Friday, September 4, 2009

Intro to C++

A C++ program is a collection of commands, which tell the computer to do "something". This collection of commands is usually called C++ source code, source code or just code. Commands are either "functions" or "keywords". Keywords are a basic building block of the language, while functions are, in fact, usually written in terms of simpler functions--you'll see this in our very first program, below. (Confused? Think of it a bit like an outline for a book; the outline might show every chapter in the book; each chapter might have its own outline, composed of sections. Each section might have its own outline, or it might have all of the details written up.) Thankfully, C++ provides a great many common functions and keywords that you can use.

But how does a program actually start? Every program in C++ has one function, always named main, that is always called when your program first executes. From main, you can also call other functions whether they are written by us or, as mentioned earlier, provided by the compiler.

So how do you get access to those prewritten functions? To access those standard functions that comes with the compiler, you include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program:
  #include   using namespace std;  int main() {   cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";   cin.get(); } 
Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called iostream into our program before actually creating the executable. By including header files, you gain access to many different functions. For example, the cout function requires iostream. Following the include is the statement, "using namespace std;". This line tells the compiler to use a group of functions that are part of the standard library (std). By including this line at the top of a file, you allow the program to use functions such as cout. The semicolon is part of the syntax of C++. It tells the compiler that you're at the end of a command. You will see later that the semicolon is used to end most commands in C++.

The next important line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ({ and }) signal the beginning and end of functions and other code blocks. You can think of them as meaning BEGIN and END.

The next line of the program may seem strange. If you have programmed in another language, you might expect that print would be the function used to display text. In C++, however, the cout object is used to display text (pronounced "C out"). It uses the << class="example" style="font-family: 'Courier New', Courier, mono; font-size: 12px; color: rgb(0, 0, 0); background-color: rgb(255, 255, 204); padding-top: 8px; padding-right: 8px; padding-bottom: 8px; padding-left: 8px; border-top-color: rgb(0, 0, 0); border-right-color: rgb(0, 0, 0); border-bottom-color: rgb(0, 0, 0); border-left-color: rgb(0, 0, 0); border-width: initial; border-style: initial; border-top-style: dashed; border-right-style: dashed; border-bottom-style: dashed; border-left-style: dashed; border-top-width: 1px; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; width: auto; "> #include using namespace std; int main() { cout<<"HEY, you, I'm alive! Oh, and Hello World!\n"; cin.get(); return 1; } The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .cpp file. Our Code::Blocks tutorial actually takes you through creating a simple program, so check it out if you're confused.

If you are not using Code::Blocks, you should read the compiler instructions for information on how to compile.
Once you've got your first program running, why don't you try playing around with the cout function to get used to writing C++?

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