It is time to get to one of the core paradigms of C++: object-oriented programming (OOP).
OOP is centered around classes
- user-defined types of data with their own set of related functions.
We will start with the basics and will cover more advanced topics further down the syllabus tree.
Classes can have member variables and member functions.
They are accessed by the member selection operator .
.
Just as variables outside of classes
, it is advisable to initialize member variables with a value upon declaration.
This value will then become the default for newly created objects from this class.
Classes offer the option to restrict access to their members.
The two basic cases are private
and public
.
private
members are not accessible from outside the class.
public
members can be called freely.
All members are private
by default and need to be explicitly marked to be usable outside of the class.
The definition of a new class
can be seen in the following example.
Notice the ;
after the definition.
class Wizzard {
public: // from here on all members are accessible publicly
int cast_spell() { // defines the public member function cast_spell
return damage;
}
std::string name{}; // defines the public member variable `name`
private: // from here on all members are private
int damage{5}; // defines the private member variable `damage`
};
You can access all variables from within the class.
Take a look at damage
inside the cast_spell
function.
You cannot read or change private
members outside of the class:
Wizzard silverhand{};
// calling the `cast_spell` function is okay, it is public:
silverhand.cast_spell();
// => 5
// name is public and can be changed:
silverhand.name = "Laeral";
// damage is private:
silverhand.damage = 500;
// => Compilation error
Constructors offer the possibility to assign values to member variables at object creation.
They have the same name as the class
and do not have a return type.
A class can have several constructors.
This is useful if you do not always have a need to set all variables.
class Wizzard {
public:
Wizzard(std::string new_name) {
name = new_name;
}
Wizzard(std::string new_name, int new_damage) {
name = new_name;
damage = new_damage;
}
int cast_spell() {
return damage;
}
std::string name{};
private:
int damage{5};
};
Wizzard el{"Eleven"}; // deals 5 damage
Wizzard vecna{"Vecna", 50}; // deals 50 damage
Constructors are a big topic and have many nuances.
If you are not explicitly defining a constructor
for your class
, then - and only then - the compiler will do the job for you.
This has happened in the first example above.
The silverhand object is created by calling the default constructor, no arguments were passed.
All variables are set to the value that was stated in the definition of the class.
If you had not given any values in that definition, the variables might be uninitialized, which might have unintended consequences.
Structs came from the language's original C roots and are as old as C++ itself.
They are effectively the same thing as classes
with one important exception.
By default, everything in a class
is private
.
Structs, on the other hand, are public
until defined otherwise.
Conventionally, the struct
keyword is often used for data-only structures.
The class
keyword is preferred for objects that need to ensure certain properties.
Such an invariant could be that the damage
of your Wizzard
class
cannot turn negative.
The damage
variable is private and any function that changes the damage would ensure the invariant is preserved.