Question

I don’t know why, but after a while working without problems I added some buttons, then I launched my app and this error appeared:

ld: duplicate symbol _x in
/Users/alexbarco/Library/Developer/Xcode/DerivedData/RecolectaDatos-ayjpqqcajbhuzvbkvernzsyunpbe/Build/Intermediates/RecolectaDatos.build/Debug-iphonesimulator/RecolectaDatos.build/Objects-normal/i386/SeconViewController.o
and
/Users/alexbarco/Library/Developer/Xcode/DerivedData/RecolectaDatos-ayjpqqcajbhuzvbkvernzsyunpbe/Build/Intermediates/RecolectaDatos.build/Debug-iphonesimulator/RecolectaDatos.build/Objects-normal/i386/ViewController.o
for architecture i386 clang: error: linker command failed with exit
code 1 (use -v to see invocation)

Answer 1

Whenever I have duplicate symbol errors, it is almost always because I have a circular #import in my headers. The solution is quite simple, use forward declarations where possible, and #import .h files from .m files instead.

There are just two cases where you need to #import one .h from another:

1. if you are extending the class in the #import
2. you are implementing a protocol in the #import

Specifically, you do not need to import files just to use a class name or protocol in your signatures; instead use forward declarations.

For example, this (in Bar.h):

#import "Foo.h"

might become this (Bar.h):

@class Foo;
@protocol FooDelegate;

and bar.m:

#import "Foo.h"

Here is a link to the documentation on forward declarations.