C++98/03 std::is_constructible implementation

问题:

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.

回答1:

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

---

This works with g++ 4.4.7

It doesn't work with g++ 4.3.6

回答2:

I think Danh's idea was great! With a minor modification we can eliminate the operator new. (I have a C++98 enable_if and remove_reference implementation). The mentioned int*, void* case works with this implementation too. No operator new required. Only the old g++ support remains...

/********** std::remove_cv replacement **********/
template< typename T >
struct remove_const
{
    typedef T type;
};

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


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

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


template< typename T >
struct remove_cv
{
    typedef typename remove_volatile< typename remove_const< T >::type >::type type;
};


/********** std::is_pointer replacement *********/
template< typename T >
struct is_pointer_helper
{
    static const bool value = false;
};

template< typename T >
struct is_pointer_helper< T* >
{
    static const bool value = true;
};

template< typename T >
struct is_pointer
{
    static const bool value = is_pointer_helper< typename remove_cv< T >::type >::value;
};


/********** std::enable_if replacement **********/
template< bool CONDITION, typename TYPE = void >
struct enable_if
{
};

template< typename TYPE >
struct enable_if< true, TYPE >
{
    typedef TYPE type;
};


/****** std::remove_reference replacement *******/
template< typename T >
struct remove_reference
{
    typedef T type;
};

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


/******* std::is_constructible replacement ******/
template< typename T, typename AT_1 = void, typename AT_2 = void, typename AT_3 = void, typename AT_4 = void >
class is_constructible_impl
{
private:
    template< typename C_T, typename C_AT_1, typename C_AT_2, typename C_AT_3, typename C_AT_4 >
    static bool test(
        typename c_std::enable_if<
            sizeof( C_T ) ==
            sizeof( C_T(
                static_cast< C_AT_1 >( *static_cast< typename c_std::remove_reference< C_AT_1 >::type* >( NULL ) ),
                static_cast< C_AT_2 >( *static_cast< typename c_std::remove_reference< C_AT_2 >::type* >( NULL ) ),
                static_cast< C_AT_3 >( *static_cast< typename c_std::remove_reference< C_AT_3 >::type* >( NULL ) ),
                static_cast< C_AT_4 >( *static_cast< typename c_std::remove_reference< C_AT_4 >::type* >( NULL ) )
            ) )
        >::type*
    );

    template< typename, typename, typename, typename, typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T, AT_1, AT_2, AT_3, AT_4 >( NULL ) ) == sizeof( bool ) );
};

template< typename T, typename AT_1, typename AT_2, typename AT_3 >
class is_constructible_impl< T, AT_1, AT_2, AT_3, void >
{
private:
    template< typename C_T, typename C_AT_1, typename C_AT_2, typename C_AT_3 >
    static bool test(
        typename c_std::enable_if<
            sizeof( C_T ) ==
            sizeof( C_T(
                static_cast< C_AT_1 >( *static_cast< typename c_std::remove_reference< C_AT_1 >::type* >( NULL ) ),
                static_cast< C_AT_2 >( *static_cast< typename c_std::remove_reference< C_AT_2 >::type* >( NULL ) ),
                static_cast< C_AT_3 >( *static_cast< typename c_std::remove_reference< C_AT_3 >::type* >( NULL ) )
            ) )
        >::type*
    );

    template< typename, typename, typename, typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T, AT_1, AT_2, AT_3 >( NULL ) ) == sizeof( bool ) );
};

template< typename T, typename AT_1, typename AT_2 >
class is_constructible_impl< T, AT_1, AT_2, void, void >
{
private:

    template< typename C_T, typename C_AT_1, typename C_AT_2 >
    static bool test(
        typename c_std::enable_if<
            sizeof( C_T ) ==
            sizeof( C_T(
                static_cast< C_AT_1 >( *static_cast< typename c_std::remove_reference< C_AT_1 >::type* >( NULL ) ),
                static_cast< C_AT_2 >( *static_cast< typename c_std::remove_reference< C_AT_2 >::type* >( NULL ) )
            ) )
        >::type*
    );

    template< typename, typename, typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T, AT_1, AT_2 >( NULL ) ) == sizeof( bool ) );
};

template< typename T, typename AT_1 >
class is_constructible_impl< T, AT_1, void, void, void >
{
private:
    template< typename C_T, typename C_AT_1 >
    static bool test(
        typename c_std::enable_if<
            sizeof( C_T ) ==
            sizeof( C_T(
                static_cast< C_AT_1 >( *static_cast< typename c_std::remove_reference< C_AT_1 >::type* >( NULL ) )
            ) )
        >::type*
    );

    template< typename, typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T, AT_1 >( NULL ) ) == sizeof( bool ) );
};

template< typename T >
class is_constructible_impl< T, void, void, void, void >
{
private:
    template< typename C_T >
    static C_T testFun( C_T );

    template< typename C_T >
    static bool test( typename c_std::enable_if< sizeof( C_T ) == sizeof( testFun( C_T() ) ) >::type* );

    template< typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T >( NULL ) ) == sizeof( bool ) );
};

template< typename T, typename AT_1 = void, typename AT_2 = void, typename AT_3 = void, typename AT_4 = void >
class is_constructible_impl_ptr
{
public:
    static const bool value = false;
};

template< typename T, typename AT_1 >
class is_constructible_impl_ptr< T, AT_1, typename enable_if< is_pointer< typename remove_reference< T >::type >::value, void >::type, void, void >
{
private:
    template< typename C_T >
    static bool test( C_T );

    template< typename >
    static int test( ... );

public:
    static const bool value = ( sizeof( test< T >( static_cast< AT_1 >( NULL ) ) ) == sizeof( bool ) );
};

template< typename T >
class is_constructible_impl_ptr< T, void, void, void, void >
{
public:
    static const bool value = true;
};

template< typename T, typename AT_1 = void, typename AT_2 = void, typename AT_3 = void, typename AT_4 = void >
class is_constructible
{
public:
    static const bool value = (
        is_pointer< typename remove_reference< T >::type >::value ?
            is_constructible_impl_ptr< T, AT_1, AT_2, AT_3, AT_4 >::value :
            is_constructible_impl< T, AT_1, AT_2, AT_3, AT_4 >::value
    );
};