Definition

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

Implementation Method

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 ( ) ;
    }
}

Contributors

dingyuqi

Changelog

10/11/25, 2:26 AM

View All Changelog

• 941fa-feat(theme): upgrade and use collectionson 10/11/25
• eb6eb-improve(docs): use pangu formatteron 8/5/25
• dd170-feat(docs): add solid principles serieson 3/27/25

Copyright

Copyright Ownership:dingyuqi

License under:Attribution-NonCommercial-NoDerivatives 4.0 International (CC-BY-NC-ND-4.0)

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