c++ - 如何在 C++ 中实现基本的 Variant（以及 Variant 上的访问者）模板？

Question

我试过阅读：

http://www.boost.org/doc/libs/1_41_0/boost/variant.hpp


http://www.codeproject.com/KB/cpp/TTLTyplist.aspx


and chapter 3 of "Modern C++ Design"

但仍然不明白变体是如何实现的。任何人都可以粘贴一个关于如何定义类似内容的简短示例：

class Foo {
  void process(Type1) { ... };
  void process(Type2) { ... };
};


Variant<Type1, Type2> v;

v.somethingToSetupType1 ...;

somethingToTrigger process(Type1);

v.somethingToSetupType2 ...;

somethingToTrigger process(Type2);

谢谢！

Answer 1

如果我必须定义一个变体对象，我可能会从以下开始：

template<typename Type1, typename Type2>
class VariantVisitor;

template<typename Type1, typename Type2>
class Variant
{
public:
   friend class VariantVisitor<Type1, Type2>;
   Variant();
   Variant(Type1);
   Variant(Type2);
   // + appropriate operators =
   ~Variant(); // deal with memory management

private:
    int type; // 0 for invalid data, 1 for Type1, 2 for Type2
    void* data;
};

template<typename Visitor, typename Type1, typename Type2>
class VariantVisitor 
{
   private:
     Visitor _customVisitor;
   public:
   void doVisit(Variant<Type1, Type2>& v)
   {
      if( v.type == 1 )
      {
          _customVisitor( *(Type1*)(v.data));
      }
      else if( v.type == 2 )
      {
          _customVisitor( *(Type2*)(v.data));
      }
      else
      {
         // deal with empty variant
      }
   }
};
template<typename Visitor, typename Type1, typename Type2>
void visit( Visitor visitor, Variant<Type1, Type2> v )
{
  VariantVisitor<Visitor, Type1, Type2>(visitor).doVisit(v);
}

然后使用MPL 向量使该方法不仅适用于两种不同的类型。

最后，您可以编写如下内容：

Variant<Type1, Type2> v;
class MyVisitor
{
  public:
  operator()(Type1);
  operator()(Type2);
};

MyVisitor visitor;
v = Type1();
visit(visitor, v);
v = Type2();
visit(visitor, v);

注意：这段代码不可能编译，但这描述了我会使用的想法。

Answer 2

I think you are asking how to use variants, not how to implement them. You may want to look at the boost documentation on variants; this will be much more helpful than looking at the header file.

Then your example might look something like this:

class v_visitor : public boost::static_visitor
{
public:
   void operator()(Type1 &t) const {...}
   void operator()(Type2 &t) const {...}
};

v = Type1(...);
boost::apply_visitor(v_visitor(), v);