错误LNK2019：函数_wmain中引用的未解析的外部符号_CreateFastString

Question

I am creating a DLL and providing the entry point to the FastString class using CreateFastString function:

FastString.h :

#define EXPORT __declspec(dllexport)
#define IMPORT __declspec(dllimport)

class FastString
{
    const int m_length;
    char* m_str;

public:
    FastString(const char* str);
    ~FastString();
    int Length()const;
    int Find(const char* str)const;
};

extern "C" FastString* CreateFastString(const char* str);

FastString.cpp :

#include "stdafx.h"
#include <string>
#include "FastString.h"

FastString* CreateFastString(const char* str)
{
    return new FastString(str);
}

FastString::FastString(const char* str): m_length(strlen(str)),
                                         m_str(new char[m_length+1])
{}

FastString::~FastString()
{
    delete[] m_str;
}

int FastString::Length()const
{
    return m_length;
}

int FastString::Find(const char* str)const
{
    return 1;
}

main.cpp :

#include "stdafx.h"
#include <iostream>
#include "FastString.h"

int _tmain(int argc, _TCHAR* argv[])
{
    FastString* str = CreateFastString("Hello Dll");
    std::cout<<"The length is "<<str->Length()<<std::endl;
    return 0;
}

During compilation, I am getting the following errors:

TeatApp.obj : error LNK2019: unresolved external symbol _CreateFastString referenced in function _wmain
D:\MFC\FastString\Debug\TeatApp.exe : fatal error LNK1120: 1 unresolved externals

In the Linker -> Input -> Additional Dependencies I have provided the path for the .lib file.

Can anyone suggest what's going wrong. Thanks in advance.

Answer 1

config.h :

#define MY_DLL_EXPORT __declspec(dllexport)
#define MY_DLL_IMPORT __declspec(dllimport)

// Should be defined when MyDLL is built (more details below).
#ifdef MY_DLL_EXPORTS
  #define MY_DLL_PUBLIC MY_DLL_EXPORT
#else
  #define MY_DLL_PUBLIC MY_DLL_IMPORT
#endif

#define MY_DLL_PRIVATE

#ifdef __cplusplus
  #define MY_DLL_FUNCTION extern "C"
#else
  #define MY_DLL_FUNCTION extern
#endif

FastString.h :

#include "config.h"

class MY_DLL_PUBLIC FastString
{
    const int m_length;
    char* m_str;

public:
    FastString(const char* str);
    ~FastString();
    int Length() const;
    int Find(const char* str) const;
};

MY_DLL_FUNCTION FastString* MY_DLL_PUBLIC CreateFastString(const char* str);

FastString.cpp :

#include "FastString.h"

#include "stdafx.h"

#include <string>

FastString* CreateFastString(const char* str)
{
    return new FastString(str);
}

FastString::FastString(const char* str): m_length(strlen(str)),
                                         m_str(new char[m_length+1])
{}

FastString::~FastString()
{
    delete[] m_str;
}

int FastString::Length()const
{
    return m_length;
}

int FastString::Find(const char* str)const
{
    return 1;
}

NOTE: Build FastString.cpp with MY_DLL_EXPORTS defined , so that the all the symbols marked with MY_DLL_PUBLIC are exported from the resulting DLL. "Exported" , means they will be visible for DLL consumers, such as your application. The best practice is to supply this definition during compilation. For example, on MSVC it can be done by adding /DMY_DLL_EXPORTS to the compiler invocation.

main.cpp :

#include "stdafx.h"

#include "FastString.h"

#include <iostream>

int _tmain(int argc, _TCHAR* argv[])
{
    FastString* str = CreateFastString("Hello Dll");
    std::cout<<"The length is "<<str->Length()<<std::endl;
    return 0;
}

NOTE: main.cpp is your application, which is basically the DLL consumer, therefore, you should not define MY_DLL_EXPORTS during its compilation, so that all the symbols marked with MY_DLL_PUBLIC are imported from the consumed DLL.

If your DLL has some private functions or classes which should not be visible to the DLL consumers, then it's good practice to mark them with MY_DLL_PRIVATE . Finally, I didn't see any include guards in your posted code, and I omitted them too in my examples, but beware that you should have them for sure, so don't forget to add them too all of your headers, if they are missing in your real code.