如何使用迭代器来解决这个问题?

编译器(VC8)错误为:

模拟错误的源代码:
[编辑]源代码已修复

#include <map>
#include <string>

struct tag_data
{
    int in;
    int on;
    std::string sn;
};

class myclass
{
private:
    typedef std::map<std::string, tag_data> TypeData; 
    TypeData MapStorage;
    typedef std::map<unsigned long, TypeData::iterator > TypeToThreadIterMapStorage; 
    TypeToThreadIterMapStorage ThreadIterMapStorage;

public:
    bool find( std::string& k) 
    {
        TypeData::const_iterator theData ; 
        theData = MapStorage.find(k);
        //ThreadIterMapStorage [ 0 ] = MapStorage.begin();// this is ok
        ThreadIterMapStorage [ 1 ] = dynamic_cast<TypeData::iterator>(theData); // the error occurs here
        return theData != MapStorage.end();
    }

    virtual ~myclass(){}
};

int _tmain(int argc, _TCHAR* argv[])
{
    myclass mc;
    return 0;
}