Why did the HelloWorld codes using cocoa's library fail to build? - objective-c

Question

The codes are like this:
#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>
int main(int argc, const char * argv[])
{
#autoreleasepool {
// insert code here...
NSLog(#"Hello, World!");
}
NSAlert *alert = [[NSAlert alloc] init];
[alert setMessageText:#"Hi there."];
[alert runModal];
return 0;
}
What I want is: when called from command line, this program pops up an alert box, when I close the alertbox. The program exits.
But when building, it complains like this:
Ld /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug/KeyCatcher normal x86_64
cd /Users/hanfei/Desktop/KeyCatcher
setenv MACOSX_DEPLOYMENT_TARGET 10.9
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -L/Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug -F/Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug -filelist /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Intermediates/KeyCatcher.build/Debug/KeyCatcher.build/Objects-normal/x86_64/KeyCatcher.LinkFileList -mmacosx-version-min=10.9 -fobjc-arc -fobjc-link-runtime -framework Foundation -Xlinker -dependency_info -Xlinker /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Intermediates/KeyCatcher.build/Debug/KeyCatcher.build/Objects-normal/x86_64/KeyCatcher_dependency_info.dat -o /Users/hanfei/Library/Developer/Xcode/DerivedData/KeyCatcher-hijnrqhwiafuxtdjmdubtsijyhwh/Build/Products/Debug/KeyCatcher
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_NSAlert", referenced from:
objc-class-ref in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
When I choose the template for new project, I select command line tool instead of CoCoa Application as I just need CoCoa to display an alertbox. Does anyone have ideas about this..

Answer 1

That's because NSAlert is not #imported.
AppKit.h is not included by default when you create a command line tool , as you can see in the link, AppKit imports NSAlert.h.
EDIT:
First, to compile you need to add the Cocoa framework in your project.
Second, all the code should be enclosed in the #autoreleasepool section
#autoreleasepool {
// insert code here...
NSLog(#"Hello, World!");
NSAlert *alert = [[NSAlert alloc] init];
[alert setMessageText:#"Hi there."];
[alert runModal];
}
Your code will compile and run, but I think you'll have some other runtime errors.

Answer 2

I got the same error in C++ project that was using a NSAlert, this worked for me:
1) Adding
#include <Cocoa/Cocoa.h>
2) Adding Cocoa Framework in project->Buld Phases->Link Binary With Libraries.
3) Setting "Type" of all my .cpp files to "Objective-C++ Source" in the most right column in xCode.

Answer 3

you need to add this before main function
#import <AppKit/AppKit.h>

Answer 4

You need to import AppKit as mentioned by others and – as important as the framework – you need a run loop. So insert before the runModal line
[NSApplication sharedApplication];
to launch the "application" and the run loop