Question

I have a file:

#import "OutletsHandler.h"


@implementation OutletsHandler

- (IBAction) pi : (id) sender
{
    extern const double PI; // here I try to import PI
    [textField setDoubleValue: PI];
}

- (id)init
{
self = [super init];
if (self) 
{
}

return self;
}

- (void)dealloc
{
    [super dealloc];
}

@end

OutletsHandler.h imports CalculatorScanner.h, this is CalculatorScanner.h:

#import <stdbool.h>

const double PI=3.141592654;

But I get semantic issues:

Command /Developer/usr/bin/clang failed with exit code 1
ld: duplicate symbol _PI in /Users/ramy/Library/Developer/Xcode/DerivedData/Calculator-   crcetknqiorefpbjxcrtydequwxf/Build/Intermediates/Calculator.build/Debug/Calculator.build/Objects-normal/x86_64/CalculatorScanner.o and /Users/ramy/Library/Developer/Xcode/DerivedData/Calculator-crcetknqiorefpbjxcrtydequwxf/Build/Intermediates/Calculator.build/Debug/Calculator.build/Objects-normal/x86_64/CalculatorAppDelegate.o for architecture x86_64

What am I doing wrong?

Answer 1

Your problem is that you are defining PI in CalculatorScanner.h, when you should just be declaring it. To make it compile and link properly, put this in CalculatorScanner.h:

extern const double PI;

and put this in CalculatorScanner.cpp:

const double PI = 3.141592654;

But that’s not really the correct solution either, in this case. The correct solution here is to use the standard constant M_PI instead of defining your own (less accurate) constant.