Liskov Substitution Principle

Subclasses should be substitutable for their base classes

Precise Definition

Let $q(x)$ be a property provable about objects $x$ of type $T$. Then $q(y)$ should be provable for objects $y$ of type $S$, where $S$ is a subtype of $T$

class Base { ... };
class Derived : public Base { ... };
void q(Base& b); // should work, even if b is an instance of Derived

Design by Contract

Method in derived class must

• have preconditions that are no stronger than those of the base's method it is overriding
• have postconditions no weaker than those of the base's method it is overriding

In other words

Derived methods should expect no more and provide no less than parent method.</blockquote