我的爱好库的基本组件必须与C++ 98和C++ 11编译器配合使用。为了学习和享受我自己，我创建了几种类型支持功能（如enable_if,conditional,is_same,is_integral等）的C++ 98实现，以便在没有C++ 11支持时使用它们。C++ 98/03 std :: is_constructible实现
但是，当我实施is_constructible我卡住了。有什么样的模板魔法（某种SFINAE），我可以在没有C++ 11支持的情况下实现它（declval）？
当然，在C++ 03中没有可变参数模板的支持，所以我将专门化实现直到一定的深度。主要问题是，是否有一种技术可以决定T是否可以从给定类型中构造出来。
这是可能的：
#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 <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(U(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(U(
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));
}
能扩大2个参数的版本为3，4，5，...参数进一步。
这适用于g++ 4.4.7
它不使用g ++ 4.3.6
我觉得DANH的想法是伟大的工作！只需稍作修改，我们就可以消除新的操作员。 （我有一个C++ 98的enable_if和remove_reference实现）。所提到的int *，void *也适用于这个实现。没有运营商需要新的。只有旧的g ++支持仍然存在...
/********** 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
);
};
几个编译器使用内置'__is_constructible'助手，所以它可能非常困难，至少。 –
相关：https://stackoverflow.com/questions/38181357/how-is-stdis-constructiblet-args-implemented – BartoszKP
是的，C++ 11的实现是明确的。我想用某种魔法消除被引用实现中的'decltype'和'std :: declval'依赖关系，或者找到一种完全不同的技术。唯一的限制是C++ 98标准。 – Broothy