C 98/03 std :: is_constructible实现
作者：互联网
我的爱好库的基本组件必须与C 98和C 11编译器一起使用.为了学习和享受自己,我创建了几种类型支持功能的C 98实现(如enable_if,conditional,is_same,is_integral等…),以便在没有C 11支持时使用它们.
然而,当我实现is_constructible时,我被卡住了.有没有任何模板魔术(某种SFINAE),我可以用它来实现它而不需要C 11支持(declval)？
当然在C 03中没有可变参数模板支持,所以我将把实现专门化到一定深度.主要问题是,是否存在可以决定T是否可以从给定类型构造的技术.
解决方法:
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<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));
}
您可以进一步扩展3个,4个,5个……参数的2个参数版本.
这适用于g++ 4.4.7
它不适用于g 4.3.6
标签：c,c11,compile-time,sfinae,c03 来源： https://codeday.me/bug/20191001/1840369.html