标签：virtual-inheritance c c11 variadic-templates multiple-inheritance

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... {};

然后我可以这样做：

typedef zipper<meas, meas2> meas_type;
meas* m = new meas_type;

这些可以级联：

typedef zipper<meas3, meas_type> meas_type2;

但是,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：

>有没有办法转换zipper< meas3,zipper< meas,meas2>>拉链< meas3,meas2>？
>在完成#1时,有没有办法删除类型列表中的重复条目？

谢谢！

解决方法:

我解决这个问题的策略是使用几个级别的间接.

>拉链< Args ...>通过继承来调度对函数process_zipper_arguments的参数处理：

例：

template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type > {};

>使用模板< typename ... Args> struct typelist {}用于跟踪要继承的对象类型.
>专门的结构拉链<类型列表< Args ...> &gt ;:公共虚拟Args …来做实际的继承

为了摆脱重复的父类型,在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是一个类型列表< ...>是否添加了CandidateType.它调用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.批评它是受欢迎的.

// 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(返回0处的断点;行)：

(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> >>}

标签：virtual-inheritance,c,c11,variadic-templates,multiple-inheritance
来源： https://codeday.me/bug/20191005/1854651.html

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