What are the differences between #import and #include in Objective-C and are there times where you should use one over the other? Is one deprecated?
I was reading the following tutorial: http://www.otierney.net/objective-c.html#preamble and its paragraph about #import and #include seems to contradict itself or at least is unclear.
#include works just like the C #include.
#import keeps track of which headers have already been included and is ignored if a header is imported more than once in a compilation unit. This makes it unnecessary to use header guards.
The bottom line is just use #import in Objective-C and don't worry if your headers wind up importing something more than once.
The #import directive was added to Objective-C as an improved version of #include. Whether or not it's improved, however, is still a matter of debate. #import ensures that a file is only ever included once so that you never have a problem with recursive includes. However, most decent header files protect themselves against this anyway, so it's not really that much of a benefit.
Basically it's up to you to decide which you want to use. I tend to #import headers for Object-C things (like class definitions and such) and #include standard C stuff that I need. For example, one of my source files might look like this:
#import <Foundation/Foundation.h>
#include <asl.h>
#include <mach/mach.h>
IF you #include a file two times in .h files than compiler will give error. But if you #import a file more than once compiler will ignore it.
import directive is used to import any java classes into the jsp. eg : import java.lang.String, whereas include directive is used to include a content of another jsp.
'include' directive is used to include content from a jsp which is static for ever once the jspServlet created for it. The include jsp will not compipled each and every time it gets loaded Header and footer jsps can be included in this method.
If any jsp is included thru include action, the content of the jsp will be compiled and the respective output will be generated and rendered to the including jsp each and every time the including jsp is getting loaded.
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.