在DLL中无法导出外部的Template类

问题描述

我正在尝试创建一个导出模板类的类库dll。我有同一个课程的本地版本，它必须是巧合，但可以正常工作。我知道这并不意味着什么，显然有什么问题。

I am trying to create a class library dll that exports a template class. I have a local version of the same class and it must be coincidental, but it works correctly. I know this doesn't really mean much, clearly something is wrong.

库dll中导出的模板类为：

The exported template class in the library dll is:

template <class CcmBlock>
class CCMSHARED_EXPORT CcmFilter
{
public:
    CcmFilter()
    {
        mBlock = nullptr;
        ////mBlockList = new std::list<CcmBlock*>();
    }

    void add(CcmFrame* frame)
    {
        if (frame == nullptr)
            return;

        mBlock = new CcmBlock(
                    frame->getFrameData(),
                    frame->getSampleRate(),
                    0xFFFFFFFF,
                    frame->getBlockSize(),
                    frame->getDomain()
                    );
        mBlockList->push_back(aBlock);
    }

    CcmBlock* get()
    {
        return mBlock;
    }
private:
    CcmBlock* mBlock;
    ////std::list<CcmBlock*>* mBlockList;
};

在应用程序方面：

    CcmFilter<FooBlock>* oneFilter = new CcmFilter<FooBlock>();
    //Filter<Block>* filter = new Filter<Block>();

    CcmFrame* frame = new CcmFrame(0, 50000, 40, 1024, Domain::Time);

    oneFilter->add(frame);
    CcmBlock* block = oneFilter->get();
    FooBlock* fooBlock = dynamic_cast<FooBlock*>(block);
    if (fooBlock == nullptr)
    { //Report Error }
    else
    { // Do the work}

，而FooBlock类是从CcmBlock派生的，如下所示：

and the FooBlock class is derived from CcmBlock as follows:

class FooBlock : public CcmBlock
{
public:
    FooBlock(int* ipblock, DWORD dwSampleRate, DWORD dwMicrophoneIndex, DWORD dwBlockSize, Domain domain);

    void process();
};

库会编译并生成dll。当我尝试构建应用程序时，得到以下消息：

The library compiles and builds the dll. When I attempt to build the application I get messages:

mainwindow.obj：-1：错误：LNK2019：未解析的外部符号 __declspec（dllimport）公共：__cdecl CcmFilter： ：CcmFilter（void）（__ imp _ ?? 0？$ CcmFilter @ VFooBlock @@@@ QEAA @ XZ）在函数 public：__cdecl MainWindow :: MainWindow（class QWidget *）中引用（?? 0MainWindow @@ QEAA @ PEAVQWidget @@@ Z）

mainwindow.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl CcmFilter::CcmFilter(void)" (__imp_??0?$CcmFilter@VFooBlock@@@@QEAA@XZ) referenced in function "public: __cdecl MainWindow::MainWindow(class QWidget *)" (??0MainWindow@@QEAA@PEAVQWidget@@@Z)

mainwindow.obj：-1：错误：LNK2019：无法解析的外部符号 __declspec（dllimport）public：void __cdecl CcmFilter :: add（class CcmFrame *）（__imp_？add @？$ CcmFilter @ VFooBlock @@@@ QEAAXPEAVCcmFrame @@@@ Z）在函数 public：__cdecl MainWindow :: MainWindow（QWidget *类）中引用（?? 0MainWindow @@ QEAA @ PEAVQWidget @ @@ Z）

mainwindow.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: void __cdecl CcmFilter::add(class CcmFrame *)" (__imp_?add@?$CcmFilter@VFooBlock@@@@QEAAXPEAVCcmFrame@@@Z) referenced in function "public: __cdecl MainWindow::MainWindow(class QWidget *)" (??0MainWindow@@QEAA@PEAVQWidget@@@Z)

推荐答案

DLL-s在运行时加载，而模板在编译时实例化。

DLL-s are loaded at runtime, while templates are instantiated at compile time.

一种不理想的解决方法，但可能在非常有限的用例中起作用，是在声明后的某个地方显式实例化模板。也就是说，添加如下内容：

A work-around which is not ideal but might work for a very limited use case is to explicitly instantiate a template somewhere after their declaration. That is, add something like:

template class CcmFilter<FooBlock>;

有关详细信息，请参见

For details, see https://msdn.microsoft.com/en-us/library/by56e477(VS.80).aspx

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