generic call of a std::function from QVariantList - c++

Question

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.

Answer 1

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);
}

Answer 2

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.

Answer 3

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)
}