我偶然发现了一个我无法解决的问题。我正在编译一个共享库,其中包含一个模板类(Derived<T>
,其基数为Base
)以及该类的一些显式实例。我希望图书馆用户能够从这个模板类扩展。当我尝试dynamic_cast
用户的实例从Base*
到Derived<T>*
时,问题就出现了。在共享库中显式实例化模板化类和dynamic_cast
我缩小了问题该MWE:
共享库包含以下文件:
Base.h
#ifndef BASE_H_
#define BASE_H_
class Base {
public:
Base();
virtual ~Base();
};
#endif /* BASE_H_ */
Derived.h
#ifndef DERIVED_H_
#define DERIVED_H_
#include <Base.h>
template <typename T>
class Derived : public Base {
public:
Derived();
virtual ~Derived();
};
#endif /* DERIVED_H_ */
Derived.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>;
Helper.h
#ifndef HELPER_H_
#define HELPER_H_
#include <Base.h>
class Helper {
public:
Helper(Base* m);
virtual ~Helper();
};
#endif /* HELPER_H_ */
Helper.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() {
}
和使用该库可以是一个简单的代码:
TEST.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;
}
的问题是,当我创建共享库和链接test.cpp
反对,该dynamic_cast
失败。下面是一个例子输出:
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,如果我编译整个库和实例一起,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:为什么是dynamic_cast
失败?
而且,在我想保持类似结构的前提下,继续使用共享库:我如何才能成功获得从Base*
转换而来?
关于缩小范围的建议...... 1)确保传递给你的编译器的选项对于你的lib组件和你的程序组件是一样的......它们会影响管理符号的形成。 2)使用'[nm](http://linux.about.com/library/cmd/blcmdl1_nm.htm)'工具从您的库对象和程序对象中转储符号...您应该看到相同的你的库组件(已定义)和你的测试代码(未定义)的损坏符号 – Andrew 2011-05-24 15:04:16