我今天偶然发现了一个我似乎无法解决的问题。我正在编译一个共享库,其中包括一个模板类(Derived<T>
,其基础是Base
)和该类的一些显式实例化。我希望图书馆用户从这个模板类扩展。当我尝试dynamic_cast
将用户的实例从Base*
to 转移到Derived<T>*
.
我已将问题缩小到这个 MWE:
共享库包含以下文件:
基数.h
#ifndef BASE_H_
#define BASE_H_
class Base {
public:
Base();
virtual ~Base();
};
#endif /* BASE_H_ */
派生的.h
#ifndef DERIVED_H_
#define DERIVED_H_
#include <Base.h>
template <typename T>
class Derived : public Base {
public:
Derived();
virtual ~Derived();
};
#endif /* DERIVED_H_ */
派生的.cpp
#include <Derived.h>
template <typename T>
Derived<T>::Derived() :
Base() {
}
template <typename T>
Derived<T>::~Derived() {
}
// explicit instantiations
template class Derived<float>;
template class Derived<double>;
template class Derived<long double>;
助手.h
#ifndef HELPER_H_
#define HELPER_H_
#include <Base.h>
class Helper {
public:
Helper(Base* m);
virtual ~Helper();
};
#endif /* HELPER_H_ */
助手.cpp
#include <Helper.h>
#include <Base.h>
#include <Derived.h>
#include <iostream>
using namespace std;
Helper::Helper(Base* m) {
cout << "after received " << m << endl;
cout << "after fom: " << dynamic_cast< Derived<float>* >(m) << endl;
cout << "after dom: " << dynamic_cast< Derived<double>* >(m) << endl;
cout << "after ldom: " << dynamic_cast< Derived<long double>* >(m) << endl;
cout << "===" << endl;
}
Helper::~Helper() {
}
使用该库的简单代码可能是:
测试.cpp
#include <Derived.h>
#include <Helper.h>
#include <iostream>
using namespace std;
class MyModel : public Derived<double> {
public:
MyModel() : Derived<double>() {
};
virtual ~MyModel() {
};
};
int main(int argc, char *argv[]) {
MyModel om1;
cout << "created mymodel " << &om1 << endl;
cout << "before fom: " << dynamic_cast< Derived<float>* >(&om1) << endl;
cout << "before dom: " << dynamic_cast< Derived<double>* >(&om1) << endl;
cout << "before ldom: " << dynamic_cast< Derived<long double>* >(&om1) << endl;
cout << "===" << endl;
Helper root(&om1);
return 0;
}
问题是当我创建一个共享库并链接test.cpp
它时,dynamic_cast
失败了。这是一个示例输出:
created mymodel 0x7fff5fbff3e0
before fom: 0
before dom: 0x7fff5fbff3e0
before ldom: 0
===
after received 0x7fff5fbff3e0
after fom: 0
after dom: 0 // <<< Here I expected it to succeed and return a non-null pointer
after ldom: 0
===
但是,如果我一起编译整个库和示例,则转换成功:
created mymodel 0x7fff5fbff3e0
before fom: 0
before dom: 0x7fff5fbff3e0
before ldom: 0
===
after received 0x7fff5fbff3e0
after fom: 0
after dom: 0x7fff5fbff3e0
after ldom: 0
===
我的问题是:为什么会dynamic_cast
失败?
并且,在我想像示例一样维护一个类结构,并继续使用共享库的前提下:我怎样才能成功地Derived<some type>*
从 a中获取强制转换Base*
?