Dynamically creating an instance of a class from a string containing the class name in C++

Question

Lets say I have a base class with 100 children:

class Base { 
  virtual void feed();
  ...   
};
class Child1 : public Base {
  void feed();  //specific proc         


        

Answer 1

It sounds like you might be using subclasses for things that should be encoded as fields.

Instead of coding the different behaviour in 100 classes, consider building a look-up table with rules/constants/function-pointers that allow you to implement the proper behaviour from one class.

For example, instead of:

class SmallRedSquare  : public Shape {...};
class SmallBlueSquare : public Shape {...};
class SmallBlueCircle : public Shape {...};
class SmallRedCircle  : public Shape {...};
class BigRedSquare    : public Shape {...};
class BigBlueSquare   : public Shape {...};
class BigBlueCircle   : public Shape {...};
class BigRedCircle    : public Shape {...};

try:

struct ShapeInfo
{
   std::string type;
   Size size;
   Color color;
   Form form;
};

class Shape
{
public:
    Shape(std::string type) : info_(lookupInfoTable(type)) {}

    void draw()
    {
        // Use info_ to draw shape properly.
    }

private:
    ShapeInfo* lookupInfoTable(std::string type) {info_ = ...;}

    ShapeInfo* info_;
    static ShapeInfo infoTable_[];
};

const ShapeInfo Shape::infoTable_[] =
{
    {"SmallRedSquare",  small,  red, &drawSquare},
    {"SmallBlueSquare", small, blue, &drawSquare},
    {"SmallRedCircle",  small,  red, &drawCircle},
    {"SmallBlueCircle", small, blue, &drawCircle},
    {"BigRedSquare",      big,  red, &drawSquare},
    {"BigBlueSquare",     big, blue, &drawSquare},
    {"BigBlueCircle",     big,  red, &drawCircle},
    {"BigRedCircle",      big, blue, &drawCircle}
}

int main()
{
    Shape s1("SmallRedCircle");
    Shape s2("BigBlueSquare");
    s1.draw();
    s2.draw();
}

This idea might not be applicable to your problem, but I figure it couldn't hurt to present it anyway. :-)

My idea is like the Replace Subclass with Fields refactoring, but I go a bit further.