How to implement a factory design pattern in C ?
Jul 17, 2025 am 12:33 AM
To implement the factory design pattern in C , first define a common interface, then create concrete product classes, implement a factory class with a static creation method, and finally use the factory to decouple object creation from client code. (1) Define an abstract base class Product with a pure virtual method use(). (2) Implement ConcreteProductA and ConcreteProductB that override use(). (3) Create ProductFactory with a static method createProduct() that returns a product based on input type. (4) In main(), use the factory to create and manage objects through the base interface, preferably using smart pointers for automatic memory management.
Implementing the factory design pattern in C is a clean way to handle object creation without exposing the instantiation logic to the client. The core idea is to use a separate class (the factory) to decide which class to instantiate based on input or context. Here's how to do it effectively.
Define a Common Interface
The first step is to define a common base class or interface that all the products (the objects you want to create) will inherit from. This ensures that each object created by the factory can be used interchangeably.
class Product {
public:
virtual void use() = 0;
virtual ~Product() = default;
};This interface defines a method use() that all derived classes must implement. Using a pure virtual function makes Product an abstract class, which enforces a contract for all subclasses.
Create Concrete Product Classes
Next, define the actual classes that implement the interface. These are the objects that the factory will create.
class ConcreteProductA : public Product {
public:
void use() override {
std::cout << "Using Product A\n";
}
};
class ConcreteProductB : public Product {
public:
void use() override {
std::cout << "Using Product B\n";
}
};These two classes implement the use() method differently, but they can both be treated as Product* pointers, which is essential for the factory to work.
Implement the Factory Class
The factory class contains a method that decides which product to return. This is often based on an input parameter like a string or enum.
class ProductFactory {
public:
static Product* createProduct(const std::string& type) {
if (type == "A") {
return new ConcreteProductA();
} else if (type == "B") {
return new ConcreteProductB();
}
return nullptr;
}
};Some points to note:
- This is a static method, so you don’t need to instantiate the factory to use it.
- You can easily extend the logic to support more product types.
- Don’t forget to handle invalid input (e.g., return
nullptror throw an exception).
Use the Factory
Now you can use the factory without knowing the exact type of object being created:
int main() {
Product* product = ProductFactory::createProduct("A");
if (product) {
product->use();
delete product;
}
return 0;
}Here’s what’s happening:
- The factory decides which object to create.
- The client code uses the returned object through the base interface.
- Memory is manually managed (
newanddelete), so be careful with ownership.
If you're using modern C , consider returning a smart pointer like std::unique_ptr<product></product> from the factory method to avoid manual memory management.
That's the basic structure. It's straightforward, but it's powerful when you need to centralize and decouple object creation logic.
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