c++ - 如何避免序列化器和容器序列化器之间的循环模板依赖？

Question

我正在为 STL 容器的反序列化编写模板类。

假设我想序列化一个 set<int> .我有一个基地 serialisation类和以下模板:

template<typename T>class serialiser;

template<>class serialiser<int>:public serialisation<int>{
public:
  void serialise ( int t );
};

序列化set ，我有:

template<typename T>class Container_serialiser:public serialisation<T>{
public:
  void serialise ( T t );
private:
  /* Notice that I must declare a serialiser.
   */
  serialiser<typename T :: value_type>value_serialiser;
};

template<typename T>void Container_serialiser<T>::serialise ( T t ){
  for(typename T :: const_iterator t_iterator = t . begin ( );t_iterator != t . end ( );++ t_iterator){
    value_serialiser . serialise ( * t_iterator );
  }
}

这仅适用于其元素本身不构成容器的容器。

问题

如果我想序列化一个 map<int,set<int> > 怎么办？ ？

因为每个元素的类型都是 pair<int,set<int> > ，我需要以下类(class):

template<>class serialiser<pair<int,set<int> > >:public serialisation<pair<int,set<int> > >{
public:
  void serialise ( const pair<int,set<int> >t );
private:
  serialiser<int>t_first_serialiser;

  /* Notice that I must declare a Container_serialiser.
   */
  Container_serialiser<set<int> >t_second_serialiser;
};

然而，Container_serialiser需要 serialiser<T>成为一个完整的类型。因此，它不能在上面的类之后定义。同样，上面的类需要 Container_serialiser<T>是一个完整的类型，不能在它之前定义。

编译器报如下错误:

prog.cpp: In instantiation of 'class Container_serialiser<std::map<int, std::set<int> > >':
prog.cpp:73:44:   required from here
prog.cpp:40:39: error: 'Container_serialiser<T>::value_serialiser' has incomplete type
   serialiser<typename T :: value_type>value_serialiser;
                                       ^
prog.cpp:25:27: note: declaration of 'class serialiser<std::pair<const int, std::set<int> > >'
 template<typename T>class serialiser;

问题代码

#include <iostream>
#include <map>
#include <set>
#include <utility>

using std :: cout;

using std :: map;

using std :: set;

using std :: make_pair;
using std :: pair;

template<typename T>class serialisation{
public:
  virtual ~serialisation ( );
protected:
  virtual void serialise ( const T t ) = 0;
};

template<typename T>serialisation<T> :: ~serialisation ( ){
}

template<typename T>class serialiser;

template<>class serialiser<int>:public serialisation<int>{
public:
  void serialise ( const int t );
};

void serialiser<int>:: serialise ( const int t ){
  cout << t << '\n';
}

template<typename T>class Container_serialiser:public serialisation<T>{
public:
  void serialise ( const T t );
private:
  serialiser<typename T :: value_type>value_serialiser;
};

template<typename T>void Container_serialiser<T>:: serialise ( const T t ){
  for(typename T :: const_iterator t_iterator = t . begin ( );t_iterator != t . end ( );++ t_iterator){
    value_serialiser . serialise ( * t_iterator );
  }
}

template<>class serialiser<pair<int,set<int> > >:public serialisation<pair<int,set<int> > >{
public:
  void serialise ( const pair<int,set<int> >t );
private:
  serialiser<int>t_first_serialiser;

  Container_serialiser<set<int> >t_second_serialiser;
};

void serialiser<pair<int,set<int> > >:: serialise ( const pair<int,set<int> >t ){
  t_first_serialiser . serialise ( t . first );
  t_second_serialiser . serialise ( t . second );
}

int main ( ){
  set<int>t;
  t . insert ( 2 );
  t . insert ( 3 );
  Container_serialiser<set<int> >t_serialiser;
  t_serialiser . serialise ( t );

  map<int,set<int> >u;
  u . insert ( make_pair ( 5,t ) );
  u . insert ( make_pair ( 7,t ) );
  Container_serialiser<map<int,set<int> > >u_serialiser;
  u_serialiser . serialise ( u );
}

Ideone

黑客

我可以通过编写四个模板来完成这项工作:

1. serialiser0<T>对于所有不是容器且不包含容器的 T
2. Container_serialiser0<T>对于 value_type 的所有 T是 serialiser0<T> 的参数
3. serialiser1<T>对于所有不是容器但包含本身不包含容器的元素的容器的 T
4. Container_serialiser1<T>对于 value_type 的所有 T是 serialiser1<T> 的参数

这种设计是重复的、令人困惑的，并且必须为每个新级别的容器手动扩展。有没有更好的模式？

不幸的是，由于我正在从事的项目，我必须避免外部依赖，例如boost.serialization。甚至以牺牲性能为代价并为 C++98 标准编写代码。

破解代码

#include <iostream>
#include <map>
#include <set>
#include <utility>

using std :: cout;

using std :: map;

using std :: set;

using std :: make_pair;
using std :: pair;

template<typename T>class serialisation{
public:
  virtual ~serialisation ( );
protected:
  virtual void serialise ( const T t ) = 0;
};

template<typename T>serialisation<T> :: ~serialisation ( ){
}

template<typename T>class serialiser0;

template<>class serialiser0<int>:public serialisation<int>{
public:
  void serialise ( const int t );
};

void serialiser0<int>:: serialise ( const int t ){
  cout << t << '\n';
}

template<typename T>class Container_serialiser0:public serialisation<T>{
public:
  void serialise ( const T t );
private:
  serialiser0<typename T :: value_type>value_serialiser;
};

template<typename T>void Container_serialiser0<T>:: serialise ( const T t ){
  for(typename T :: const_iterator t_iterator = t . begin ( );t_iterator != t . end ( );++ t_iterator){
    value_serialiser . serialise ( * t_iterator );
  }
}

template<typename T>class serialiser1;

template<>class serialiser1<pair<const int,set<int> > >:public serialisation<pair<int,set<int> > >{
public:
  void serialise ( const pair<int,set<int> >t );
private:
  serialiser0<int>t_first_serialiser;

  Container_serialiser0<set<int> >t_second_serialiser;
};

void serialiser1<pair<const int,set<int> > >:: serialise ( const pair<int,set<int> >t ){
  t_first_serialiser . serialise ( t . first );
  t_second_serialiser . serialise ( t . second );
}

/* This is the same as Container_serialiser0!
 */
template<typename T>class Container_serialiser1:public serialisation<T>{
public:
  void serialise ( const T t );
private:
  serialiser1<typename T :: value_type>value_serialiser;
};

template<typename T>void Container_serialiser1<T>:: serialise ( const T t ){
  for(typename T :: const_iterator t_iterator = t . begin ( );t_iterator != t . end ( );++ t_iterator){
    value_serialiser . serialise ( * t_iterator );
  }
}

int main ( ){
  set<int>t;
  t . insert ( 2 );
  t . insert ( 3 );
  Container_serialiser0<set<int> >t_serialiser;
  t_serialiser . serialise ( t );

  map<int,set<int> >u;
  u . insert ( make_pair ( 5,t ) );
  u . insert ( make_pair ( 7,t ) );
  Container_serialiser1<map<int,set<int> > >u_serialiser;
  u_serialiser . serialise ( u );
}

Ideone

Answer 1

(我必须承认我忘记了这个问题。)

感谢@michael-gopshtein 的评论，我最终偶然发现了 partial template specialization ，很好的解决了这个问题。请注意，我必须同时使用 template <typename SerialisedType> class Serialiser和 template <typename SerialisedType> inline void serialise(const SerialisedType &x) .函数本身不能部分特化，但是 Serialiser::serialise 的大部分特化取决于对 ::serialise 的一般调用.

序列化 std::map<int, std::set<int>> ，我会写以下内容:

#include <map>
#include <set>
#include <utility>

// For the sake of having a MWE, I'll just write int-s to stdout.
#include <iostream>

template <typename SerialisedType> class Serialiser;

template <typename SerialisedType>
inline void serialise(const SerialisedType &x) {
  Serialiser<SerialisedType>::serialise(x);
}

template <> class Serialiser<int> {
public:
  inline static void serialise(const int &x);
};

template <typename Key> class Serialiser<std::set<Key>> {
public:
  inline static void serialise(const std::set<Key> &x);
};

template <typename T1, typename T2> class Serialiser<std::pair<T1, T2>> {
public:
  inline static void serialise(const std::pair<T1, T2> &x);
};

template <typename Key, typename T>
class Serialiser<std::map<Key, T>> {
public:
  inline static void serialise(const std::map<Key, T> &x);
};

void Serialiser<int>::serialise(const int &x) {
  std::cout << x << "\n";
}

template <typename Key>
void Serialiser<std::set<Key>>::serialise(const std::set<Key> &x) {
  {
    const typename std::set<Key>::const_iterator x_cend = x.cend();

    for (typename std::set<Key>::const_iterator x_iterator = x.cbegin();
         x_iterator != x_cend; ++x_iterator)
      ::serialise(*x_iterator);
  }
}

template <typename T1, typename T2>
void Serialiser<std::pair<T1, T2>>::serialise(const std::pair<T1, T2> &x) {
  ::serialise(x.first);
  ::serialise(x.second);
}

template <typename Key, typename T>
void Serialiser<std::map<Key, T>>::serialise(const std::map<Key, T> &x) {
  {
    const typename std::map<Key, T>::const_iterator x_cend = x.cend();

    for (typename std::map<Key, T>::const_iterator x_iterator = x.cbegin();
         x_iterator != x_cend; ++x_iterator)
      ::serialise(*x_iterator);
  }
}

int main() {
  std::map<int, std::set<int>> u {
    {5, {2, 3}},
    {7, {2, 3}}
  };
  serialise(u);
}

输出:

5
2
3
7
2
3