Notes on advanced points of C language 4

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Catalog

• 5、 ... and 、 Structure

1、 Definitions and concepts
2、 Array of structs
3、 Structure pointer
4、 Structure memory allocation
5、 Bit segment （ Bit field ）

• 6、 ... and 、 Shared body
• 7、 ... and 、 Enumeration type

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• 5、 ... and 、 Structure

1、 Definitions and concepts

struct Structure type name

{

          Member list

};

You can explain first and then define , You can also define... Before the semicolon

Define structure variables before semicolons , At the same time, omit Structure type name , Then you can no longer define this structure variable , At this point, the definition is the only , It is often used for global variables that are used a limited number of times

typedef struct （ Structure type name is optional ）

{

        Member list

} Type the name ;    Variables cannot be defined before semicolons

To define a variable, just   Type name plus variable name   

Variable name plus dot plus member name for assignment

When a structure is described, its member list can make another structure , Multi level references can be used

Structural variables of the same type can be assigned to each other directly

2、 Array of structs

Structure array is an array , A collection of several structural variables of the same type

struct Structure type name Array name [ Element number ];

3、 Structure pointer

The structure pointer is defined and assigned in the same way as ordinary variables

The overall reference of structure is the same as that of ordinary variables

But it's different when referring to structure members , You need brackets ,. The priority ratio * high ,(*ps).num

Bracketed trouble , Available pointing operators （->） Instead of ,ps->num

When there is nesting , When referencing a member of an inner structure , The first layer uses the pointing operator, and the second layer uses the dot operator

4、 Structure memory allocation

#pragma pack (value) The specified alignment value for value.

value Can only be ： 1 2 4 8 etc.

Byte alignment , The current member is value Integer multiples remain unchanged ; The current member is less than value, But with the latter member, it happens to be value The integral multiple of is also unchanged ; The current member is less than value, Plus the latter member is also less than value The integral multiple of is supplemented by value Integer multiple .

You can use sizeof Measure the space occupied

5、 Bit segment （ Bit field ）

struct packed_data
{
    unsigned int a:2;
    unsigned int b:6;
    unsigned int c:3;
    unsigned int  :1; //  Meaningless paragraph 
    unsigned int d:4;
    unsigned int i;
} data;

The number after the colon indicates how many digits , Cannot take address for bit segment member

Do not exceed the range defined by the bit segment when assigning , If the assignment is out of range, it will be intercepted

Meaningless bit segments can be defined , Simply take up space , To ensure that the following members occupy the required bits

In the same structure , Member variables can include both bit segments and ordinary member variables

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• 6、 ... and 、 Shared body

The common body is similar to the structure , It's also a construction type of data structure

The definition method of common body is also very similar to that of structure , take struct Change to union that will do , Other methods of use are the same , But no bit segment is allowed

When performing some algorithms , Several different types of variables need to be stored in the same memory unit , The spaces used by several variables overlap each other

The size of a community is the size of its member with the largest memory length

Only one member of a community is valid at a time , It is often used in the sending process of network protocol

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• 7、 ... and 、 Enumeration type

List the values of variables one by one , The value of a variable is limited to the values listed

enum Enumeration type name

{

          Enumerate the list of values , （ Members are separated by commas ）

};

Also available typedef

Where enumeration is used, macro definition can be used instead of , Not vice versa

Enumeration members can be any content , Its actual value is an integer , The default from the 0 Began to increase , You can also change the integer value corresponding to the member by assigning a value to the member

Definition of enumeration variables ：enum Enumeration type name Enumerate variable names