所以我一直在玩类型清单，男孩们很有趣。我想做的一件事就是尝试实施我自己的variant课程，只是作为一个关于类型列表如何工作以及它们如何有用的教育实验。这是我的代码目前的样子：
#include <cstddef>
#include <typeinfo>
#ifndef VARIANT_H_
#define VARIANT_H_
struct NullType {};
template <class T, class U>
struct TypeList {
typedef T Head;
typedef U Tail;
};
#define TYPELIST_1(T1) TypeList<T1, NullType>
#define TYPELIST_2(T1, T2) TypeList<T1, TYPELIST_1(T2) >
#define TYPELIST_3(T1, T2, T3) TypeList<T1, TYPELIST_2(T2, T3) >
#define TYPELIST_4(T1, T2, T3, T4) TypeList<T1, TYPELIST_3(T2, T3, T4) >
#define TYPELIST_5(T1, T2, T3, T4, T5) TypeList<T1, TYPELIST_4(T2, T3, T4, T5) >
#define TYPELIST_6(T1, T2, T3, T4, T5, T6) TypeList<T1, TYPELIST_5(T2, T3, T4, T5, T6) >
#define TYPELIST_7(T1, T2, T3, T4, T5, T6, T7) TypeList<T1, TYPELIST_6(T2, T3, T4, T5, T6, T7) >
#define TYPELIST_8(T1, T2, T3, T4, T5, T6, T7, T8) TypeList<T1, TYPELIST_7(T2, T3, T4, T5, T6, T7, T8) >
#define TYPELIST_9(T1, T2, T3, T4, T5, T6, T7, T8, T9) TypeList<T1, TYPELIST_8(T2, T3, T4, T5, T6, T7, T8, T9) >
namespace util {
namespace {
template <class TL> struct MaxSize;
template <class TL> struct Length;
template <class TL, class T> struct IndexOf;
template <class TL, unsigned int i> struct TypeAt;
template <>
struct MaxSize<NullType> {
static const size_t value = 0;
};
template <class Head, class Tail>
struct MaxSize<TypeList<Head, Tail> > {
static const size_t value = (sizeof(Head) > MaxSize<Tail>::value) ? sizeof(Head) : MaxSize<Tail>::value;
};
template <>
struct Length<NullType> {
enum { value = 0 };
};
template <class Head, class Tail>
struct Length<TypeList<Head, Tail> > {
enum { value = 1 + Length<Tail>::value };
};
template <class T>
struct IndexOf<NullType, T> {
enum { value = -1 };
};
template <class Tail, class T>
struct IndexOf<TypeList<T, Tail>, T> {
enum { value = 0 };
};
template <class Head, class Tail, class T>
struct IndexOf<TypeList<Head, Tail>, T> {
enum { value = (IndexOf<Tail, T>::value == -1) ? -1 : 1 + IndexOf<Tail, T>::value };
};
template <class Head, class Tail>
struct TypeAt<TypeList<Head, Tail>, 0> {
typedef Head type;
};
template <class Head, class Tail, unsigned int i>
struct TypeAt<TypeList<Head, Tail>, i> {
typedef typename TypeAt<Tail, i - 1>::type type;
};
}
template <class TL>
class variant;
template<class U, class TL>
U *get(variant<TL> *v);
template<class U, class TL>
const U *get(const variant<TL> *v);
template<class U, class TL>
U &get(variant<TL> &v);
template<class U, class TL>
const U &get(const variant<TL> &v);
// this stuff is a visitation pattern used to make sure
// that contained objects get properly destroyed
namespace {
template <class TL>
struct apply_visitor;
struct destroy_visitor {
template <class T>
void operator()(T *p) {
p->~T();
}
};
template <class H, class T>
struct visitor_impl {
template <class U, class Pred>
static void visit(U *p, Pred pred) {
if(H *x = get<H>(p)) {
pred(x);
} else {
apply_visitor<T>::visit(p, pred);
}
}
};
template <class H>
struct visitor_impl<H, NullType> {
template <class U, class Pred>
static void visit(U *p, Pred pred) {
if(H *x = get<H>(p)) {
pred(x);
} else {
throw std::bad_cast();
}
}
};
template <class TL>
struct apply_visitor {
typedef typename TL::Head H;
typedef typename TL::Tail T;
template <class U, class Pred>
static void visit(U *p, Pred pred) {
visitor_impl<H, T>::visit(p, pred);
}
};
}
template <class TL>
class variant {
template<class U, class X> friend U *get(variant<X> *v);
template<class U, class X> friend const U *get(const variant<X> *v);
template<class U, class X> friend U &get(variant<X> &v);
template<class U, class X> friend const U &get(const variant<X> &v);
public :
variant() : type_index_(0){
new (&storage_) typename TypeAt<TL, 0>::type();
}
~variant() {
apply_visitor<TL>::visit(this, destroy_visitor());
}
template <class T>
variant(const T &x) : type_index_(IndexOf<TL, T>::value) {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
new (&storage_) value_type(x);
}
template <class T>
variant(T &x) : type_index_(IndexOf<TL, T>::value) {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
new (&storage_) value_type(x);
}
template <class T>
variant &operator=(const T &rhs) {
variant(rhs).swap(*this);
return *this;
}
variant &operator=(const variant &rhs) {
variant(rhs).swap(*this);
return *this;
}
public:
void swap(variant &other) {
using std::swap;
swap(storage_, other.storage_);
swap(type_index_, other.type_index_);
}
private:
template <class T>
const T &get_ref() const {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
if(IndexOf<TL, T>::value != type_index_) {
throw std::bad_cast();
}
return *reinterpret_cast<const value_type *>(&storage_);
}
template <class T>
T &get_ref() {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
if(IndexOf<TL, T>::value != type_index_) {
throw std::bad_cast();
}
return *reinterpret_cast<value_type *>(&storage_);
}
template <class T>
const T *get_ptr() const {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
if(IndexOf<TL, T>::value != type_index_) {
return 0;
}
return reinterpret_cast<const value_type *>(&storage_);
}
template <class T>
T *get_ptr() {
typedef typename TypeAt<TL, IndexOf<TL, T>::value>::type value_type;
if(IndexOf<TL, T>::value != type_index_) {
return 0;
}
return reinterpret_cast<value_type *>(&storage_);
}
public:
int which() const {
return type_index_;
}
bool empty() const {
return false;
}
const std::type_info &type() const;
private:
struct { unsigned char buffer_[MaxSize<TL>::value]; } storage_;
int type_index_;
};
// accessors
template<class U, class TL>
U *get(variant<TL> *v) {
return v->template get_ptr<U>();
}
template<class U, class TL>
const U *get(const variant<TL> *v) {
return v->template get_ptr<U>();
}
template<class U, class TL>
U &get(variant<TL> &v) {
return v.template get_ref<U>();
}
template<class U, class TL>
const U &get(const variant<TL> &v) {
return v.template get_ref<U>();
}
}
#endif
这非常有效！我可以写下面的内容，效果很好：
typedef util::variant<TYPELIST_3(std::string, int, double)> variant;
variant x = std::string("hello world");
variant y = 10;
variant z = 123.45;
std::cout << util::get<std::string>(x) << std::endl;
std::cout << util::get<int>(y) << std::endl;
std::cout << util::get<double>(z) << std::endl;
所有工作都按预期进行:-)。这是我的问题。使用boost::variant我可以编写以下内容而不会出现任何问题：
boost::variant<int, std::string> v = "hello world";
使用我的版本，如果我写得类似：
util::variant<TYPELIST_2(int, std::string)> v = "hello world";
我收到这样的错误：
variant.hpp: In instantiation of 'util::<unnamed>::TypeAt<TypeList<std::basic_string<char>, NullType>, 4294967294u>':
variant.hpp:76:47: instantiated from 'util::<unnamed>::TypeAt<TypeList<int, TypeList<std::basic_string<char>, NullType> >, 4294967295u>'
variant.hpp:161:61: instantiated from 'util::variant<TL>::variant(const T&) [with T = char [12], TL = TypeList<int, TypeList<std::basic_string<char>, NullType> >]'
test.cc:27:50: instantiated from here
variant.hpp:76:47: error: invalid use of incomplete type 'struct util::<unnamed>::TypeAt<NullType, 4294967293u>'
variant.hpp:32:46: error: declaration of 'struct util::<unnamed>::TypeAt<NullType, 4294967293u>'
基本上，它在变体中的类型列表中找不到char[12]。这是有道理的，因为char[12]实际上并未明确列为其中一种类型......
boost::variant如何无缝地完成这项工作？我觉得这是我理解boost::variant如何运作的唯一真正缺失的部分。想法？
答案 0 :(得分：3)
你不想做is_convertible作为另一个答案建议。您基本上将使用C ++类型特征重新实现C ++转换机制。相反，您可以使用已有的C ++基础结构。
boost的方法是通过一个具有函数的类来获取变量可以接受的每种类型。我不确定它是如何用C ++ 03完全提升的，但在C ++ 11语法中：
template <typename First, typename... Rest>
class constructor : public constructor<Rest...>
{
using constructor<Rest...>::construct;
static void
construct(variant& v, First&& value);
};
然后你的operator =和其他函数调用constructor<Types...>::construct(*this, value)，如果有明确的转换，那么C ++会为你找到它。
我写了一篇相当详细的博客文章，剖析了这一切是如何运作的：http://thenewcpp.wordpress.com/2012/02/15/variadic-templates-part-3-or-how-i-wrote-a-variant-class/
答案 1 :(得分：2)
您可以使用is_convertible（或C ++ 11 stdlib版本）等类型特征。
正如@Andreas的评论所说，你需要修改你的模板化构造函数/赋值运算符，比如，不要搜索特定类型，而是搜索第一个匹配。
#include <boost/mpl/if.hpp>
#include <boost/type_traits/is_convertible.hpp>
template<class T, class TList>
struct FirstMatch;
template<class T, class Head, class Tail>
struct FirstMatch<T, TypeList<Head, Tail>>{
static bool const is_conv = boost::is_convertible<T, Head>::value;
typedef typename boost::mpl::if_c<is_conv, Head,
typename FirstMatch<T, Tail>::type>::type type;
};
template<class T>
struct FirstMatch<T, NullType>{
typedef struct ERROR_no_convertible_type_found type;
};
template<class T, class TList>
struct FirstOrExactMatch{
static int const idx = IndexOf<TList, T>::value;
typedef typename boost::mpl::if_c<idx != -1,
TypeAt<TList, idx>,
FirstMatch<T, TList>
>::type::type type;
};
代码未经测试，但应该有效（减去拼写错误）。