Windows DLL 链接器错误与模板类

Question

template <typename SenderType__, typename... Args__>
class CORE_API Event
{
public:
  typedef typename std::function<void(const SenderType__*, Args__ ...)> EventHandler;  
  Event& operator+=(const EventHandler& toSubscribe)
  {
    Subscribe(toSubscribe);
    return *this;
  }
  void operator()(const SenderType__* sender, Args__ ... args) const
  {
    Invoke(sender, args...);
  }
  void Subscribe(const EventHandler& toSubscribe)
  {
    std::lock_guard<std::mutex> locker(m_callbackMutex);
    m_callbacks.push_back(toSubscribe);
  }
  void Clear()
  {
    std::lock_guard<std::mutex> locker(m_callbackMutex);
    m_callbacks.clear();
  }
  void Invoke(const SenderType__* sender, Args__ ... args) const
  {
    std::lock_guard<std::mutex> locker(m_callbackMutex);
    for (auto iter = m_callbacks.begin(); iter != m_callbacks.end(); ++iter)
    {
      (*iter)(sender, args...);
    }
  }
private:
  std::vector<EventHandler> m_callbacks;
  mutable std::mutex m_callbackMutex;
};
template class CORE_API Event<std::string, std::string>;

在 DLL 的使用者中。

TEST(EventTest, TestEventFiresAndPassesArgs)
{
  Event<std::string, std::string> event;
  event += &TestFunction;
  event += &TestFunction2;
  std::string sender = "TestEventFiresAndPassesArgs";
  std::string arg = "boo!";
  event.Invoke(&sender, arg);
  ASSERT_EQ(sender, testEventFiresAndPassesArgsSenderName);
  ASSERT_EQ(arg, testEventFiresAndPassesArgsTestArg);
  ASSERT_EQ(sender + "Function2", testEventFiresAndPassesArgsSenderName2);
  ASSERT_EQ(arg + "Function2", testEventFiresAndPassesArgsTestArg2);
}

则链接器输出为：

EventTest.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: class CompanyName::Utils::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > & __cdecl CompanyName::Utils::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >::operator+=(class std::function<void __cdecl(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const *,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)> const &)" (__imp_??Y?$Event@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@Utils@CompanyName@@QEAAAEAV012@AEBV?$function@$$A6AXPEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@Z@std@@@Z) referenced in function "private: virtual void __cdecl EventTest_TestEventFiresAndPassesArgs_Test::TestBody(void)" (?TestBody@EventTest_TestEventFiresAndPassesArgs_Test@@EEAAXXZ)
2>EventTest.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: void __cdecl CompanyName::Utils::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >::Invoke(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const *,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)const " (__imp_?Invoke@?$Event@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@Utils@CompanyName@@QEBAXPEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V45@@Z) referenced in function "private: virtual void __cdecl EventTest_TestEventFiresAndPassesArgs_Test::TestBody(void)" (?TestBody@EventTest_TestEventFiresAndPassesArgs_Test@@EEAAXXZ)
2>EventTest.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl CompanyName::Utils::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >(void)" (__imp_??0?$Event@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@Utils@CompanyName@@QEAA@XZ) referenced in function "private: virtual void __cdecl EventTest_TestEventFiresAndPassesArgs_Test::TestBody(void)" (?TestBody@EventTest_TestEventFiresAndPassesArgs_Test@@EEAAXXZ)
2>EventTest.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl CompanyName::Utils::Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >::~Event<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >(void)" (__imp_??1?$Event@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@Utils@CompanyName@@QEAA@XZ) referenced in function "private: virtual void __cdecl EventTest_TestEventFiresAndPassesArgs_Test::TestBody(void)" (?TestBody@EventTest_TestEventFiresAndPassesArgs_Test@@EEAAXXZ)
2>..runCoreUnitTests.exe : fatal error LNK1120: 4 unresolved externals

知道我在这里做错了什么吗？

Answer 1

创建 DLL 时，需要使用__declspec(dllexport)导出函数。

可以通过使用 __declspec(dllimport) 声明另一个 DLL 中的此类函数来使用此类函数。

这些非常适合常规功能。

但是，对于类模板和函数模板，模板会根据需要实例化。它们不会导出到在其中定义它们的 DLL。因此，它们也无法从 DLL 导入。因此，不要将__declspec(dllexport)或__declspec(dllimport)与类模板和函数模板一起使用。