Dependency Inversion Principle

About 251 wordsLess than 1 minute

2025-03-27

Definition

High-level modules should not depend on low-level modules, both should depend on their abstractions.
Abstractions should not depend on details, details should depend on abstractions.

Introduce an abstract interface layer between high-level modules and low-level modules. The abstract interface is an abstraction of low-level modules, and low-level modules inherit or implement the abstract interface. In this way, high-level modules do not directly depend on low-level modules but on the interface layer.

Depend on abstraction.
Design interfaces rather than implementations. Use inheritance to avoid direct binding to classes.
Avoid transitive dependencies. Use inheritance and abstract classes to eliminate dependencies.

Example

Counter Example

Consider a furnace regulator that reads the current temperature from one I/O channel and controls the furnace to continue heating or stop by sending instructions to another I/O channel.

Regulate.h

const byte THERMONETER=0x86;
const byte FURNACE=0x87;
const byte ENGAGE=1;
const byte DISENGAGE=0;

void Regulate(double minTemp,double maxTemp)
{
	for(;;)
	{
		while (in(THERMONETER) > minTemp)
			wait(1);
		out(FURNACE,ENGAGE);

		while (in(THERMONETER) < maxTemp)
			wait(1);
		out(FURNACE,DISENGAGE);
		}
}

Obviously, this code contains a lot of low-level implementation details, and this code is not very reusable.

Positive Example

Regulate.h

void Regulate(Thermometer t, Heater h, double minTemp,
	double maxTemp)
{
	for(;;)
	{
		while (t.Read() > minTemp)
			wait(1);
		h.Engate();

		while (t.Read() < maxTemp)
			wait(1);
		h.Disengage();
	}
}