuple_cat (std::make_tuple(std::get<std::tuple_size<Tuple>::value - N>(tuple)), TupleTail<N-1, Tuple>::get(tuple));
}
};
template <typename Tuple>
struct TupleTail<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <typename Tuple, typename F, std::size_t... Is>
auto functionOnTupleHelper (const Tuple& tuple, F f, const std::index_sequence<Is...>&,
std::enable_if_t< !std::is_void<std::result_of_t<F(std::tuple_element_t<Is, Tuple>...)>>::value >* = nullptr) { // This overload is called only if f's return type is not void.
return std::make_tuple(f(std::get<Is>(tuple)...)); // Thanks to T.C.'s advice on returning a single tuple and then calling std::tuple_cat on all the single tuples.
}
template <typename Tuple, typename F, std::size_t... Is>
auto functionOnTupleHelper (const Tuple& tuple, F f, const std::index_sequence<Is...>&,
std::enable_if_t< std::is_void<std::result_of_t<F(std::tuple_element_t<Is, Tuple>...)>>::value >* = nullptr) { // This overload is called only if f's return type is void.
f(std::get<Is>(tuple)...);
return std::tuple<NoReturnValue>(); // Thanks to T.C.'s advice on returning std::tuple<NoReturnValue>() if the return type of 'f' is void.
}
template <typename Tuple, typename F>
auto functionOnTuple (const Tuple& tuple, F f) {
return functionOnTupleHelper (tuple, f, std::make_index_sequence<std::tuple_size<Tuple>::value>{});
}
template <typename Tuple, typename... Functions> struct MultiFunction;
template <typename Tuple, typename F, typename... Fs>
struct MultiFunction<Tuple, F, Fs...> {
template <std::size_t I, std::size_t... Is>
static inline auto execute (const Tuple& tuple, const std::index_sequence<I, Is...>&, F f, Fs... fs) {
const auto headTuple = TupleHead<I, Tuple>::get(tuple);
const auto tailTuple = TupleTail<std::tuple_size<Tuple>::value - I, Tuple>::get(tuple);
const auto r = functionOnTuple(headTuple, f); // Which overload of 'functionOnTupleHelper' is called dedends on whether f's return type is void or not.
return std::tuple_cat (r, MultiFunction<std::remove_const_t<decltype(tailTuple)>, Fs...>::execute (tailTuple, std::index_sequence<Is...>{}, fs...)); // T.C.'s idea of tuple_cat with all the single return tuples.
}
};
template <>
struct MultiFunction<std::tuple<>> {
static auto execute (const std::tuple<>&, std::index_sequence<>) { return std::tuple<>(); }
};
template <std::size_t... Is, typename Tuple, typename... Fs>
auto multiFunction (const Tuple& tuple, Fs... fs) {
return MultiFunction<Tuple, Fs...>::execute (tuple, std::index_sequence<Is...>{}, fs...);
}
// Testing
template <typename T> int foo (int, char) {std::cout << "foo<T>\n"; return 0;}
double bar (bool, double, long) {std::cout << "bar\n"; return 3.5;}
double bar (bool, int) {return 1.4;}
void voidFunction() {std::cout << "voidFunction\n";}
template <int...> bool baz (char, short, float) {std::cout << "baz<int...>\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
const auto firstBar = [](bool b, double d, long l) {return bar(b, d, l);};
const auto t = multiFunction<2,3,0,3> (tuple, foo<bool>, firstBar, voidFunction, baz<2,5,1>); // Note that since 'bar' has an overload, we have to define 'firstBar' to indicate which 'bar' function we want to use.
std::cout << std::boolalpha << std::get<0>(t) << ' ' << std::get<1>(t) << ' ' << std::get<2>(t) << ' ' << std::get<3>(t) << '\n';
// 0 3.5 [no value returned] true
}

This solution should have linear time complexity. It uses std::tie instead of std::make_tuple, so neither the functions nor the arguments are copied unnecessarily. I think it should be fairly easy to follow compared to some other answers here.
First, we need a utility to invoke a function using a std::tuple of arguments.
template <typename F, typename Args, std::size_t... Is>
auto invoke_impl(F const& f, Args const& args, std::index_sequence<Is...>)
{
return f(std::get<Is>(args)...);
}
template <typename F, typename Args>
auto invoke(F const& f, Args const& args)
{
return invoke_impl(f, args, std::make_index_sequence<std::tuple_size<Args>::value>());
}
Secondly, we need a utility to std::tie a sub-range of tuple elements.
template <std::size_t Offset, typename Tuple, std::size_t... Is>
auto sub_tie_impl(Tuple const& tuple, std::index_sequence<Is...>)
{
return std::tie(std::get<Offset + Is>(tuple)...);
}
template <std::size_t Offset, std::size_t Count, typename Tuple>
auto sub_tie(Tuple const& tuple)
{
return sub_tie_impl<Offset>(tuple, std::make_index_sequence<Count>());
}
Now we can create our utility to consume a std::tuple of arguments using a sequence of functions.
First we std::tie the functions into a tuple, then we split the argument list into a parameter pack of sub-argument lists, and finally we invoke a function for each sub-argument list, packing the results into a tuple which we then return.
template <typename Fs, std::size_t... Is, typename... SubArgs>
auto consume_impl(Fs const& fs, std::index_sequence<Is...>, SubArgs const&... sub_args)
{
return std::make_tuple(invoke(std::get<Is>(fs), sub_args)...);
}
template <std::size_t, typename Args, typename Fs, typename... SubArgs>
auto consume_impl(Args const&, Fs const& fs, SubArgs const&... sub_args)
{
return consume_impl(fs, std::make_index_sequence<sizeof...(SubArgs)>(), sub_args...);
}
template <std::size_t Offset, std::size_t Count, std::size_t... Counts,
typename Args, typename Fs, typename... SubArgs>
auto consume_impl(Args const& args, Fs const& fs, SubArgs const&... sub_args)
{
return consume_impl<Offset + Count, Counts...>(args, fs, sub_args...,
sub_tie<Offset, Count>(args));
}
template <std::size_t... Counts, typename Args, typename... Fs>
auto consume(Args const& args, Fs const&... fs)
{
return consume_impl<0, Counts...>(args, std::tie(fs...));
}

Here's another solution borrowing Barry's partial_apply idea but avoiding the use of his partial_sum function altogether. It is shorter as a result. I think this is linear in time complexity.
#include <iostream>
#include <tuple>
#include <utility>
template <std::size_t Offset, typename F, typename Tuple, std::size_t... Is>
auto partial_apply_impl (F f, const Tuple& tuple, const std::index_sequence<Is...>&) {
return f(std::get<Offset + Is>(tuple)...);
}
template <typename Off, typename F, typename Tuple> // Off must be of type OffsetIndexSequence<A,B> only.
auto partial_apply (F f, const Tuple& tuple) {
return partial_apply_impl<Off::value>(f, tuple, typename Off::sequence{});
}
template <std::size_t Offset, std::size_t Size>
struct OffsetIndexSequence : std::integral_constant<std::size_t, Offset> {
using sequence = std::make_index_sequence<Size>;
};
template <typename Output, std::size_t... Is> struct OffsetIndexSequenceBuilder;
template <template <typename...> class P, typename... Out, std::size_t Offset, std::size_t First, std::size_t... Rest>
struct OffsetIndexSequenceBuilder<P<Out...>, Offset, First, Rest...> :
OffsetIndexSequenceBuilder<P<Out..., OffsetIndexSequence<Offset, First>>, Offset + First, Rest...> {};
template <template <typename...> class P, typename... Out, std::size_t Offset>
struct OffsetIndexSequenceBuilder<P<Out...>, Offset> {
using type = P<Out...>;
};
template <std::size_t... Is>
using offset_index_sequences = typename OffsetIndexSequenceBuilder<std::tuple<>, 0, Is...>::type;
template <typename> struct MultiFunction;
template <template <typename...> class P, typename... Offs>
struct MultiFunction<P<Offs...>> {
template <typename ArgsTuple, typename... Fs>
static auto execute (const ArgsTuple& argsTuple, Fs... fs) {
using ResultTuple = std::tuple<decltype(partial_apply<Offs>(fs, argsTuple))...>;
return ResultTuple{partial_apply<Offs>(fs, argsTuple)...};
}
};
template <std::size_t... Is, typename ArgsTuple, typename... Fs>
auto multiFunction (const ArgsTuple& argsTuple, Fs... fs) {
return MultiFunction<offset_index_sequences<Is...>>::execute(argsTuple, fs...);
}
// Testing
int foo (int, char) {std::cout << "foo\n"; return 0;}
double bar (bool, double, long) {std::cout << "bar\n"; return 3.5;}
bool baz (char, short, float) {std::cout << "baz\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
const std::tuple<int, double, bool> t = multiFunction<2,3,3> (tuple, foo, bar, baz); // foo bar baz
std::cout << std::boolalpha << std::get<0>(t) << ' ' << std::get<1>(t) << ' ' << std::get<2>(t) << '\n'; // 0 3.5 true
}