11

我在 C++ 中使用具有多个虚拟继承的可变参数模板将类型聚合到单个结构定义中。

这是一组结构示例：

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聚合这些：

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;

然后这些可以级联：

typedef zipper<meas3, meas_type> meas_type2;

然而，生成的对象相当笨拙：

$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有一个次要问题：

typedef zipper<meas, meas> meas_type2;

以上在 G++ 4.6.3 下会产生编译器错误“duplicate base class 'meas' is invalid”。

因此，问题是双重的：

  1. 有没有办法zipper<meas3, zipper<meas, meas2>>变身zipper<meas3, meas2>？
  2. 有没有办法在完成 #1 的同时删除类型列表中的重复条目？

谢谢！

4

1 回答 1

11

我解决这个问题的策略是使用一些间接级别。

  • zipper < Args...> 通过继承将其参数的处理分派给函数 process_zipper_arguments：

例子：

template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type > {};
  • 使用 atemplate < typename... Args > struct typelist {}来跟踪要继承的对象类型。
  • 专门struct zipper < typelist < Args... > >: public virtual Args...做实际的继承

为了摆脱重复的父类型，使用了两个辅助函数process_zipper_arguments：

  • is_in < CandidateType, typelist< Args... > >::type是true_type或false_type并且可以递归定义
  • add_unique < CandidateType, typelist< Args... > >::type是一个typelist <...>添加或不添加 CandidateType 的。它要求is_in确定这一点。

这是至少使用 g++ (GCC) 4.6.3 和 --std=c++0x 编译的完整代码。欢迎批评。

// 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;行处的断点）：

(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> >>}
于 2012-12-12T13:16:20.640 回答