C++ Object Oriented langage

Now that we know well the C language, learning C + + is much easier. This language makes some improvements over the language such as C strings easier, greater use of programming and classes.

C or C++

All you can do in C, you also can do in C++.

C++ is based on C, so you can write a code in C and compile it with a C++ compiler.

Hello world!! Reloaded

We now edit the file main.cpp (cpp means C++).

#include <iostream>
using namespace std;

int main(int argc, char *argv[]){
    /*I write Hello World !!*/
    cout << "Hello world !!\n" << endl;
    return 0;
}

Some difference with C code:

We don’t include stdio.h but iostream, a C++ library, which is composed of several namespaces. We choose to use the namespace std (to use cout).
The main function is the same
We use flux for display output, here cout.
The end of the main function is the same.

We will now compile and run the code:

make main
./main

The make command know you want to compile a C++ file.

Hello world !!

Pointers

Pointers are naturally enable in C too. You can do it in the same way than in C, or in the C way

#include <iostream>

int main(void){
    int N = 10;
    double *d = NULL;
    d = new double[N];
    for (int i = 0; i < N; i++){
        d[i] = i+1;
    }
    for (int i = 0; i < N; i++){
        std::cout << d[i] << std::endl;
    }
    delete[] d;
    d = NULL;
    return 0;
}

The result of this code is:

Here, we don’t define the std namespace, so we have to define it when we use a function of the std namespace: std::cout for example.
In C++, we can declare a variable directly in a loop.
The keyworks new and delete manage the pointers.

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Classes

Classes are very useful in C++, but in FreeFem they are not useful (excepted for programmers). We just have a look on that.

#include <iostream>
using namespace std;

class Object{
    private:
    int id;
    std::string txt;

    public:
    Object();
    int get_id();
    std::string get_txt();
};

Object::Object(){
    id = 1;
    txt = "Hello";
}

int Object::get_id(){
    return id;
}

std::string Object::get_txt(){
    return txt;
}

int main(void){
    Object test_object = Object();
    cout << test_object.get_id() << endl;
    cout << test_object.get_txt() << endl;
    return 0;
}

We define a class using the keyword class. In this class, we have defined attributes : id and txt, and equally methods : get_id() and get_txt(). Object() function is the initializer.

Methods are describes with the name of the class Object::.

The class can be used in the main function.

We speak about OOP: Object-Oriented Programming

Result:

1
Hello

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Read and write

To read or write a file in C++, you can use the flux.

#include <iostream>
#include <fstream>
using namespace std;

int main(void){
    ofstream file2write("test.txt", ios::out | ios::trunc);
    if (file2write){
        file2write << "Test";
        file2write.close();
    }

    ifstream file2read("test.txt", ios::in);
    if (file2read){
        string line;
        getline(file2read, line);
        cout << line << endl;
        file2read.close();
    }

    return 0;
}

You must include the fstream library for using files.
For open a file to write, use ofstream.
You can write in a file using flux, or with others functions.
For open a file to read, use ifstream.
You can use getline to read a file, or flux too.
Don’t forget to close the file when you have finished.

Results:

Test

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