如何通过迭代器的强制转换来解决这个问题？
编译器（VC8）错误是：
错误 C2680：'std::_Tree<_Traits>::iterator'：dynamic_cast 的目标类型无效
模拟错误的源代码：
[编辑]源代码现已修复
#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;
}
The compiler (VC8) error is:
error C2680: 'std::_Tree<_Traits>::iterator' : invalid target type for dynamic_cast
the source code to simulate the error :
[EDIT]source fixed now
#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;
}
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评论（2）
首先，您的dynamic_cast语法是错误的（或者可能是格式问题？）：
Dynamic_cast 语法：
dynamic_cast(BaseClass*) 或此代码对我来说看起来非常可疑。你想达到什么目的？为什么要动态强制转换迭代器？这没有任何意义。
编辑
map 的
begin()有两个重载，一个返回 const_iterator，另一个返回非 const_iterator。在您注释的代码中，由于您的TypeToThreadIterMapStorage映射的 value_type 是非常量迭代器，因此使用 begin() 的第二个版本。但是，theData iteartor 的类型是 const_iterator，并且编译器抱怨 const_iterator 无法转换为非常量迭代器。您需要的是一个const_cast来删除对象的常量，而不是dynamic_cast。但请注意，这是一件危险的事情。更简单的方法是将 theData 声明为非常量迭代器。
First of all your dynamic_cast syntax is wrong (or may be formatting issue?):
dynamic_cast syntax:
dynamic_cast<DerivedClassName*>(BaseClass*)ORThis code looks very suspcious to me. What are you trying to achieve? Why do you want to dynamic_cast an iterator? That doesn't make any sense.
EDIT
map's
begin()has two overloads, one which returns the const_iterator and the other which returns a non const_iterator. In your commented code, since yourTypeToThreadIterMapStoragemap's value_type is a non-const iterator the second version of begin() is used. However, the type of theData iteartor is a const_iterator and the compiler is complaining that const_iterator can not be converted into a non-const iterator. What you require is aconst_castto remove the constness of the object and notdynamic_cast. But note that this is a dangerous thing to do.The simpler way is to declare theData as a non-const iterator.
是什么让您认为 TypeData::iterator 和 TypeData::const_iterator 甚至是相关的？
为什么不将“theData”的类型更改为迭代器？
What makes you think that TypeData::iterator and TypeData::const_iterator are even related?
Why not change the type of 'theData' to an iterator?