Не могу получить доступ к члену класса

//A.h 
#include "SDL.h" 
#include <string> 
class A{ 
public: 
   SDL_Surface *mSurface; //Поверхность
   string mPath; 
   A(string path); 
}; 
 
//A.cpp 
#include "A.h" 
A::A(string path){ 
   mPath=path; 
   mSurface = nullptr; 
} 
 
//B.h 
#include "SDL.h" 
#include <string> 
class B{ 
public: 
   B(); 
   SDL_Surface *thisFunctionWorks (string file); 
   void thisFunctionDoesntWork (string file, SDL_Surface *surface); 
}; 
 
//B.cpp 
#include "B.h" 
B::B(){ 
} 
 
//Определение SDL_loadBMP из SDL.h такое SDL_Surface* SDL_LoadBMP(const char* file). 
 
SDL_Surface *B::thisFunctionWorks(string file){ 
   SDL_Surface *surface = nullptr; 
   surface = SDL_LoadBMP(file.c_str()); 
   return surface; 
} 
 
B::thisFunctionDoesntWork (string file, SDL_Surface *surface){ 
   surface = SDL_LoadBMP(file.c_str()); 
} 
 
//main.cpp 
#include "A.h" 
#include "B.h" 
int main(int argc, char* argv[]) { 
   A *a; 
   a = new A("file.bmp"); 
   B *b; 
   b = new B(); 
   a->mSurface=b->thisFunctionWorks(a->mPath); 
   b->thisFunctionDoesntWork (a->mPath, a->mSurface); 
   return 0; 
}

B::thisFunctionDoesntWork (string file, int s)
{
    s = 20;
}

B::thisFunctionDoesntWork (string file, SDL_Surface **surface)
{
   *surface = SDL_LoadBMP(file.c_str()); 
}

B::thisFunctionDoesntWork (string file, SDL_Surface * & surface)
{
   surface = SDL_LoadBMP(file.c_str()); 
}