[Study Notes-Objective-C] Chapter 10 variables and Data Types in Objective-C Programming 6th, objectivec static variables

[Study Notes-Objective-C] Chapter 10 variables and Data Types in Objective-C Programming 6th, objectivec static variables
Part 1. instance variables and attributes

• The current trend is to use (_) as the start character of the instance variable name.
• In the implementation part, explicitly declare that the instance variable is private
  
  • ThereforeNoIn the subclass, you can directly obtain the instance variable value by name.
  • OnlyInherited access methodObtains the value of an instance variable.
• Not Required@synthesizeCommand to generate an access method, as long as it is declared in the attribute.

Part 2. Global Variables

• Lowercase g is the first letter of the global variable:int gMoveNumber = 1;Is also an external variable.
• External variables: variables that are accessed and changed by any other method or function
• Access: add keywordsextern:extern int gMoveNumber;
  
  • AddexternIs the declaration of the variable, not the definition. The Declaration will not cause variable storage space allocation, but will be defined.
  • If you use many methods to access global variables, perform the following operations at the beginning of the file:extern
  • If there are only a few accesses, it is best to separate them within each method.externStatement

Part 3. Static variables

• Defined as a global variable, but not an external variable: Except for methods in a specific class, no other method needs to access this specific variable:static int gGlobalVar = 0;: Methods or functions after this statement can access the value of gGlobalVar, but not methods and functions in other files.

Part 4. enumeration data type

• Enumeration type: provides a method to match an integer with a symbolic name.
  
  • To change the value of this integer, you can only change it in the place where the enumeration is defined.
  • Example:enum direction {up, down, left = 10, right};
  • The addition of element sequence, integer + 1, and so on.

Part 5. typedef statement

Typedef: change the data type name.

typedef int Counter;Counter j,n;

• Added the readability of variable definitions and clearly showed the purpose of using these variables in the program.

Part 6. Data Type Conversion

Calculation of different data types:

• If one of the operations islong double,double,floatType, the result is alsolong double,double,floatType.
• If one of the operands is Bool, char, short int, bit field, or enumeration type, all are converted to int type.
• If one of the operations islong long int,long intType, and the other operand is convertedlong long int,long intType, the result is alsolong long int,long intType.
• Both operands areintType, the result is alsointType.

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