Inheritance: Modeling IS-A

Inheritance expresses "is-a": a derived class is a kind of its base, and a derived object can be used where a base is expected. Begin with the shape:

#include <string>

class Employee {
public:
    explicit Employee(std::string name) : name_{std::move(name)} {}
    const std::string& name() const { return name_; }

private:
    std::string name_;
};

class Engineer : public Employee {     // Engineer IS-A Employee
public:
    Engineer(std::string name, std::string specialty)
        : Employee{std::move(name)}, specialty_{std::move(specialty)} {}
    const std::string& specialty() const { return specialty_; }

private:
    std::string specialty_;
};

What public inheritance means

• The derived object contains a full base subobject.
• Base private members exist in the derived object but are not accessible to the derived class — they belong to the base's implementation.
• Construction runs base first, then derived; destruction runs derived first, then base.

The default is private inheritance — use public explicitly

class D : B inherits privately (implementation detail, almost never what you want). Always write : public B for the substitutability relationship, and if you find yourself reaching for private/protected inheritance, ask whether composition says it better.

What inheritance is NOT for

Reusing a helper. That is composition or a free function. Reach for inheritance only when you need substitutability — different objects used through one base interface (next lessons).