Two options:
static const int thisIsGlobal = 123;
or
#define thisIsGlobal 123
Stephen Canon
2010-05-02 23:58:54
+1
A:
A:
What you can do is put in your header (MyConstants.h):
extern const int MyConstant;
extern NSString * const MyStringConstant;
And in a source file, include the header above but define the constants (MyConstants.m):
const int MyConstant = 123;
NSString * const MyStringConstant = @"SomeString";
Then, you simply need to include the header in any other source file that uses either of these constants. The header is simply declaring that these constants exist somewhere, so the compiler won't complain, because it's the linker's job to resolve these constant names. The source file that contains your constant definitions gets compiled, and the linker sees that this is where the constants are, and resolves all of the references found in the other source files.
The problem with declaring and defining a constant in a header (that is not declared as static) is that the compiler treats it as an independent global for each file that includes that header. When the linker tries to link all of your compiled sources together it encounters the global name as many times as you have included MyConstants.h.
dreamlax
2010-05-03 00:19:14
I used your example but the linker gives this error:Undefined symbols: "_abc", referenced from: _abc$non_lazy_ptr in FirstView.old: symbol(s) not foundcollect2: ld returned 1 exit status
Susanna
2010-05-11 02:23:39
@Kimberly: Are you sure that the file containing the definitions of your constants (i.e. `MyConstants.m`) is being compiled and linked?
dreamlax
2010-05-11 02:50:50
I have absolutely NO idea if the file is being compiled and linked. How would I tell? I just hit BUILD AND GO in xcode. Always compiles fine... but never when I try my MyConstants.h include.
Susanna
2010-05-12 00:24:53
@Kimberly: Look at the build results (i.e. go to the Build menu and click Build Results, and make sure it is set to display All Messages (instead of Issues Only, the default). To make sure `MyConstants.m` is being built, right click on it (or Ctrl+Click for one-button mouse), and click "Get Info". Then, at the top, click on "Targets", and then make sure that the target you're working on is checked.
dreamlax
2010-05-12 00:50:00
A:
This is because the symbol name in question (thisIsGlobal) is being emitted into every object file created, where the header containing the declaration for thisIsGlobal is included and visible.
The examples provided by another poster: 'extern const int MyConstant;' is the best way, unless you need the value to be visible, in which case you can use an enum:
int thisIsGlobal = 123; // bad
enum { thisIsGlobal = 123 }; // ok
using static will emit a lot of hidden symbols in a large program -- don't use it. Using a define is scary as well (considering there are safer alternatives available, why not use them?).
Justin
2010-05-04 07:01:18
> 'extern const int MyConstant;' is the best way, unless you need the value to be visible Why would anyone *NOT* want a global var to ever be visible?
Susanna
2010-05-12 00:28:18
@Fran There are many reasons to define it such that it is not visible to clients or translations. Private implementation and compilation times are the most common reasons. One int won't grossly impact compilation times, but it would affect compilation times for more complex types (i.e. larger C++ objects), or if the (visible) definition is early in a large interface/package -- then changing the value would require all clients to recompile all dependencies including the declaration - critical for large projects. As well, there are many good reasons to define privately.
Justin
2010-05-14 06:18:35
A:
I usually put my application constants file in the Xcode project's MyApplication_Prefix.pch file, usually located within the Other Sources group. Any header file included in this pch file will be included from all files in your project.
After adding this include statement, you would then no longer need to include your MyConstants.h file from every file in your project — it will be included automatically.
Alex Reynolds
2010-05-04 07:06:31