我在C ++中使用具有多个虚拟继承的可变参数模板，以将类型聚合到单个结构定义中。
以下是一组结构示例：
struct meas { int i; };
struct meas2 : public virtual meas { int j; };
struct meas3 : public virtual meas { int k; };
然后我使用多个虚拟继承来聚合它们：
template <typename... Args>
struct zipper : public virtual Args... {};
我可以这样做：
typedef zipper<meas, meas2> meas_type;
meas* m = new meas_type;
然后可以级联：
typedef zipper<meas3, meas_type> meas_type2;
然而，生成的对象相当笨重：
$46 = (zipper<meas3, zipper<meas, meas2> >) {
<meas3> = {
<meas> = {
i = 0
},
members of meas3:
_vptr.meas3 = 0x400ec8,
k = 0
},
<zipper<meas, meas2>> = {
<meas2> = {
members of meas2:
_vptr.meas2 = 0x400ee0,
j = 6299120
},
members of zipper<meas, meas2>:
_vptr.zipper = 0x400eb0
}, <No data fields>}
根据gdb。
尝试压缩相同的基本类型时还存在次要问题：
typedef zipper<meas, meas> meas_type2;
以上在G ++ 4.6.3下产生编译错误“重复基类'meas'无效”。
因此问题是双重的：
zipper<meas3, zipper<meas, meas2>>转换为zipper<meas3, meas2>？ 谢谢！
答案 0 :(得分：10)
我解决这个问题的策略是使用几个间接层。
示例：
template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type > {};
template < typename... Args > struct typelist {}跟踪要继承的对象类型。struct zipper < typelist < Args... > >: public virtual Args...进行实际继承为了摆脱重复的父类型，process_zipper_arguments中使用了两个辅助函数：
is_in < CandidateType, typelist< Args... > >::type是true_type或false_type，可以递归定义add_unique < CandidateType, typelist< Args... > >::type是typelist <...>，是否添加了CandidateType。它会调用is_in来确定。这是完整的代码，至少用g ++（GCC）4.6.3编译，其中--std = c ++ 0x。对它的批评是值得欢迎的。
// Forward declarations
template < typename... Args >
struct zipper;
// Two types meaning true and false
struct true_type {};
struct false_type {};
// The only purpose of this struct is to be associated with Types...
template < typename... Types >
struct typelist {};
// ===================================================
// is_in < type, typelist<...> >::type
// is true_type if type is in typelist
// is false_type if type is not in typelist
// Assume TElement is not in the list unless proven otherwise
template < typename TElement, typename TList >
struct is_in {
typedef false_type type;
};
// If it matches the first type, it is definitely in the list
template < typename TElement, typename... TTail >
struct is_in < TElement, typelist < TElement, TTail... > >
{
typedef true_type type;
};
// If it is not the first element, check the remaining list
template < typename TElement, typename THead, typename... TTail >
struct is_in < TElement, typelist < THead, TTail... > >
{
typedef typename is_in < TElement, typelist < TTail... > >::type type;
};
// ===================================================
// add_unique < TNew, typelist<...> >::type
// is typelist < TNew, ... > if TNew is not already in the list
// is typelist <...> otherwise
// Append a type to a type_list unless it already exists
template < typename TNew, typename TList,
typename Tis_duplicate = typename is_in < TNew, TList >::type
>
struct add_unique;
// If TNew is in the list, return the list unmodified
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, true_type >
{
typedef typelist < TList... > type;
};
// If TNew is not in the list, append it
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, false_type >
{
typedef typelist < TNew, TList... > type;
};
// ===================================================
// process_zipper_arguments < Args... >::type
// returns a typelist of types to be inherited from.
//
// It performs the following actions:
// a) Unpack zipper<...> and typelist <...> arguments
// b) Ignore values that are already in the list
template < typename... Args >
struct process_zipper_arguments;
// Unpack a zipper in the first argument
template < typename... ZipperArgs, typename... Args >
struct process_zipper_arguments < zipper < ZipperArgs... >, Args... >
{
typedef typename process_zipper_arguments < ZipperArgs..., Args... >::type type;
};
// Unpack a typelist in the first argument
template < typename... TypeListArgs, typename... Args >
struct process_zipper_arguments < typelist < TypeListArgs... >, Args... >
{
typedef typename process_zipper_arguments < TypeListArgs..., Args... >::type type;
};
// End the recursion if the list is empty
template < >
struct process_zipper_arguments < >
{
typedef typelist < > type;
};
// Construct the list of unique types by appending them one by one
template < typename THead, typename... TTail >
struct process_zipper_arguments < THead, TTail... >
{
typedef typename
add_unique < THead,
typename process_zipper_arguments < TTail... >::type
>::type type;
};
// ===================================================
// The zipper class that you might want
// If the list of types is not yet known, process it.
// The inheritance is ugly, but there is a workaround
template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type >
{
// // Instead of inheriting, you can use zipper as a factory.
// // So this:
// typedef zipper < meas2, zipper < meas1, meas > > mymeas;
// // Turns to:
// typedef typename zipper < meas2, zipper < meas1, meas > >::type mymeas;
typedef zipper < typename process_zipper_arguments < Args... >::type > type;
};
// If the list of types is known, inherit from each type
template < typename... Args >
struct zipper < typelist < Args... > >
: public virtual Args...
{};
// ===================================================
// Short usage demo, replace with your own code
struct meas {
int i;
};
struct meas2 {
int j;
};
struct meas3 {
int k;
};
typedef zipper < meas, meas, meas3 > meas_type;
typedef zipper < meas2, meas_type, meas2 > meas_type2;
typedef typename zipper < meas_type2 >::type nicer_meas_type2;
int main ( int, char** )
{
meas * m = new meas_type2;
meas_type2 n;
nicer_meas_type2 o;
return 0;
}
调试它会得到以下结果（return 0;行的断点）：
(gdb) print *m
$1 = {i = 0}
(gdb) print n
$2 = {<zipper<typelist<meas, meas3, meas2> >> = {<meas> = {i = 4196320}, <meas3> = {k = 0}, <meas2> = {j = 0},
_vptr.zipper = 0x400928}, <No data fields>}
(gdb) print o
$3 = {<meas> = {i = 4195719}, <meas3> = {k = 0}, <meas2> = {j = 1}, _vptr.zipper = 0x4009a8 <VTT for zipper<typelist<meas, meas3, meas2> >>}