"C + + Primer"----about variables and basic types

type is the basis of all programs;

C + + defines several basic types: character type (char and wchar_t), integer (short int long bool), float (float Doubel) and provides a mechanism for customizing data types Class St ruct;;

The standard library uses these mechanisms to define many more complex data types, such as variable-length string vectors, and so on;

An object is an area in memory that has a type, and in other words: The calculation of an lvalue expression produces an object;

--------about Initialization

Initialization is divided into copy initialization and direct initialization

Copy initialization syntax using equals (=), direct initialization is to put the initialization in parentheses;;

int value (1024); Direct initialization

int value = 1024; Replication initialization

"Initialization is not assignment" in C + + initialization refers to the creation of a variable and assigns it an initial value, while the assignment is to erase the current value of the object and replace it with a new value;//So the copy initialization is actually a two-step action, the direct initialization syntax is more flexible and more efficient;

For built-in types, there is not much difference between assignment initialization and direct initialization.

But for objects of class type, some initialization can only be done with direct initialization, and a member function that defines how to initialize is called a constructor, a class can define multiple constructors, and each constructor must accept different numbers or different types of arguments;

Rules for initializing-------variables:

1. Initialization of built-in type variables

Whether the built-in type variable is automatically initialized depends on the position defined by the variable;

Variables defined outside the body of the function are initialized to 0

Variables defined in the function body are not initialized automatically;

2. Initialization of class-type variables

Use an explicit initialization

Using the default constructor

Regardless of where the variable is defined, the default function is used, and some class types do not have a default constructor, and for these types, each definition must provide an explicit initialization, and no initial value is at all impossible to define a variable of this type;

----------declarations and definitions

The definition of a variable is used to allocate storage space for a variable, and you can also specify an initial value for the variable, but the variable has only one definition; (the definition is typically accompanied by an initialization)

A declaration is used to indicate to a program the type and name of a variable. Definition is also a declaration (using extern but not initialized)

extern int pi = 3.1415; Defined

extern int pi; declared but not defined;

----------Const Qualifier;

The Const qualifier provides a way to convert a variable to a constant, or a const modifier to indicate that it cannot be modified;

Default condition:

When a non-const variable is defined in the global scope, it can be accessed throughout the program;

A const variable declared in the global scope is a local variable that defines the object file, which exists only in this file and cannot be accessed by another file

By default, similar to the C language static scope;

* The non-const variable defaults to extern, and the cosnt constant that you want to access in other files must be explicitly specified as extern;

------------references;

A reference is another name of the object, and the reference is mainly used as the formal parameter of the function before the & symbol is defined;

The reference must be initialized with an object of the same type as the reference; int val = 1025;     int &var = val; Ok

References are only different names, but point to the same address, the same object; Objects of the same data type are associated with each other;

-----------typedef

Used to define a synonym for a type; typedef int QINT32; Use Qint32 to represent the int data type;

-----------enumeration; enum

Define a set of selectable values for some properties;

Default: The first enumeration member is assigned a value of 0, and each subsequent enumeration is 1 greater than the previous one;

Enum STAT {input, output, append}; Default input = 0; output = 1; append = 2;

Enum Stat {input =2, output, append}; Other members will be implicitly initialized;

--------struct and CALSS

Class The first Access ID all previous members are private:

The first member of a struct is public;//This is the only difference between class and struc;

"C + + Primer"----about variables and basic types