What is the meaning of the #define in the objective C?

I want to ask a question about th开发者_C百科e objective-C or may be the C language. I want to ask what is the meaning of the following code of #define? Is it like to declare a variable?

#define kMountainNameString         @"name"
#define kMountainHeightString           @"height"
#define kMountainClimbedDateString      @"climbedDate"

It's a simple text substitution macro. Works the same as in C, C++.

Where kMountainNameString appears the compiler will "paste in" @"name". Technically speaking this occurs before the compiler by a mechanism called the preprocessor.

#define is a preprocessor directive inherited from C that takes the form

#define identifier [value]

In general, it is used to tell the preprocessor to replace all instances of identifier in the code with the given text before passing it on to the compiler. Identifiers can also be defined without values to be used as compiler flags to prevent multiple definitions of the same variables, or to branch on machine details that will not change during execution. For example, to pass different code to the compiler based on the architecture of your processor you could do something like:

#ifdef INTEL86
//some 32-bit code
#else
//some 64-bit code
#endif

When assigning values in these definitions, it's often a good idea to surround the value with parentheses so as to preserve it as one unit, regardless of the context it exists in.

For example, #define FOO 3 + 7 has a different effect than #define FOO (3 + 7) on the result of the following line, due to the order of arithmetic operations:

a = 3 * FOO

See this link for more details on preprocessor directives in general or this link for information more focused on Objective C.

The preprocessor replaces all occurences of kMountainNameString with @"name" before compilation begins.

#define is the preprocessor directive in c and c++ language.

It is used to define the preprocessor macros for the texts. The #define is used to make substitutions throughout the file in which it is located.

#define <macro-name> <replacement-string>