Step 1 of 9

Inheritance

Often several classes share most of their data and behavior: every vehicle has a name and a speed, every account has an owner and a balance. Inheritance lets a new class (the derived class) start from an existing one (the base class), getting all its members, and then add more.

The relationship must be is-a: a Car is a Vehicle, so anything you can do with a vehicle you can do with a car.

#include <iostream>
#include <string>

class Vehicle {
public:
    Vehicle(std::string name, int wheels) : name_(name), wheels_(wheels) {}
    std::string summary() const { return name_ + " on " + std::to_string(wheels_) + " wheels"; }

private:
    std::string name_;
    int wheels_;
};

class Truck : public Vehicle {
public:
    Truck(std::string name, int load) : Vehicle(name, 6), load_(load) {}
    int load() const { return load_; }

private:
    int load_;
};

int main() {
    Truck t("hauler", 12);
    std::cout << t.summary() << ", carrying " << t.load() << " tons\n";
    Vehicle v("bike", 2);
    std::cout << v.summary() << "\n";
}

hauler on 6 wheels, carrying 12 tons
bike on 2 wheels

How it works

• class Truck : public Vehicle means "Truck inherits from Vehicle". Always write public here; without it, the inherited members become private to the outside world, which is almost never what you want.
• A Truck object contains a complete Vehicle inside it, plus its own load_.
• t.summary() works because summary was inherited.
• The base part must be constructed first, and the derived class does that in its initializer list: : Vehicle(name, 6). That's the only way to call the base constructor, and it's required when the base has no default constructor.
• name_ and wheels_ are private to Vehicle, so even Truck's own code can't touch them directly. It uses Vehicle's public functions like everyone else.

Common mistakes

• Trying to set base members in the derived constructor's body (name_ = name;). They're private to the base; pass them to the base constructor instead.
• Using inheritance just to reuse code when there's no is-a relationship. That's what members are for (more in step 7).

Your turn: make Sniper inherit from Weapon and set the base part through the base constructor.

Next: virtual and override