DP: Factory Method

The Factory Method defines an interface for creating objects but lets subclasses or functions decide which object to create. It removes direct dependencies on concrete classes.

This is useful when selecting drivers, sensors, or communication modules at runtime.

  • Decouples creation from usage
  • Supports runtime selection

C Example (Factory function)

Using pointer to function

c 
#include <stdio.h>
#include <string.h>

typedef struct {
    void (*draw)();
} Shape;

void drawCircle() { printf("Circle\n"); }
void drawSquare() { printf("Square\n"); }

Shape createShape(const char* type) {
    Shape s;

    if (strcmp(type, "circle") == 0)
        s.draw = drawCircle;
    else
        s.draw = drawSquare;

    return s;
}

int main() {
    Shape s = createShape("circle");
    s.draw();
}

C++ Example

Using function override

cpp 
#include <iostream>
using namespace std;

class Shape {
public:
    virtual void draw() = 0;
};

class Circle : public Shape {
public:
    void draw() override { cout << "Circle\n"; }
};

class Square : public Shape {
public:
    void draw() override { cout << "Square\n"; }
};

class ShapeFactory {
public:
    static Shape* create(const string& type) {
        if (type == "circle") return new Circle();
        if (type == "square") return new Square();
        return nullptr;
    }
};

int main() {
    Shape* s = ShapeFactory::create("square");
    s->draw();
    delete s;
}