Introduction to Classes

reading

C++ classes from Wikipedia
Cyganek section 3.15

Classes are a fundamental part of the object-oriented programming paradigm. A class is an objects that holds both data and functions that know how to operate on the data.

Tip

In C++, both struct and class create a class, with the difference being whether the data is publicly accessible by default.

We’ll start by doing a struct with a constructor—this will behave like a class, except by default all of the data will be publicly available.

Here’s the basic format of a struct / class:

struct ClassName {

    // data members are declared here

    ClassName(int param1, int param2) {
        // do any initialization for when we create a ClassName object
    }

    // other member functions here

};

The most important function is the constructor. When we create an instance of our class, the constructor function is called and does any initialization / setup that the class requires.

Note

The constructor always has the same name as the class/struct.

Tip

There can be more than 1 constructor—each can take different arguments.

Here’s a concrete example—we’ll build on our vector of planets, but making a class/struct that holds the data and member functions that know how to operate on that data.

First well define the struct called SolarSystem and write the constructor:

Listing 46 solar_system.H

#ifndef SOLAR_SYSTEM_H
#define SOLAR_SYSTEM_H

#include <vector>
#include <cassert>

#include "planet.H"

struct SolarSystem {

    double star_mass;
    std::vector<Planet> planets;

    SolarSystem(const double mass)
       : star_mass{mass}
    {
        assert(mass > 0.0);
    }

    void add_planet(const std::string& name, const double a, const double e=0.0);

    void print_planets();

};
#endif

Keeping with our strategy to reuse code, this uses our planet.H header from before (and will rely on the planet.cpp source file that implemented some functions).

Some notes:

After the constructor name and arguments there is a : and an initialization:
```
SolarSystem(const double mass)
  : star_mass{mass}
{
   ...
```
this is called the initialization list. Any member data specified in that (comma-separated) list is initialized when an object is created as an instance of the class.
In the constructor body we have an assert() statement—this will abort the code if it is false. This is a way to add runtime checking to the code to ensure that it is being used properly / matching your assumptions.
We end with some forward declarations of member functions of the class. These functions will have access to the class data directly.

To instantiate the class, i.e., create a SolarSystem object, we would do:

SolarSystem ss(1.0);

where we pass the constructor arguments at the time we create our object.

Note

A class can have multiple constructors. A constructor that takes no arguments is called the default constructor.

We provide implementations of the member functions in a separate file:

Listing 47 solar_system.cpp

#include <iostream>
#include <vector>
#include <cmath>

#include "planet.H"
#include "solar_system.H"

void SolarSystem::add_planet(const std::string& name, const double a, const double e) {

    // make sure a planet with that name doesn't already exist
    // if the pointer returned here is not null, then the planet exists

    for (auto p : planets) {
        if (name == p.name) {
            std::cout << "planet already exists" << std::endl;
            return;
        }
    }

    Planet p;
    p.name = name;
    p.a = a;
    p.e = e;

    planets.push_back(p);

}

void SolarSystem::print_planets() {

    for (auto p : planets) {
        std::cout << p << std::endl;
    }
}

Notes:

For each of the member functions, since we are defining them outside of the struct SolarSystem {};, we need to include the namespace and scope operator, ::, to reference the class that they belong to.
Our add_planet() member function checks to see if a planet already exists.

Finally, we can use our SolarSystem class. Here’s a main() function:

Listing 48 test_solar_system.cpp

#include <iostream>
#include "solar_system.H"

int main() {

    SolarSystem ss(2.0);

    ss.add_planet("alpha", 1.0);
    ss.add_planet("beta", 1.5, 0.1);
    ss.add_planet("gamma", 3.0, 0.24);

    // this generates an error
    ss.add_planet("alpha", 2.0);

    ss.print_planets();

}

We can compile this using the same general GNUmakefile we developed previously. We’ll add two features to it:

Listing 49 GNUmakefile

ALL: test_solar_system

# find all of the source files and header files

SOURCES := $(wildcard *.cpp)
HEADERS := $(wildcard *.H)

# create a list of object files by replacing .cpp with .o

OBJECTS := $(SOURCES:.cpp=.o)

CFLAGS := -Wall -Wextra -Wpedantic -Wshadow -g

# a recipe for making an object file from a .cpp file
# Note: this makes every header file a dependency of every object file,
# which is not ideal, but it is safe.

%.o : %.cpp ${HEADERS}
	g++ ${CFLAGS} -c $<

# explicitly write the rule for linking together the executable

test_solar_system: ${OBJECTS}
	g++ -o $@ ${OBJECTS}

clean:
	rm -f *.o test_solar_system

The first is a new target: clean. By doing

make clean

We will remove all of the object files (*.o) and the executable.

The second new feature is the addition of some compilation flags:

CFLAGS := -Wall -Wextra -Wpedantic -Wshadow -g

These are specific to the GNU compilers, and turn on some warnings that help spot code mistakes. See GCC warning options for more details.

The main advantage to using a class here is that we don’t need to know how the planet data is actually stored (in this case in a std::vector<Planet>). By creating member functions that are part of the class, we hide the implementation details from the user. Instead they are given a simple set of functions to interact with the data.

try it…

Let’s make our class more useful. Lets implement the following functions:

int get_planet(const std::string& name, Planet& p_return);

double get_period(const std::string& name);

The first will take a Planet in the argument list (by reference) and if name exists, it will return its properties through the reference. The return value here is int and is meant to return a status that we can check to see if the name was a valid planet.

The second will take a planet name and compute and return its period.

try it…

To understand the difference between a struct, where everything is public, and a class where everything is private by default, let’s edit solar_system.H and change struct to class.

What happens when we compile?

Now try adding a public: statement to the code.

Tip

Instead of doing a get_planet() to return a copy of a planet from our solar system object, we can return an iterator using std::find_if() as:

std::vector<Planet>::iterator SolarSystem::find_planet(const std::string& name) {

    auto it = std::find_if(planets.begin(), planets.end(),
                           [&] (const Planet& p) {return p.name == name;});

    return it;
}

Then we can do a check like:

auto it = find_planet("mars");
if (it != planets.end()) {
    // planet already exists
}

In this simple example, we really did not need to use an initialization list, but there are situations when it is desired or even required:

if any of our member data is const or a reference.
if we have other classes as our member data (including things like std::vector) and initializing them involves some overhead. It can be faster to do the initialization via the initialization list.

See the stack overflow Why should I prefer to use member initialization lists? discussion.