c++ - error LNK2019: unresolved external symbol _CreateFastString referenced in function _wmain -

i creating dll , providing entry point 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 "<<str->length()<<std::endl;     return 0; } 

during compilation, getting 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 linker -> input -> additional dependencies have provided path .lib file.

can suggest what's going wrong. in advance.

config.h:

#define my_dll_export __declspec(dllexport) #define my_dll_import __declspec(dllimport)  // should defined when mydll 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 my_dll_exports defined, symbols marked my_dll_public exported resulting dll. "exported", means visible dll consumers, such application. best practice supply definition during compilation. example, on msvc can done adding /dmy_dll_exports 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 "<<str->length()<<std::endl;     return 0; } 

note: main.cpp application, dll consumer, therefore, should not define my_dll_exports during compilation, symbols marked my_dll_public imported consumed dll.

if dll has private functions or classes should not visible dll consumers, it's practice mark them my_dll_private. finally, didn't see include guards in posted code, , omitted them in examples, beware should have them sure, don't forget add them of headers, if missing in real code.