Error “unresolved external symbol” when library is dynamically loaded at runtime

Question

I have a big project where an executable A.exe is generated (with a cmake toolchain) to dynamically load B.dll at run-time . But without compile-time linking with B.dll , I got "unresolved external symbol" when building the executable.

The project organized like this:

superproject | |--- build/ |----CMakeLists.txt |--- src/ | |---a : a1.cpp, CMakeLists.txt |---b : bh, b.cpp, export.def, CMakeLists.txt

file src/b/bh :

class Machine
{
    int u;
    static Machine* sing;
public:
    Machine();
    static Machine* GetInstance();
    int answer();
};

file src/b/b.cpp :

#include "b.h"

Machine* Machine::sing;

Machine::Machine() : u(1) {}

Machine* Machine::GetInstance()
{
   if (!sing)
      sing = new Machine();
   return sing;
}

int Machine::answer(){ return u + 33; }

Machine* GetMach()
{
   return Machine::GetInstance();
}

file src/a/a1.cpp :

#include <windows.h>
#include <iostream>

#include "b.h"

using namespace std;
typedef Machine* (*func_t)();
int main(int argc, char* argv[])
{
   cout << "hello\n";
   HMODULE lib = LoadLibrary("b.dll");
   if (lib)
   {
      void* rawfunc = GetProcAddress(lib, "GetMach");
      if (rawfunc)
      {
         func_t func = (func_t)(rawfunc);
         Machine* m = func();
         cout << "The exported number is " << m->answer();
      }
      else
      {
         cout << "Failed to get the symbol";
      }
   }
   else
   {
      cout << "Failed to load library";
   }
   getchar();
   return 0;
}

file src/b/export.def :

EXPORTS
    GetMach

file src/b/CMakeLists.txt :

file(GLOB_RECURSE SRC_FILES *.cpp)
file(GLOB_RECURSE DEF_FILES *.def)
add_library(b MODULE ${SRC_FILES} ${DEF_FILES})

file src/a/CMakeLists.txt :

file(GLOB_RECURSE SRC_FILES *)
include_directories(../b)
add_executable(A ${SRC_FILES})

Visual Studio complains of:

error LNK2019: unresolved external symbol "public: int __cdecl Machine::answer(void)" (?answer@Machine@@QEAAHXZ) referenced in function main    C:\....\superproj\build\a\a1.obj    A

Why have I got this error and how to eliminate it?

Answer 1

From the linker's point of view, non-virtual method ( Machine::answer ) is the same as global function (say, GetMach() ).

That is, for being able to use this method, A should either:

• link ( target_link_libraries ) with a library defining the method or
• request the method at runtime ( GetProcAddress() ).

If both of these approaches seems inconvinent, you may declare the method as virtual :

virtual int answer();

All virtual methods are resolved at runtime . A linker doesn't bother about using of virtual methods.