C++ call function with many different types

Question

I have an overloaded function that I have to call with many different types. The simple approach is:

uint8_t a;
uint16_t b;
//....
double x;

doSomething(a);
doSomething(b);
//...
doSomething(x);

expressing those calls succinctly can be done with a variadic template as explained at this Q&A. The code will look somewhat like this:

auto doSomethingForAllTypes = [](auto&&... args) {
    (doSomething(args), ...);
};

uint8_t a;
uint16_t b;
//....
double x;

doSomethingForAllTypes(a, b, ... ,x);

But I'll have to do this at many places in the code, so I'd like to define the list of types only once. I'd like to have code that looks conceptually like this:

auto doSomethingForAllTypes = [](auto&&... args) {
    (doSomething(args), ...);
};

someContainer allTypesNeeded(uint8_t, uint16_t, ... double);

doSomethingForAllTypes(allTypesNeeded);

How could this be done?

Answer 1

With some boiler-plate to obtain a tuple foreach, you can implement what you want as follows:

#include <tuple>
#include <utility>

namespace detail
{
    template<typename T, typename F, std::size_t... Is>
    void for_each(T&& t, F f, std::index_sequence<Is...>)
    {
        ( static_cast<void>(f(std::get<Is>(std::forward<T>(t)))),... );
    }
}

template<typename... Ts, typename F>
void for_each_in_tuple(std::tuple<Ts...> const& t, F f)
{
    detail::for_each(t, f, std::index_sequence_for<Ts...>{});
}

int main() {
    std::tuple<uint8_t, uint16_t, double> tup{};
    for_each_in_tuple(tup, [](auto&& arg) {
        doSomething(arg);
    });
}

Live Example

Answer 2

如果要具有预定义的类型序列，并且不想创建带有参数的元组，则可以使用TypeList方法：

#include <type_traits>
#include <utility>

void doSomething(int)
{
}

void doSomething(double)
{
}

template <typename... Args>
void doSomething(Args&&... args)
{
    (doSomething(std::forward<Args>(args)), ...);
}

template <typename ...Args>
struct TypeList{};

template <typename T>
struct DoSomethingHelper;

template <typename ...Args>
struct DoSomethingHelper<TypeList<Args...>>
{
    static void doSomething(Args&&... args)
    {
        ::doSomething(std::forward<Args>(args)...);
    }
};

template <typename T, typename ...Args>
void doSomethingForTypes(Args&&... args)
{
    DoSomethingHelper<T>::doSomething(std::forward<Args>(args)...);
}

int main()
{
    using MyTypeList = TypeList<int, double, int>;

    doSomethingForTypes<MyTypeList>(1, 1.0, 2);
}

Answer 3

与std::tuple和std::apply

std::tuple<uint8_t, uint16_t, double> tup{};
std::apply([](const auto&... arg) { (doSomething(arg), ...); }, tup);