[Study note-OBJECTIVE-C] "OBJECTIVE-C Programming 6th Edition" chapter tenth variables and data types

Part 1. Instance variables and properties

• The current trend is to use (_) as the starting character for the instance variable name.
• Explicitly declaring an instance variable in the implementation section is private
  
  • Therefore, you cannot get the value of an instance variable directly in a subclass by its name.
  • The value of an instance variable can only be obtained through an inherited access method in a subclass.
• You do not have @synthesize to use directives to generate access methods, as long as you declare them in attributes.

Part 2. Global Variables

• The lowercase g is the first letter of the global variable: int gMoveNumber = 1; It is also an external variable.
• External variable: A variable that accesses and changes a value by any other method or function
• Access: Add keywords extern : extern int gMoveNumber;
  
  • Add extern a declaration to a variable, not a definition. The declaration does not cause variable storage space allocations, and the definition will.
  • If there are many ways to access a global variable, the file starts onceextern
  • Only a few visits, it is best to declare them separately within each method. extern

Part 3. Static variables

• is defined as a global variable, but not an external variable: in addition to the methods in a particular class, there is no other way to access this particular variable: the static int gGlobalVar = 0; method or function behind this statement can access the value of Gglobalvar, and the methods and functions in other files do not work.

Part 4. Enumeration data Types

• Enum type: Provides a way to match an integer value to a symbolic name.
  
  • If you need to change the value of this integer, it can only be changed where the enumeration is defined.
  • Example:enum direction {up, down, left = 10, right};
  • An increase in the sequence of elements, an integer value of +1, and so on.

Part 5. typedef statements

typedef: Changing the name of a data type

typedefint Counter;Counter j,n;

• The readability of variable definitions is increased, and the purpose of these variables in the program is clearly seen.

Part 6. Data type Conversions

Different types of data type operations:

• If one of the operands is long double , double float type, the result is also, long double double float type.
• If one of the operands is Bool,char,short int, bit field or enum type, all are converted to int.
• If one of the operands is long long int , type, and the long int other operand is converted to long long int , type long int , the result is also long long int , long int type.
• Two operands are all int type, the result is also int type.

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[Study note-OBJECTIVE-C] "OBJECTIVE-C Programming 6th Edition" chapter tenth variables and data types