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Programming in Objective-C

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This chapter is from the book
In order to help you understand the steps involved in keying in a program and compiling and running it, this chapter shows you how to write your first simple Objective-C program.

In this chapter, we dive right in and show you how to write your first Objective-C program. You won’t work with objects just yet; that’s the topic of the next chapter. We want you to understand the steps involved in keying in a program and compiling and running it.

To begin, let’s pick a rather simple example: a program that displays the phrase “Programming is fun!” on your screen. Without further ado, Program 2.1 shows an Objective-C program to accomplish this task:

Program 2.1.

// First program example 

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
   @autoreleasepool {
      NSLog (@"Programming is fun!");
   }
   return 0;
}

Compiling and Running Programs

Before we go into a detailed explanation of this program, we need to cover the steps involved in compiling and running it. You can both compile and run your program using Xcode, or you can use the Clang Objective-C compiler in a Terminal window. Let’s go through the sequence of steps using both methods. Then you can decide how you want to work with your programs throughout the rest of this book.

Using Xcode

Xcode is a sophisticated application that enables you to easily type in, compile, debug, and execute programs. If you plan on doing serious application development on the Mac, learning how to use this powerful tool is worthwhile. We just get you started here. Later we return to Xcode and take you through the steps involved in developing a graphical application with it.

Xcode is located in the Developer folder inside a subfolder called Applications. Figure 2.1 shows its icon.

Figure 2.1.

Figure 2.1. Xcode icon

Start Xcode. You can then select “Create a New Xcode Project” from the startup screen. Alternatively, under the File menu, select New, New Project... (see Figure 2.2).

Figure 2.2.

Figure 2.2. Starting a new project

A window appears, as shown in Figure 2.3.

Figure 2.3.

Figure 2.3. Starting a new project: selecting the application type

In the left pane, you’ll see a section labeled Mac OS X. Select Application. In the upper-right pane, select Command Line Tool, as depicted in the previous figure. On the next pane that appears, you pick your application’s name. Enter prog1 for the Product Name and make sure Foundation is selected for the Type. Also, be sure that the Use Automatic Reference Counting box is checked. Your screen should look like Figure 2.4.

Figure 2.4.

Figure 2.4. Starting a new project: specifying the product name and type

Click Next. The dropdown that appears allows you to specify the name of the project folder that will contain the files related to your project. Here, you can also specify where you want that project folder stored. According to Figure 2.5 we’re going to store our project on the Desktop in a folder called prog1.

Figure 2.5.

Figure 2.5. Selecting the location and name of the project folder

Click the Create button to create your new project. Xcode will open a project window such as the one shown in Figure 2.6. Note that your window might look different if you’ve used Xcode before or have changed any of its options.

Figure 2.6.

Figure 2.6. Xcode prog1 project window

Now it’s time to type in your first program. Select the file main.m in the left pane (you may have to reveal the files under the project name by clicking the disclosure triangle). Your Xcode window should now appear as shown in Figure 2.7.

Figure 2.7.

Figure 2.7. File main.m and edit window

Objective-C source files use .m as the last two characters of the filename (known as its extension). Table 2.1 lists other commonly used filename extensions.

Table 2.1. Common Filename Extensions

Extension

Meaning

.c

C language source file

.cc, .cpp

C++ language source file

.h

Header file

.m

Objective-C source file

.mm

Objective-C++ source file

.pl

Perl source file

.o

Object (compiled) file

Returning to your Xcode project window, the right pane shows the contents of the file called main.m, which was automatically created for you as a template file by Xcode, and which contains the following lines:

//
//  main.m
//  prog1
//
//  Created by Steve Kochan on 7/7/11.
//  Copyright 2011 ClassroomM, Inc.. All rights reserved.
//
#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
   @autoreleasepool {

      // insert code here...
      NSLog (@"Hello World!");
   }
   return 0;
}

You can edit your file inside this window. Make changes to the program shown in the Edit window to match Program 2.1. The lines that start with two slash characters (//) are called comments; we talk more about comments shortly.

Your program in the edit window should now look like this (don’t worry if your comments don’t match):

Program 2.1.

// First program example

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
   @autoreleasepool {
      NSLog (@"Programming is fun!");
   }
   return 0;
}

Now it’s time to compile and run your first program—in Xcode terminology, it’s called building and running. Before doing that, we need to reveal a window pane that will display the results (output) from our program. You can do this most easily by selecting the middle icon under View in the toolbar. When you hover over this icon, it says “Hide or show the Debug area.” Your window should now appear as shown in Figure 2.8. Note that XCode will normally reveal the Debug area automatically whenever any data is written to it.

Figure 2.8.

Figure 2.8. Xcode Debug area revealed

Now, if you press the Run button located at the top left of the toolbar or select Run from the Product menu, Xcode will go through the two-step process of first building and then running your program. The latter occurs only if no errors are discovered in your program.

If you do make mistakes in your program, along the way you’ll see errors denoted as red stop signs containing exclamation points—these are known as fatal errors and you can’t run your program without correcting these. Warnings are depicted by yellow triangles containing exclamation points—you can still run your program with them, but in general you should examine and correct them. After running the program with all the errors removed, the lower right pane will display the output from your program and should look similar to Figure 2.9. Don’t worry about the verbose messages that appear. The output line we’re interested in is the one you see in bold.

Figure 2.9.

Figure 2.9. Xcode Debug output

You’re now done with the procedural part of compiling and running your first program with Xcode (whew!). The following summarizes the steps involved in creating a new program with Xcode:

  1. Start the Xcode application.
  2. If this is a new project, select File, New, New Project... or choose Create a New Xcode Project from the startup screen.
  3. For the type of application, select Application, Command Line Tool, and click Next.
  4. Select a name for your application and set its Type to Foundation. Make sure Use Automatic Reference Counting is checked. Click Next.
  5. Select a name for your project folder, and a directory to store your project files in. Click Create.
  6. In the left pane, you will see the file main.m (you might need to reveal it from inside the folder that has the product's name). Highlight that file. Type your program into the edit window that appears in the rightmost pane.
  7. In the toolbar, select the middle icon under View. This will reveal the Debug area. That’s where you’ll see your output.
  8. Build and run your application by clicking the Run button in the toolbar or selecting Run from the Product menu.

  9. If you get any compiler errors or the output is not what you expected, make your changes to the program and rerun it.

Using Terminal

Some people might want to avoid having to learn Xcode to get started programming with Objective-C. If you’re used to using the UNIX shell and command-line tools, you might want to edit, compile, and run your programs using the Terminal application. Here we examine how to go about doing that.

The first step is to start the Terminal application on your Mac. The Terminal application is located in the Applications folder, stored under Utilities. Figure 2.10 shows its icon.

Figure 2.10.

Figure 2.10. Terminal program icon

Start the Terminal application. You’ll see a window that looks like Figure 2.11.

Figure 2.11.

Figure 2.11. Terminal window

You type commands after the $ (or %, depending on how your Terminal application is configured) on each line. If you’re familiar with using UNIX, you’ll find this straightforward.

First, you need to enter the lines from Program 2.1 into a file. You can begin by creating a directory in which to store your program examples. Then you must run a text editor, such as vi or emacs, to enter your program:

sh-2.05a$ mkdir Progs   Create a directory to store programs in
sh-2.05a$ cd Progs      Change to the new directory
sh-2.05a$ vi main.m     Start up a text editor to enter program
 --

For Objective-C files, you can choose any name you want; just make sure the last two characters are .m. This indicates to the compiler that you have an Objective-C program.

After you’ve entered your program into a file (and we’re not showing the edit commands to enter and save your text here), you can use the LLVM Clang Objective-C compiler, which is called clang, to compile and link your program. This is the general format of the clang command:

clang -fobjc-arc –framework Foundation files -o progname

This option says to use information about the Foundation framework:

-framework Foundation

Just remember to use this option on your command line. files is the list of files to be compiled. In our example, we have only one such file, and we’re calling it main.m. progname is the name of the file that will contain the executable if the program compiles without any errors.

We’ll call the program prog1; here, then, is the command line to compile your first Objective-C program:

$ clang -fobjc-arc -framework Foundation main.m -o prog1     Compile main.m & call it prog1
$

The return of the command prompt without any messages means that no errors were found in the program. Now you can subsequently execute the program by typing the name prog1 at the command prompt:

$ prog1        Execute prog1
sh: prog1: command not found
$

This is the result you’ll probably get unless you’ve used Terminal before. The UNIX shell (which is the application running your program) doesn’t know where prog1 is located (we don’t go into all the details of this here), so you have two options: One is to precede the name of the program with the characters ./ so that the shell knows to look in the current directory for the program to execute. The other is to add the directory in which your programs are stored (or just simply the current directory) to the shell’s PATH variable. Let’s take the first approach here:

$ ./prog1      Execute prog1
2008-06-08 18:48:44.210 prog1[7985:10b] Programming is fun!
$

You should note that writing and debugging Objective-C programs from the terminal is a valid approach. However, it’s not a good long-term strategy. If you want to build Mac OS X or iOS applications, there’s more to just the executable file that needs to be “packaged” into an application bundle. It’s not easy to do that from the Terminal application, and it’s one of Xcode’s specialties. Therefore, I suggest you start learning to use Xcode to develop your programs. There is a learning curve to do this, but the effort will be well worth it in the end.

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