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
);
};
来源：https://stackoverflow.com/questions/40439909/c98-03-stdis-constructible-implementation