CODEWITHSUNDEEP
c++c++17template-meta-programming
gerum
gerum

Reputation: 1134

Template Class return tuple or value, depend on parameter pack

I want to write a class which has a parameter pack and a function. This function should return the value itself, if there is only one type in the pack and otherwise it should return a tuple. If there pack is empty it can return an empty tuple or void, I don't mind.

Something like this(pseudocode):

template<typename ...Args>
class A{
std::tuple<Args...> data;

#if sizeof...(Args)
std::tuple<Args...> get();
#else
Args[0] get();
#endif

};

I think my current solution is very complicated and it doesn't work if Args is empty.

Current solution:

template<bool EnableBool = true>
    typename std::enable_if<
            EnableBool && std::tuple_size_v<decltype(data)> == 1, typename std::tuple_element<0, decltype(data)>::type>::type
get();

Upvotes: 2

Views: 145

Answers (1)

rafix07
rafix07

Reputation: 20936

Since C++17 you can use if constexpr and return type will be deduced by compiler:

template<typename ...Args>
class A{
    std::tuple<Args...> data;
public:
    auto get()
    {
        if constexpr (sizeof...(Args) > 1)
            return data;
        else if constexpr (sizeof...(Args) == 1)
            return std::get<0>(data);
        else 
            return;
    }
};

int main (int argc, char *argv[])
{
    A<int,double> a;
    static_assert(std::is_same_v<decltype(a.get()),std::tuple<int,double>>);
    A<> b;
    static_assert(std::is_same_v<decltype(b.get()),void>);
    A<int> c;
    static_assert(std::is_same_v<decltype(c.get()),int>);

Demo

Upvotes: 4

Related Questions