c++20 How to make a constrained tuple like container that contains only the allowed types and an instance of itself

Question

Imagine I want to make a tuple like template container as part of api interface. And I want to constraint it to the list of allowed types plus an instance of this template container. Now I have something like this:

#include <string>
#include <tuple>
#include <utility>
#include <type_traits>

template<typename T>
constexpr const bool bIsAllowedType =
    std::is_same<T, bool>::value ||
    std::is_same<T, void*>::value ||
    std::is_same<T, double>::value ||
    std::is_same<T, int64_t>::value ||
    std::is_same<T, std::string>::value;

template<typename T, typename... Args>
constexpr const bool bIsAllowedArgList =
    bIsAllowedType<std::remove_reference_t<T>> &&
    bIsAllowedArgList<Args...>;

template<typename T>
constexpr const bool bIsAllowedArgList<T> =
    bIsAllowedType<std::remove_reference_t<T>>;

template<typename... Args>
concept CAllowedArgList =
    bIsAllowedArgList<Args...>;

template<CAllowedArgList... Args>
class Container
{
private:
    using TupleT = std::tuple<std::decay_t<Args>...>;
    TupleT data;
public:
    Container() = default;
    Container(Args&&... args) : data(std::forward<Args>(args)...) {}
};

int main()
{
    auto test_cont = Container(1LL, 2.0, std::string("three")); // Ok
    auto err_cont = Container(std::wstring(L"wide string")); // Not ok, type's not allowed
    return 0;
}

So, how do I make it to also accept an instance of itself? Like this:

int main()
{
    auto test_cont = Container(1LL, Container(2.0, std::string("three"))); 

    return 0;
}

I want it to be as simple as possible, so please restraint using third party libs (like Boost) if possible

Answer 1

You can check this by using partial template specialization.

template <typename T>
constexpr const bool bIsContainerType = false;

template <typename... Types>
constexpr const bool bIsContainerType<Container<Types...>> = true;

Then we just add it to you list of checks in bIsAllowedType

template<typename T>
constexpr const bool bIsAllowedType =
    std::is_same<T, bool>::value ||
    std::is_same<T, void*>::value ||
    std::is_same<T, double>::value ||
    std::is_same<T, int64_t>::value ||
    std::is_same<T, std::string>::value ||
    bIsContainerType<T>;

Since bIsAllowedType now needs to know about Container which needs to know about our concept I changed bIsAllowedType to a struct so we can forward declare it.

#include <string>
#include <tuple>
#include <utility>
#include <type_traits>

template<typename T>
struct bIsAllowedType;

template<typename... T>
constexpr const bool bIsAllowedArgList =
    (bIsAllowedType<std::remove_reference_t<T>>::value && ...);

template<typename... Args>
concept CAllowedArgList =
    bIsAllowedArgList<Args...>;

template<CAllowedArgList... Args>
class Container
{
private:
    using TupleT = std::tuple<std::decay_t<Args>...>;
    TupleT data;
public:
    Container() = default;
    Container(Args&&... args) : data(std::forward<Args>(args)...) {}
};

template <typename T>
constexpr const bool bIsContainerType = false;

template <typename... Types>
constexpr const bool bIsContainerType<Container<Types...>> = true;

template<typename T>
struct bIsAllowedType {
    static constexpr bool value =
        std::is_same<T, bool>::value ||
        std::is_same<T, void*>::value ||
        std::is_same<T, double>::value ||
        std::is_same<T, int64_t>::value ||
        std::is_same<T, std::string>::value ||
        bIsContainerType<T>;
};


int main()
{
    auto test_cont = Container(static_cast<int64_t>(1), 2.0, std::string("three")); // Ok
    //auto err_cont = Container(std::wstring(L"wide string")); // Not ok, type's not allowed
    auto test_cont2 = Container(static_cast<int64_t>(1), Container(2.0, std::string("three"))); 
    return 0;
}

Live demo