C++ Basics

You write C++ using a simple text editor, like nano. Open a command prompt and use a text editor to open a file called hello.cpp, e.g.

nano hello.cpp

C++ source files traditionally end in .cpp. This isn’t a requirement, but it does make it easier to recognise the file.

Now type the following into the file;

#include <iostream>

int main()
{
    std::cout << "Hello from C++" << std::endl;

    return 0;
}

Save the file. You have just written a simple C++ program! C++ is a compiled programming language, meaning that we now need to compile the C++ source file you have just written to create an executable program that you can run.

To compile your C++ source file, type;

g++ hello.cpp -o hello

This will use your C++ compiler (in this case g++) to compile your C++ source file (hello.cpp) to create a new executable program called hello. You can run the program by typing

./hello

You should see that this prints out Hello from C++. If so, then congratulations as you have now written, compiled and run your first C++ program.

This was a simple program. C++ is a language designed to help you write powerful and full-featured programs, and as such, it is a large language with a lot of concepts and features.

This program introduced some of the basic building blocks of C++;

• The main function. All C++ programs must have a function called main. This is the function that is executed when you start the program. The return type of this function is an integer, with 0 returned by the function if the program ran correctly (hence the return 0; at the end of main).
• A command std::cout. Commands are instructions to tell C++ to do something, in this case to print the passed strings to the screen.
• A string Hello from C++. A string is just a piece of text. Strings are always enclosed in either double quotes or single quotes.
• A line of code std::cout << "Hello from C++!" << std::endl;. A line of code forms a complete instruction which will be executed by the C++ program. The C++ program executes each line of code, one at a time in order, from the first line of the main function to the last. Note that every line of code must end with a semicolon.
• A header file iostream. This contains a set of functions, classes and objects that extend C++. In this case, the header file iostream is part of the standard C++ library, and provides here std::cout and std::endl. These objects are used for printing text to the screen.

In C++ you use std::cout to print text to the screen. It is an object provided by the C++ standard library (hence the std:: before the name). You use it to print things to the screen using the << operator. For example, use a text editor to write a new C++ source file called variables.cpp

nano variables.cpp

Type into the source file the following lines;

#include <iostream>
#include <string>

int main()
{
    std::string a = "Hello";
    std::string b = "from";
    std::string c = "C++!";

    std::cout << a << " " << b << " " << c << std::endl;

    return 0;
}

What do you think will be printed when you compile and run this program? Have a go by typing;

g++ variables.cpp -o variables
./variables

Did you see what you expected? In this program we created three variables of type std::string, called a, b and c. The line std::string a = "Hello"; sets the variable a to type std::string and sets it equal to the string Hello. The next lines set b equal to from and c equal to C++!.

The three variables are printed to the screen using std::cout. They are printed by pushing them into std::cout using its << operator. We push the value of a first (printing ‘Hello’ to the screen). We follow this with a space, before pushing the value of b, then a space, then the value of c. We finish by pushing std::endl, which represents the end of line character (\n on Linux/OS X or \r\n on Windows).

Note that a, b and c are all of type std::string. This type is provided in the string header file that is included at the top of the script. Like std::cout, std::string is part of the standard library of C++. Also note that every complete line of code ends with a semicolon. C++ uses semicolons to separate different lines, and (mostly) ignores actual line breaks. This means that you could write the above program as a small number of real lines of text, e.g.

#include <iostream>
#include <string>
int main(){ std::string a = "Hello"; std::string b = "from"; std::string c = "C++!"; std::cout << a << " " << b << " " << c << std::endl; return 0; }

While you could write the program as a small number of lines, it makes it difficult to read and is not recommended. It is therefore good practice to write only one line of code per line of text, i.e. finish every line of code with a semicolon and a newline (return).

As you can see, in C++ you have to specify the type of every variable. C++ has many different variable types, e.g.

• int : Integer (whole number)
• float : Floating point number
• double : Floating point number that has double the precision of a float
• bool : Boolean (logical true/false) value
• std::string : String (text)

You can use std::cout to print out values of any of these variable types, e.g. create a new C++ source file called variables2.cpp and type into it;

#include <iostream>
#include <string>

int main()
{
    int a = 42;
    float b = 3.14159265;
    std::string c = "Spot the dog";
    bool d = true;

    std::cout << "Use the '<<' operator to push things to print to std::cout, e.g. " 
              << a << std::endl;

    std::cout << "You can print anything you like, e.g. floats such as " << b << std::endl;
    std::cout << " or other strings, like " << c << " or booleans such as " << d << std::endl
              << "and when you type, you can go over "
              << "multiple lines, as 'std::endl' is used to print an 'end of line'."
              << std::endl;
              
    return 0;
}

What do you think will be printed to the screen when you compile and run this program? Do this by typing;

g++ variables2.cpp -o variables2
./variables2

Did you see what you expected? Play with this program by breaking the text over different source file lines, and moving the std::endl so that the text is printed cleanly to the screen (e.g. the newlines are placed so that each line printed has about the same length).

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