pragma keylist关键字有什么作用？

Question

While reading about various IoT messaging protocols I came across a structure defined as below:

enum TempScale {
   CELSIUM,
   KELVIN,
   FARENHEIT
};

struct TempSensorType {
   short id;
   float temp;
   float hum;
   TempScale scale;
};
#pragma keylist TempSensorType id

My question is: What does this #pragma keylist keyword do and where can I find some documentation about using #pragma preprocessor directives (I believe it is such directive..).

Thanks.

Answer 1

The #pragma you are looking at is the PrismTech method for defining a key value within an OMG-DDS (Data Distribution Service for Real-Time Systems) Type structure. In this case, it is defining the short 'id' as a key value. The comparable RTI definition would be

struct TempSensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}

For interoperability between vendors' implementations, you can safely do

struct TempSensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}
#pragma keylist TempSensorType id

because the RTI compiler ignores the pragmas, and the PT compiler ignores the //@key.

This will change with future versions of the specification for Extensible Types, which will define a standard method for all vendors to support.

Note that if you were looking at a generic list of IoT messaging protocols, the concept of a "key" value may not exist in the other messaging protocols you were looking at.

Answer 2

请注意，通过编译指示的显式键列表规范允许在键中定义一个排序 - 根据用例 - 可能对维护（填充/读取/查询/过滤）'多维 - 具有显着的性能影响存储'用于dataReader（和/或耐久性服务）

Answer 3

Note that for DDS implementations that comply with the recently-adopted OMG DDS-XTYPES specification ( http://www.omg.org/spec/DDS-XTypes/ ) the standard portable way to specify keys is either:

struct SensorType {
    @key short id;
    float temp;
    float hum;
    TempScale scale;
}

Or alternatively (to avoid breaking IDL compilers that do not understand the IDL annotations):

struct SensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}