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Structure of a program

Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:

// my first program in C++ #include using namespace std; int main () { cout << "Hello World!"; return 0; }

Hello World!

The first panel shows the source code for our first program. The second one shows the result of the program once compiled and executed. The way to edit and compile a program depends on the compiler you are using. Depending on whether it has a Development Interface or not and on its version. Consult the compilers section and the manual or help included with your compiler if you have doubts on how to compile a C++ console program.

The previous program is the typical program that programmer apprentices write for the first time, and its result is the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in C++, but it already contains the fundamental components that every C++ program has. We are going to look line by line at the code we have just written:

// my first program in C++

This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is.

#include

Lines beginning with a hash sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include tells the preprocessor to include the iostream standard file. This specific file (iostream) includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is going to be used later in the program.

using namespace std;

All the elements of the standard C++ library are declared within what is called a namespace, the namespace with the name std. So in order to access its functionality we declare with this expression that we will be using these entities. This line is very frequent in C++ programs that use the standard libary, and in fact it will be included in most of the source codes included in these tutorials.

int main ()

This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function.

The word main is followed in the code by a pair of parentheses (()). That is because it is a function declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them.

Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.

cout << "Hello World";

This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program.

cout represents the standard output stream in C++, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen).

cout is declared in the iostream standard file within the std namespace, so that's why we needed to include that specific file and to declare that we were going to use this specific namespace earlier in our code.

Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).

return 0;

The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ console program.

You may have noticed that not all the lines of this program perform actions when the code is executed. There were lines containing only comments (those beginning by //). There were lines with directives for the compiler's preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case, the main function) and, finally lines with statements (like the insertion into cout), which were all included within the block delimited by the braces ({}) of the main function.

The program has been structured in different lines in order to be more readable, but in C++, we do not have strict rules on how to separate instructions in different lines. For example, instead of

int main () { cout << " Hello World "; return 0; }

We could have written:

int main () { cout << "Hello World"; return 0; }

All in just one line and this would have had exactly the same meaning as the previous code.

In C++, the separation between statements is specified with an ending semicolon (;) at the end of each one, so the separation in different code lines does not matter at all for this purpose. We can write many statements per line or write a single statement that takes many code lines. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it.

Let us add an additional instruction to our first program:

Input: Output:

// my second program in C++ #include using namespace std; int main () { cout << "Hello World! "; cout << "I'm a C++ program"; return 0; }

Hello World! I'm a C++ program

In this case, we performed two insertions into cout in two different statements. Once again, the separation in different lines of code has been done just to give greater readability to the program, since main could have been perfectly valid defined this way:

int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; }

We were also free to divide the code into more lines if we considered it more convenient:

int main () { cout << "Hello World!"; cout << "I'm a C++ program"; return 0; }

And the result would again have been exactly the same as in the previous examples.

Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and processed by the preprocessor and do not produce any code by themselves. Preprocessor directives must be specified in their own line and do not have to end with a semicolon (;).

Comments

Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.

C++ supports two ways to insert comments:

// line comment /* block comment */

The first of them, known as line comment, discards everything from where the pair of slash signs (//) is found up to the end of that same line. The second one, known as block comment, discards everything between the /* characters and the first appearance of the */ characters, with the possibility of including more than one line.
We are going to add comments to our second program:

Input:

/* my second program in C++ with more comments */ #include using namespace std; int main () { cout << "Hello World! "; // prints Hello World! cout << "I'm a C++ program"; // prints I'm a C++ program return 0; }

Output:

Hello World! I'm a C++ program

If you include comments within the source code of your programs without using the comment characters combinations //, /* or */, the compiler will take them as if they were C++ expressions, most likely causing one or several error messages when you compile

Tutorial Introduction

Instructions for use:

To whom is this tutorial directed?

This tutorial is for those people who want to learn programming in C++ and do not necessarily have any previous knowledge of other programming languages. Of course any knowledge of other programming languages or any general computer skill can be useful to better understand this tutorial, although it is not essential.

It is also suitable for those who need a little update on the new features the language has acquired from the latests standards.

If you are familiar with the C language, you can take the first 3 parts of this tutorial as a review of concepts, since they mainly explain the C part of C++. There are slight differences in the C++ syntax for some C features, so I recommend you its reading anyway.

The 4th part describes object-oriented programming.

The 5th part mostly describes the new features introduced by ANSI-C++ standard.

Structure of this tutorial

The tutorial is divided in 6 parts and each part is divided in its turn into different sections covering a topic each one. You can access any section directly from the section index available on the left side bar, or begin the tutorial from any point and follow the links at the bottom of each section.

Many sections include examples that describe the use of the newly acquired knowledge in the chapter. It is recommended to read these examples and to be able to understand each of the code lines that constitute it before passing to the next chapter.

A good way to gain experience with a programming language is by modifying and adding new functionalities on your own to the example programs that you fully understand. Don't be scared to modify the examples provided with this tutorial, that's the way to learn!

Compatibility Notes

The ANSI-C++ standard acceptation as an international standard is relatively recent. It was first published in November 1997, and revised in 2003. Nevertheless, the C++ language exists from a long time before (1980s). Therefore there are many compilers which do not support all the new capabilities included in ANSI-C++, specially those released prior to the publication of the standard.

This tutorial is thought to be followed with modern compilers that support -at least on some degree- ANSI-C++ specifications. I encourage you to get one if yours is not adapted. There are many options, both commercial and free.

Compilers

The examples included in this tutorial are all console programs. That means they use text to communicate with the user and to show their results.All C++ compilers support the compilation of console programs. Check the user's manual of your compiler for more info on how to compile them.

C++: A brief Description

Programs

Nowadays computers are able to perform many different tasks, from simple mathematical operations to sophisticated animated simulations. But the computer does not create these tasks by itself, these are performed following a series of predefined instructions that conform what we call a program.

A computer does not have enough creativity to make tasks which it has not been programmed for, so it can only follow the instructions of programs which it has been programmed to run. Those in charge of generating programs so that the computers may perform new tasks are known as programmers or coders, who for that purpose use a programming language.

Programming languages

A programming language is a set of instructions and a series of lexical conventions specifically designed to order computers what to do.

When choosing a programming language to make a project, many different considerations can be taken. First, one must decide what is known as the level of the programming language. The level determines how near to the hardware the programming language is. In the lower level languages, instructions are written thinking directly on interfacing with hardware, while in "high level" ones a more abstract or conceptual code is written.

Generally, high level code is more portable, that means it can work in more different machines with a smaller number of modifications, whereas a low level language is limited by the peculiarides of the hardware which it was written for. Nevertheless, the advantage of low level code is that it is usually faster due to the fact that it is indeed written taking advantage of the possibilities of a specific machine.

A higher or lower level of programming is to be chosen for a specific project depending on the type of program that is being developed. For example, when a hardware driver is developed for an operating system obviously a very low level is used for programming. While when big applications are developed usually a higher level is chosen, or a combination of critic parts written in low level languages and others in higher ones.

Although there are languages that are clearly thought to be low level, like Assembly, whose instruction sets are adapted to each machine the code is made for, and other languages are inherently high level, like the Java, that is designed to be totally independent of the platform where is going to run. The C++ language is in a middle position, since it can interact directly with the hardware almost with no limitations, and can as well abstract lower layers and work like one of the most powerful high level languages.

Why C++?

C++ has certain characteristics over other programming languages. The most remarkable ones are:

Object-oriented programming

The possibility to orientate programming to objects allows the programmer to design applications from a point of view more like a communication between objects rather than on a structured sequence of code. In addition it allows a greater reusability of code in a more logical and productive way.

Portability

You can practically compile the same C++ code in almost any type of computer and operating system without making any changes. C++ is the most used and ported programming language in the world.

Brevity

Code written in C++ is very short in comparison with other languages, since the use of special characters is preferred to key words, saving some effort to the programmer (and prolonging the life of our keyboards!).

Modular programming

An application's body in C++ can be made up of several source code files that are compiled separately and then linked together. Saving time since it is not necessary to recompile the complete application when making a single change but only the file that contains it. In addition, this characteristic allows to link C++ code with code produced in other languages, such as Assembler or C.

C Compatibility

C++ is backwards compatible with the C language. Any code written in C can easily be included in a C++ program without making any change.

Speed

The resulting code from a C++ compilation is very efficient, due indeed to its duality as high-level and low-level language and to the reduced size of the language itself.

C++ Language FAQ

Frequently Asked Questions

What is C++?

C++ is a programming language. It literally means "increased C", reflecting its nature as an evolution of the C language.

Is it necessary to already know another programming language before learning C++?

Not necessarily. C++ is a simple and clear language in its expressions. It is true that a piece of code written with C++ may be seen by a stranger of programming a bit more cryptic than some other languages due to the intensive use of special characters ({}[]*&!|...), but once one knows the meaning of such characters it can be even more schematic and clear than other languages that rely more on English words.

Also, the simplification of the input/output interface of C++ in comparison to C and the incorporation of the standard template library in the language, makes the communication and manipulation of data in a program written in C++ as simple as in other languages, without losing the power it offers.

How can I learn C++?

There are many ways. Depending on the time you have and your preferences. The language is taught in many types of academic forms throughout the world, and can also be learnt by oneself with the help of tutorials and books. The documentation section of this Website contains an online tutorial to help you achieve the objective of learning this language.

What is OOP: Object-oriented programming?

It is a programming model that treats programming from a perspective where each component is considered an object, with its own properties and methods, replacing or complementing strutured programming paradigm, where the focus was on procedures and parameters.

Is C++ a propietary language?

No. No one owns the C++ language. Anyone can use the language royalty-free.

What is ANSI-C++?

ANSI-C++ is the name by which the international ANSI/ISO standard for the C++ language is known. But before this standard was published, C++ was already widely used and therefore there is a lot of code out there written in pre-standard C++. Referring to ANSI-C++ explicitly differenciates it from pre-standard C++ code, which is incompatible in some ways.

How may I know if my compiler supports ANSI-C++?

The standard was published in 1998, followed by a revision in 2003. Some compilers older than the standard implement already some of its features, and many newer compilers don't implement all ANSI-C++ features. If you have doubts on whether your compiler will be able to compile ANSI-C++ code, you can try to compile a piece of code with some of the new features introduced mainly after the publication of the standard. For example, the following code fragment uses the bool type, and uses namespaces and templates.

#include using namespace std; template <class T> bool ansisupported (T x) { return true; } int main() { if (ansisupported(0)) cout << "ANSI OK"; return 0; }

ANSI OK

If your compiler is able to compile this program, you will be able to compile most of the existing ANSI-C++ code.

How can I make windowed programs?

You need a C++ compiler that can link code for your windowing environment (Windows, XWindow, MacOS, ...). Windowed programs do not generally use the console to communicate with the user. They use a set of functions or classes to manipulate windows instead, which are specific to each environment. Anyway the same principles apply both for console and windowed programs, except for communicating with the user.

What is Visual C++? And what does "visual programming" mean?

Visual C++ is the name of a C++ compiler with an integrated environment from Microsoft. It includes special tools that simplify the development of large applications as well as specific libraries that improve productivity. The use of these tools is generally known as visual programming. Other manufacturers also develop these types of tools and libraries, like Borland C++, Visual Age, etc...