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Editorial Team
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Asked: 2026-05-28T04:41:56+00:00

Please help me understand the meaning of this code. I have seen this kind

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Please help me understand the meaning of this code. I have seen this kind of usage for the first time

typedef enum {

        E_1_DEFAULT = 0,
        E_1_1,
        E_1_2,
        E_1_3,
        E_1_4,
        E_1_5,
        E_1_255 = 255           //needs at least 8Bit

} APPLICATION_ENUM_1;                           

typedef enum {

        E_2_DEFAULT = 0,
        E_2_1,
        E_2_2,
        E_2_3                   //needs at least 2Bit

} APPLICATION_ENUM_2;                           

typedef enum {

        E_3_DEFAULT = 0,
        E_3_1,
        E_3_2,
        E_3_3,
        E_3_4,
        E_3_5,
        E_3_666 = 666           //needs at least 10Bit

} APPLICATION_ENUM_3;



typedef struct {

        APPLICATION_ENUM_3      var3:10;                // 10Bit
        APPLICATION_ENUM_1      var1:8;         // 18Bit
        APPLICATION_ENUM_2      var2:2;         // 20Bit
        uint8                   unnused_1:4;   // fill up the last whole byte -> 24Bit = 3byte

} APPLICATION_RAM;;
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1 Answer

  1. Editorial Team

    According to the C++ standard

    The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration.

    Now lets look at the first definition

    typedef enum {

        E_1_DEFAULT = 0,
        E_1_1,
        E_1_2,
        E_1_3,
        E_1_4,
        E_1_5,
        E_1_255 = 255           //needs at least 8Bit

} APPLICATION_ENUM_1;

    definition E_1_255 = 255 ensures that all values from 0 to 255 can be represented by this enum type and you need at least 8bits to represent all values from 0 to 255

    typedef struct {

        APPLICATION_ENUM_3      var3:10;                // 10Bit
        APPLICATION_ENUM_1      var1:8;         // 18Bit
        APPLICATION_ENUM_2      var2:2;         // 20Bit
        uint8                   unnused_1:4;   // 24Bit = 3byte

} APPLICATION_RAM;

    The above struct is using a seldom used bitfield construct. Basically declaring the APPLICATION_RAM as a struct that has a

    • var3 member that is 10 bits
    • var1 member that is 8 bits
    • var2 member that is 2 bits
    • unnused_1 member that is 4 bits
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