Catalog

1. What is a bit segment ?

Grammatical rules

2. Bit segment memory allocation

Using examples

3. The cross platform problem of bit segment

4. Application of bit segment

1. What is a bit segment ?

        Bit segments are used to describe complex objects , When creating a member list, some objects don't need much space , So save space by setting bit segments , As its name suggests , Segment by bit .

Grammatical rules ：

        1. Bit segment Members must be plastic families int、unsigned int or signed int 、char、unsigned char or signed char.

        2. Bit segment After the member name There is one The colon And a Numbers .

struct A
{
    int a:2;
    int b:5;
    int c:10;
    int d:30;
};

2. Bit segment memory allocation

        You can see , The meaning of numbers is The size of the space , In bits , Create a space from the type of the first member variable , other Next to each other Bit segment of member Variable Use this space together , Use the set number of digits to occupy .

         The space of the bit segment is based on 4 Bytes （ int ） perhaps 1 Bytes （ char ） Of Way to open up Of , Namely It belongs to the plastic family .

         Bit segments involve many uncertainties , Bit segments are not cross platform , Pay attention to portable program should avoid using bit segment .

Using examples ：

 3. The cross platform problem of bit segment

        1. int It's uncertain whether a bit segment is treated as a signed number or an unsigned number .
        2. The number of the largest bits in the bit segment cannot be determined .（16 The machine is the largest 16,32 The machine is the largest 32, It's written in 27, stay 16 A bit of the machine will go wrong .
        3. The members in the bit segment are allocated from left to right in memory , Or right to left allocation criteria have not yet been defined .
        4. When a structure contains two bit segments , The second segment is relatively large , Cannot hold the remaining bits of the first bit segment , Whether to discard the remaining bits or to use , This is uncertain .VS Will abandon and open up a new space .

4. Application of bit segment

        After the above simple understanding , Bit segment can save a lot of space , As many programs look small , In fact, there are variable types in the source code that can meet the conditions, which can save most of the useless space , This way of saving space is also one of them .