2 variables and basic types

2.1 Words and bytes

1、 The bit （bit）: Computers store data in bit sequences , Each bit is not 0 namely 1.

2、 byte （byte）: Most computers operate in 2 The integer power of bits is used as a block to process memory , The smallest addressable block of memory is called a byte

3、 word （word）： The basic unit of storage is called a word , It usually consists of several bytes .
Generally, the bytes of a computer consist of 8 Bit formation , And a word is usually 4 Bytes or 8 Bytes .

2.2 quote （ lvalue reference ）

1、 A reference is an alias for an object ;

2、 References must be initialized , And the initial value of the reference must be a object ;

What is an object ？
Generally speaking, an object is a piece of memory space that can store data and has some type of memory . Some people distinguish between named objects and unnamed objects , They call named objects variables . Others distinguish between objects and values , The object refers to the data that can be modified by the program , And value (value) Refers to read-only data .

int &a = 3  // error 

3、 The reference itself is not an object , So you can't define references to references ;

4、 Multiple references are allowed to be defined in one statement , Each reference identifier must be represented by the symbol & start

Key points ： An operation on a referenced variable is an operation on the object it references ！！ So the formal parameters of some functions are often reference types .

2.3 The pointer

1、 Understand pointers , Need to understand * Symbols and & Symbol ,* Can be used to define pointers , It can also be used to dereference ,& It means to get the address .

2、 A null pointer doesn't point to anything .int *p=nullptr Define null pointer .

3、void* The pointer can store the address of any object , You can compare the size , Because there is no type , Only operate on memory space , Unable to access or dereference the stored object , Cannot force offset , Can be replaced by forced type static_cast Replace with other types of pointers .void * Pointer variables are mainly used to pass different types of pointer variables to functions , Cast it to a specific pointer type inside the function .

4、 The pointer to the pointer , Pointer to the variable p It is also a variable occupying an independent memory block , Its own address can also be saved in a In pointer variables , This pointer variable is often referred to as a pointer to a pointer , representative ** Symbol .

5、 Reference to pointer , The reference itself is not an object , So you can't define a pointer to a reference . But the pointer is the object , So there are references to pointers ,*& Symbol , Look from right to left .

int i=42;
int *p;
int *&r = p;  // Look from right to left , Pointer type reference 

r = &i;  // establish , be relative to p = &i

Comparison of references and pointers ：
Reference variables are just aliases for other variables , No independent memory , The same reference variable cannot be modified to reference different variables . Pointer variables store the addresses of other variables , There is independent memory , Can point to different objects at different times .

2.4const Modifier

1、const I want to define some A variable whose value cannot be changed .

2、const Object must be initialized , And cannot be changed .

3、const Variables cannot be accessed by other files by default , No access , Must specify const Add... Before definition extern. To use in multiple files const Variable sharing , Definitions and declarations are added const Key words can be used .

4、const And a pointer , Include pointer to const and Constant of pointer type .

5、 pointer to const （pointer to const）, Cannot be used to change the value of the object it refers to , There are two ways to define it , Take the integer constant as an example , The definition includes int const* p = &i and const int* p = &i. Indicates that the object pointed to by the pointer is a constant , The pointed object cannot be modified .

5、 Constant of pointer type is also called constant pointer （const pointer）, The pointer itself is a constant , There are several ways to define it int *const p = &i;, That is, the pointer is of constant type , That is, the pointer is fixed to the object , The address stored in the pointer remains unchanged , But the object value corresponding to the address can be modified .

6、const And quotes , A reference to a constant （reference to const）, Point to const References to objects , Such as const int i =1; const int &r = i;, Variables can be read but not modified r.

Provisional quantity （temporary） object ： When the compiler needs a space to temporarily store the evaluation results of the expression , An unnamed object created temporarily . A reference to a temporary quantity is illegal .

2.5 Type the alias typedef

1、 Type the alias (type alias) It's a name , It is a synonym of some kind . There are many benefits to using type aliases , It makes complex type names simple 、 Easy to understand and use , It also helps programmers to be clear about the real purpose of using this type .

2、 Traditional definition , Use keywords typedef, Such as typedef Sales_item SI

2、C++11 New feature definitions ,using SI = Sales_item

3、 Understand pointer types typedef The difference between ： This object that matches the type , It cannot be understood by substituting the original formula, e.g ：

typedef char *pstring;  // pstring yes char* Another name for 
const pstring cstr = 0; //  Constant of pointer type of , Point to char type 
const pstring *ps;  // here ps It's a point char Pointer to type constant 
//  It's like const char *cstr = 0; Incorrect , To point to const char The pointer to 

//  To assist in understanding （ It can be replaced by parentheses ）
// const pstring cstr = 0; After replacement const (char *) cstr = 0;
// const char *cstr = 0; That is to say (const char *) cstr = 0;

Again, pay attention to ：
1、 How pointers to constants are defined const int* p = &i perhaps int const* p = &i,
2、 The constant of pointer type is defined as int const* p = &i,
3、 If you use typedef Words ,cons A constant that represents a pointer type , namely const pstring p Is a constant of pointer type , and const pstring *p Is a pointer to a constant .