Installation
This is specific to Arch, but you'll just need to install your distro's packages for gcc Objective-C support and the GNUstep base:
pacman -S gcc-objc gnustep-base
On Arch GNUstep installs to /opt/GNUstep.
Interface
We need to declare the interface for our class.
HelloWorld.h:
// use the GNUstep Foundation library
#import <Foundation/NSObject.h>
// declare interface for HelloWorld class as a subclass of NSObject
@interface HelloWorld : NSObject {
// instance variables
char *string;
}
// method definitions
- (id)setString: (char *)s;
- (char *)string;
- (void)speak;
// end interface declaration
@end
Implementation
Now to implement our simple class by writing the methods. Objective-C implementation files traditionally use the .m extension.
HelloWorld.m:
// include our interface declaration
#import <HelloWorld.h>
// start the implementation of the HelloWorld class
@implementation HelloWorld
// constructor
- (HelloWorld *)init
{
[super init]; // call init from superclass
[self setString: "hello world"]; // set default value
}
// accessor method to get value of string
- (char *)string
{
return(string);
}
// set string value
- (id)setString: (char *)s
{
string = s;
}
// print current value of string
- (void)speak
{
printf("%s\n", string);
}
// end the implementation
@end
Main
As with C, the entry point to our program is the main() function.
main.m
#import <HelloWorld.h>
int main (int argc, const char *argv[]) {
// create instance of our class
HelloWorld *hw = [[HelloWorld alloc] init];
// print the default
[hw speak];
// set new value and print
[hw setString: "goodbye"];
[hw speak];
// release memory
[hw release];
return 0;
}
Makefile
An example Makefile, showing required GNUstep libs. You will need to change the value of GNUSTEP to your distro's install location.
Makefile:
GNUSTEP=/opt/GNUstep
EXEC=hello
OBJS=HelloWorld.o main.o
LIBS=-lgnustep-base
LIBDIRS=-L${GNUSTEP}/Local/Library/Libraries
INCLUDES=-I${GNUSTEP}/Local/Library/Headers -I.
all: ${EXEC}
${EXEC}: ${OBJS}
gcc -o ${EXEC} ${OBJS} ${LIBDIRS} ${LIBS}
main.o: main.m
gcc -c ${INCLUDES} main.m
HelloWorld.o: HelloWorld.m HelloWorld.h
gcc -c ${INCLUDES} HelloWorld.m
clean:
rm -f ${OBJS} ${EXEC}
Compile and run
$ make
$ ./hello
hello world
goodbye
Further reading
