尽管具有正确的dllExport，也没有函数导出到DLL中-Visual Studio [英] No functions get exported into DLL despite proper dllExport - Visual Studio

问题描述

我有一个基类（ QIndicator ），我想在DLL中实现派生类。一个示例派生类的Visual Studio 2012中的DLL项目具有以下代码：

I have a base class (QIndicator) and I want to implement derived classes in DLLs. The DLL project in Visual Studio 2012 for a sample derived class has the following code:

基类的头文件

#ifndef _DLL_COMMON_INDICATOR_
#define _DLL_COMMON_INDICATOR_

// define the DLL storage specifier macro
#if defined DLL_EXPORT
    #define DECLDIR __declspec(dllexport)
#else
    #define DECLDIR __declspec(dllimport)
#endif

class QIndicator 
{
    private:
        int x;
        int y;
};

extern "C"      
{
    // declare the factory function for exporting a pointer to QIndicator
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}

#endif 

源文件派生类

#define DLL_EXPORT

#include "indicator.h"

class QIndicatorDer : public QIndicator
{
    public:
        QIndicatorDer       (void) : QIndicator(){};
        ~QIndicatorDer      (void){};

    private:
        // list of QIndicatorDer parameters
        int x2;
        int y2;
};

extern "C"     
{
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void)
    {
        return new QIndicatorDer();
    };
}

我遇到的问题是，在成功构建后，生成的DLL文件不会包含导出的 getIndicatorPtr 函数（如DependencyWalker所示）。我检查了 dllexport 关键字是否正确传播到 getIndicatorPtr 的声明中，并且确实如此。

The problem I have is that upon successful build, the produced DLL file does not contain the exported getIndicatorPtr function (as shown by DependencyWalker). I checked whether the dllexport keyword gets propagated properly into the declaration of getIndicatorPtr and it does.

另一个有趣的问题是，我几个月前创建的另一个DLL项目中已经有了另一个这样的派生类。这个较旧的项目基本上是相同的，并且一切正常。我检查了旧项目和当前项目的所有属性，它们似乎相同。因此，我没了主意，为什么我无法导出 getIndicatorPtr 。

Another interesting problem is that I already have another derived class like this, in another DLL project, that I created some months ago. This older project is basically the same and everything works well there. I checked all properties of both the old and the current projects, and they seem identical. So I ran out of ideas, why I can't get getIndicatorPtr to export.

任何帮助深表感谢，
Daniel

Any help is much appreciated, Daniel

推荐答案

这是因为未导出。为什么？

That's because it's not being exported. Why?

__ declspec 说明符只能放在函数的声明中，不是它的定义。另外，请避免使用 #define DLL_EXPORT 之类的方法。预处理程序定义应该在项目属性（MSVC）或命令行选项（例如，GCC中的 -D ）中定义。

__declspec specifier should only be placed in the declaration of a function, not it's definition. Also, avoid something like #define DLL_EXPORT. Preprocessor definitions should either defined in project properties (MSVC) or command line option (-D in GCC, for example).

查看您的代码：

标题

extern "C"      
{
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}

当编译器解析此标头时，会看到 DECLDIR 作为 dllimport （因为您在 .cpp中定义了 DLL_EXPORT ）。然后在 .cpp 中，它突然显示为 dllexport 。使用哪一个？第一个。

When compiler parses this header, is sees DECLDIR as dllimport (because you define DLL_EXPORT in .cpp). Then in .cpp, it suddenly appears as dllexport. Which one is used? The first one.

因此，保留标题（很好），但是更改源：

So, leave your header (it's fine), but change your source:

//#define DLL_EXPORT -> remove this!

#include "indicator.h"

class QIndicatorDer : public QIndicator
{
    //...
};

extern "C"     
{
    /* DECLDIR -> and this! */ QIndicator * __stdcall getIndicatorPtr(void)
    {
        return new QIndicatorDer();
    };
}

然后，转到项目属性（假设您使用Visual Studio），然后 C / C ++ -> 预处理器-> 预处理器定义和在此处添加 DLL_EXPORT = 1 。

Then, go to project properties (I assume you use Visual Studio) and then C/C++ -> Preprocessor -> Preprocessor Definitions and add there DLL_EXPORT=1.

应该可以。

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