C++98/03 std::is_constructible implementation

Question

The base components of my hobby library has to work with C++98 and C++11 compilers. To learn and to enjoy myself I created the C++98 implementations of several type support functionality (like enable_if, conditional, is_same, is_integral etc. ...) in order to use them when there is no C++11 support.

However while I was implementing is_constructible I got stuck. Is there any kind of template magic (some kind of SFINAE) with which I can implement it without C++11 support (declval)?

Of course there is no variadic template support in C++03, so I will specialise the implementation till some depth. The main question is if there is a technique that can decide whether T is constructible from the given types or not.

Answer 1

To implement a fully conforming is_constructible, compiler support is necessary. The issue is not variadic template simulation or select idiom(sizeof over decltype).

Actually even before gcc 8.x(4.x to 7.x), there is a bug on is_constructible<To, From>, because it is implemented purely by library code. The bug happens when To is a reference type(i.e. T& or T&&). The same applies to clang libc++'s library version __libcpp_is_constructible<To, From>, but clang has compiler support for __is_constructible() since the support of c++11 so that's never a real issue.

The non-conforming cases are, when constructing a reference, the pure library implementation used by both clang(libc++) and gcc(libstdc++) uses SFINAE to check if static_cast<To>(declval<From>()) is well-formed. But there are two scenarios where you must explicitly use cast instead of initialization syntax(i.e. T t(args...)):

1. When cast from a reference of base class to a reference of derived class:
   static_cast<Derived&>(declval<Base&>()) is valid, but you must always explicitly use cast, i.e. Base& bref; Derived& dref = bref; doesn't work, you must use Derived& dref = static_cast<Derived&>(bref).
2. When cast from a lvalue reference to a rvalue reference:
   static_cast<A&&>(declval<A&>()) is valid (your familiar std::move()), but you must always explicitly use cast, i.e. A& lref; A&& ref = lref; doesn't work, you must use A&& ref = static_cast<A&&>(lref);(i.e. A&& ref = std::move(lref);)

In order to address such false positives, in addition to SFINAE conversion check, extra checks already exist in libc++ and libstdc++ to ensure the cast conversion is neither of the scenarios above.

But this introduces a new problem: if there exists user-defined (explicit) conversion, __is_constructible() is valid. But when the conversion is also one of the scenarios above, false negative happens.

For example, the code below demonstrates the base to derived reference conversion scenario. convert Base& to D1& or D2& needs explicit cast, but, there is also an user-defined explicit conversion that converts Base(&) to D1&. So is_constructible<D1&, Base&>::value evaluates to true, whereas is_constructible<D2&, Base&>::value evaluates to false.

struct D1;
struct D2;
struct Base {
    explicit operator D1&();
};

struct D1 : Base {
    D1(const D1&) = delete;
};
struct D2 : Base {};

int BtoD1() { // should be true
    return std::is_constructible<D1&, Base&>::value;
}
int BtoD2() { // should be false
    return std::is_constructible<D2&, Base&>::value;
}

But the library implementation reported both as false. godbolt link try it at you own. You can switch between clang / gcc(<7) / gcc(>=8) to see how the results change.

Answer 2

It's possible:

#include <iostream>

template<typename T, T Val>
struct integral_constant {
    typedef integral_constant type;
    typedef T value_type;
    enum {
        value = Val
    };
};

typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;

template<typename T>
struct remove_ref {
    typedef T type;
};

template<typename T>
struct remove_ref<T&> {
    typedef T type;
};

// is_base_of from https://stackoverflow.com/questions/2910979/how-does-is-base-of-work
namespace aux {
    typedef char yes[1];
    typedef char no[2];

    template <typename B, typename D>
    struct Host
    {
        operator B*() const;
        operator D*();
    };
}
template <typename B, typename D>
struct is_base_of
{
  template <typename T> 
  static aux::yes& check(D*, T);
  static aux::no& check(B*, int);

  static const bool value = sizeof(check(aux::Host<B,D>(), int())) == sizeof(aux::yes);
};

template<typename T>
struct remove_cv {
    typedef T type;
};
template<typename T>
struct remove_cv<const T> {
    typedef T type;
};
template<typename T>
struct remove_cv<volatile T> {
    typedef T type;
};
template<typename T>
struct remove_cv<const volatile T> {
    typedef T type;
};

template<typename T>
struct is_void : integral_constant<bool, false> {};
template<>
struct is_void<void> : integral_constant<bool, true> {};

template<class T>
struct type_identity {
    // Used to work around Visual C++ 2008's spurious error: "a function-style conversion to a built-in type can only take one argument"
    typedef T type;
};

template <bool, typename T, typename>
struct conditional {
    typedef T type;
};
template <typename T, typename U>
struct conditional<false, T, U> {
    typedef U type;
};


namespace aux {

template<typename T, typename U>
struct is_more_const : integral_constant<bool, false> {};

template<typename T, typename U>
struct is_more_const<const T, U> : integral_constant<bool, true> {};

template<typename T, typename U>
struct is_more_const<const T, const U> : integral_constant<bool, false> {};

template<typename T, typename U>
struct is_more_volatile : integral_constant<bool, false> {};

template<typename T, typename U>
struct is_more_volatile<volatile T, U> : integral_constant<bool, true> {};

template<typename T, typename U>
struct is_more_volatile<volatile T, volatile U> : integral_constant<bool, false> {};

template<typename T, typename U>
struct is_more_cv : integral_constant<bool, is_more_const<T,U>::value && is_more_volatile<T,U>::value> {};


    template<typename T>
    struct is_default_constructible {
        template<typename U>
        static yes& test(int(*)[sizeof(new U)]);
        template<typename U>
        static no& test(...);
        enum {
            value = sizeof(test<T>(0)) == sizeof(yes)
        };
    };    

    template<typename T, typename Arg>
    struct is_constructible_1 {
        template<typename U, typename Arg_>
        static yes& test(int(*)[sizeof(typename type_identity<U>::type(static_cast<Arg_>(*((typename remove_ref<Arg_>::type*)0))))]);
        template<typename U, typename Arg_>
        static no& test(...);
        enum {
            value = sizeof(test<T, Arg>(0)) == sizeof(yes)
        };
    };   

    // Base pointer construct from Derived Pointer
    template<typename T, typename U>
    struct is_constructible_1<T*, U*>
        : conditional<
            is_void<typename remove_cv<T>::type>::value,
            integral_constant<bool, true>,
            typename conditional<
                is_void<typename remove_cv<U>::type>::value,
                integral_constant<bool, false>,
                typename conditional<
                    is_more_cv<T, U>::value,
                    integral_constant<bool, false>,
                    is_base_of<T,U>
                >::type
            >::type
        >::type
    {};

    // Base pointer construct from Derived Pointer
    template<typename T, typename U>
    struct is_constructible_1<T&, U&>
        : conditional<
            is_more_cv<T, U>::value,
            integral_constant<bool, false>,
            is_base_of<T,U>
        >::type
    {};


    template<typename T, typename Arg1, typename Arg2>
    struct is_constructible_2 {
        template<typename U, typename Arg1_, typename Arg2_>
        static yes& test(int(*)[
            sizeof(typename type_identity<U>::type(
                static_cast<Arg1_>(*((typename remove_ref<Arg1_>::type*)0)),
                static_cast<Arg2_>(*((typename remove_ref<Arg2_>::type*)0))
                ))
            ]);
        template<typename U, typename Arg1_, typename Arg2_>
        static no& test(...);
        enum {
            value = sizeof(test<T, Arg1, Arg2>(0)) == sizeof(yes)
        };
    };
}

template<typename T, typename Arg1 = void, typename Arg2 = void>
struct is_constructible : integral_constant<bool, aux::is_constructible_2<T, Arg1, Arg2>::value> {

};

template<typename T, typename Arg>
struct is_constructible<T, Arg> : integral_constant<bool, aux::is_constructible_1<T, Arg>::value> {

};
template<typename T>
struct is_constructible<T> : integral_constant<bool, aux::is_default_constructible<T>::value> {

};

struct Foo {};
struct fuzz_explicit {};
struct fuzz_implicit {};
struct Fuzz {
    explicit Fuzz(fuzz_explicit);
    Fuzz(fuzz_implicit);
};
struct buzz_explicit {};
struct buzz_implicit {};
struct Buzz {
    explicit Buzz(buzz_explicit);
    Buzz(buzz_implicit);
};
struct Bar {
    Bar(int);
    Bar(int, double&);
    Bar(Fuzz);
    explicit Bar(Buzz);
};

struct Base {};
struct Derived : Base {};

#define TEST(X) std::cout << #X << X << '\n'

int main() {
    TEST((is_constructible<Foo>::value));
    TEST((is_constructible<Bar>::value));
    TEST((is_constructible<Foo, int>::value));
    TEST((is_constructible<Bar, int>::value));
    TEST((is_constructible<Foo, const Foo&>::value));
    TEST((is_constructible<Bar, Bar>::value));
    TEST((is_constructible<Bar, int, double>::value));
    TEST((is_constructible<Bar, int, double&>::value));
    TEST((is_constructible<Bar, int, const double&>::value));
    TEST((is_constructible<int*, void*>::value));
    TEST((is_constructible<void*, int*>::value));
    TEST((is_constructible<Base&, Derived&>::value));
    TEST((is_constructible<Derived*, Base*>::value));
    // via Fuzz
    TEST((is_constructible<Bar, fuzz_explicit>::value));
    TEST((is_constructible<Bar, fuzz_implicit>::value));
    // via Buzz
    TEST((is_constructible<Bar, buzz_explicit>::value));
    TEST((is_constructible<Bar, buzz_implicit>::value));
    // integer promotion
    TEST((is_constructible<Bar, char>::value));
    // integer conversion
    TEST((is_constructible<Bar, unsigned long>::value));
}

You can expand the 2 parameters version for 3, 4, 5, ... parameters further more.

Live Demo

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This works with g++ 4.4.7

It doesn't work with g++ 4.3.6