c++: definition of dllimport function not allowed, building with visual studio 2010

Question

I'm using visual studio 2010 to build a .dll. I wrote up a trial as:

// trialDLL.h
#ifndef TRIALDLL_H_
#define TRIALDLL_H_

// ... MyMathFuncs class definition omitted

#ifdef __cplusplus
extern "C"{
#endif

#ifdef TRIALDLL_EXPORT
#define TRIALDLL_API __declspec(dllexport)
#else
#define TRIALDLL_API __declspec(dllimport)
#endif

TRIALDLL_API MyMathFuncs* __stdcall new_MyMathFuncs(double offset);
TRIALDLL_API void __stdcall del_MyMathFuncs(MyMathFuncs *myMath);
TRIALDLL_API double __stdcall MyAdd(MyMathFuncs* myMath, double a, double b);
// some other similar stuff

#ifdef __cplusplus
}
#endif

#endif

And the triallDLL.cpp file:

// trialDLL.cpp
#include "trialDLL.h"

TRIALDLL_API MyMathFuncs* __stdcall new_MyMathFuncs(double offset)
{
    return new MyMathFuncs(offset);
}

TRIALDLL_API void __stdcall del_MyMathFuncs(MyMathFuncs *myMath)
{
    delete myMath;
}

TRIALDLL_API double __stdcall MyAdd(MyMathFuncs *myMath, double a, double b)
{
    return myMath->Add(a, b);
}
// ... some other definitions

With these two files in the project, I added a property sheet to the project through visual studio 2010 property manager and added TRIALDLL_EXPORT to user macros. After all these, the nice Intellisense gives me error for each function defined in the .cpp file and complains "error: a function declared 'dllimport' may not be defined". So it appears that Intellisense doesn't find TRIALDLL_EXPORT defined. I thought it might make a difference if I actually build the project, but the result suggests the same error: "error C2491: 'new_MyMathFuncs' : definition of dllimport function not allowed". Then it is clear that the macro TRIALDLL_EXPORT is still not defined in compiling time.

After failing to add macro through visual studio, I also tried putting code line: #define TRIALDLL_EXPORT in trialDLL.cpp but it didn't help either. I wonder what's the proper way to do this? How do I inform the compiler that the micro is defined so that TRIALDLL_API evaluates to dllexport rather than dllimport?

Also, if I can build the .dll successfully, is there any systematic way to test/verify the functionality of the .dll?

Thanks for any help in advance! (Although I know it's an issue here on stackoverflow to put appreciation in the question, I feel myself impolite not to do so. Forgive me for any inefficiency caused by these lines.)

Solution 1

"User macros" in VS property sheets have nothing to do with preprocessor macros. Put TRIALDLL_EXPORT into the property sheet's section C/C++ > Preprocessor > Preprocessor Definitions

"User macros," which can only be defined in property sheets, allow you to create your own "variables" usable in Visual Studio properties, similar to the built-in ones $(TargetName), $(SolutionDir) etc.

Solution 2

Put

#error Where is my macro?

in the #else block of the header. Then experiment with the project settings or the #define until you get it right. Did you perhaps only add the property sheet to one configuration? Did you put the #define at the very top of the file? Do you have any PCH stuff that causes it to ignore your settings? And so on.

Solution 3

As said in this Microsoft article, you cannot apply the __declspec(dllimport) keyword to implement a function. You should use it only in the declaration. For example:

    // function declaration
    void __declspec(dllimport) funcB();

    // function definition
    void funcB() {
        //funcB code
    }

Comments

[springRoll]

Thanks! This works perfectly! By the way, for my curiosity, what does "User macros" do then?

[Angew is no longer proud of SO]

@springRoll They're "variables" for properties you can set in Visual Studio. Like the built-in ones $(TargetName), $(SolutionDir) etc. Property sheets allow you to create your own.

[springRoll]

Thanks! It works if the #define is put on the very top of the .cpp file. What I did was putting it after the #include in the .cpp file. Can I ask why does it make a difference?

[Sebastian Redl]

Macros are defined from the point where they are #defined until the point where they are #undefed (or the end of the translation unit). If you put the #define below the #include, the macro won't be defined in the area of your header.

[springRoll]

Sorry for the stupid question, I didn't ask myself before I asked you. I figured it out myself when I had a rest minutes ago. Thank you very much for the detailed explanation!