Question

I am trying to implement factory pattern by registering the function pointers of the derived class to the factory in a static map(member of the factory)and creating objects by looking up the map. But I am getting a segmentation fault in doing this.

Code Snippet:

factory.cpp

typedef Shape* (*Funcptr)();

std::map<int,Funcptr> Factory::funcmap;

int Factory::registerCreator(int ShapeID, Shape *(*CFuncptr)()) {
    Factory::funcmap[ShapeID] = CFuncptr;
return 1;
} 

Shape* Factory::CreateObject(int ShapeID) {
    std::map<int,Funcptr>::iterator iter;
    iter = funcmap.find(ShapeID);
    if(iter != funcmap.end()){
        return iter->second();
    }
    return NULL;
}

factory.h

class Factory {
public:
    Factory();
    virtual ~Factory();
    static int registerCreator(int, Shape *(*CFuncptr)());
    Shape* CreateObject(int);
private:
    static  std::map<int,Funcptr> funcmap;
};

Square.cpp

static Shape *SquareCreator() { 
    return new Square; 
}
static int SquareAutoRegHook = Factory::registerCreator(1,SquareCreator);

On creating the object for Factory in the main file a segmentation fault occurs.
Can you please suggest if I am doing something wrong. I am using CppUTest for TDD and not sure how to debug this.

Answer 1

There is no guarantee about the order of creation of static objects that are defined in different translations uints* so you have a 50/50 shot as to which would happen first, the initializtion of Factory::funcmap or Factory::registerCreator(1,SquareCreator) and Undefined Behavior Russian Roulette is not a good game to play.

A common approach to deal with this, and one that described in Item 4 of the third edition of Scott Meyer’s Effective C++ is to use local static objects instead of global static objects. In this case it means changing Factory to look like this:

class Factory { 
public: 
    Factory(); 
    virtual ~Factory(); 
    static int registerCreator(int, Shape *(*CFuncptr)()); 
    Shape* CreateObject(int); 
private: 
    static std::map<int,Funcptr> & GetFactoryMap() {
        static std::map<int,Funcptr> funcmap;
        return funcmap;
    } 
}; 

and changing Factory::registerCreator to this:

int Factory::registerCreator(int ShapeID, Shape *(*CFuncptr)()) {   
    GetFactoryMap()[ShapeID] = CFuncptr;   
    return 1;   
}

This way funcmap will be initialized the first time registerCreator is called and will never be used uninitialized.

*Or, roughly speaking, different .cpp files if you are not familar with the term translation unit