There seems to be a lot of confusion regarding the preprocessor.
What the compiler does when it sees a #include
that it replaces that line with the contents of the included files, no questions asked.
So if you have a file a.h
with this contents:
typedef int my_number;
and a file b.c
with this content:
#include "a.h"
#include "a.h"
the file b.c
will be translated by the preprocessor before compilation to
typedef int my_number;
typedef int my_number;
which will result in a compiler error, since the type my_number
is defined twice. Even though the definition is the same this is not allowed by the C language.
Since a header often is used in more than one place include guards usually are used in C. This looks like this:
#ifndef _a_h_included_
#define _a_h_included_
typedef int my_number;
#endif
The file b.c
still would have the whole contents of the header in it twice after being preprocessed. But the second instance would be ignored since the macro _a_h_included_
would already have been defined.
This works really well, but has two drawbacks. First of all the include guards have to be written, and the macro name has to be different in every header. And secondly the compiler has still to look for the header file and read it as often as it is included.
Objective-C has the #import
preprocessor instruction (it also can be used for C and C++ code with some compilers and options). This does almost the same as #include
, but it also notes internally which file has already been included. The #import
line is only replaced by the contents of the named file for the first time it is encountered. Every time after that it is just ignored.
The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic
-- atomic
was added as a keyword in recent versions of llvm/clang).
Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _
prepended to their name to prevent accidental direct access).
With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic
, no such guarantees are made. Thus, nonatomic
is considerably faster than "atomic".
What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.
Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.
Adding to this:
atomicity
of a single property also cannot guarantee thread safety when multiple dependent properties are in play.
Consider:
@property(atomic, copy) NSString *firstName;
@property(atomic, copy) NSString *lastName;
@property(readonly, atomic, copy) NSString *fullName;
In this case, thread A could be renaming the object by calling setFirstName:
and then calling setLastName:
. In the meantime, thread B may call fullName
in between thread A's two calls and will receive the new first name coupled with the old last name.
To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName
while the dependent properties are being updated.
Best Answer
I believe there is a difference between
bool
andBOOL
, check out this webpage for an explanation of why:http://iosdevelopertips.com/objective-c/of-bool-and-yes.html
Because
BOOL
is anunsigned char
rather than a primitive type, variables of typeBOOL
can contain values other thanYES
andNO
.Consider this code:
The output is:
For most people this is an unnecessary concern, but if you really want a boolean it is better to use a
bool
. I should add: the iOS SDK generally usesBOOL
on its interface definitions, so that is an argument to stick withBOOL
.