Accessing a struct field using an int or enum value

Question

I currently use a logging system that uses a tag value to identify the parameter that it will store. The format we are using is the following: Tag + Time + value.

The next step is to take a logged message and send it to a server using Protocol Buffers to serialize the data.

The protocol buffer .proto file, provides a struct with all the fields each corresponding to a tag in the original logging system.

The problem comes when I need to efficiently read the tag and assign the value to a field in the protocol buffer struct. In essence I would for example like to take in tag 5 and be able to find the field 5 in the struct and write the value.

I know this could be done with switch cases, but we are using around 50 tags so I would like to avoid this solution if possible. I attach an example struct to illustrate the problem.

/* Struct definitions */
typedef struct _Profiles {
    int32_t param1;
    int32_t param2;
    int32_t param3;
    int32_t param4;
    int32_t param5;
    int16_t param6;
    int32_t param7;
    uint32_t param8;
    int32_t param9;
    int32_t param10;
    uint32_t param11;
    int32_t time;
/* @@protoc_insertion_point(struct:Profiles) */
} Profiles;

The expected result would be that I can store a logged line like the following: 5 1345643 1500 (tag, time, value)

to a protocol buffer struct:

profiles.param5 = 1500
profiles.time  = 1345643

without the need of 12 switch cases (in this example). Basically, how can I access the 5th declared field of a struct using an integer/enum.

Bare in mind that each field of the struct could potentially have a different type.

user_4431643 · Answer

My approach would be having pointers to each of the fields as below.

int *ptr[11] = {&profiles.param1, &profiles.param2,  &profiles.param3,,,,,, &profiles.param11};

And when the message arrives, I will update the field using ptr.

  *ptr[tag-1] = 1500; //tag-1 because ptr[4] points to profiles.param5

user_3555525 · Answer

Improving on the previous solution by @kiran.

In case you have multiple types then you need to store void pointers. But for typecasting you again need to have a switch case.

This can be avoided in two ways.

Store the type along with the data. i.e, store (tag, type, time, value). Then you can have a limited no of conditions for the types.

void *ptr[11] = {&profiles.param1, &profiles.param2,  &profiles.param3,,,,,, &profiles.param11};

switch (type)
{
    case 0:
        *(char*)ptr[tag-1] = value;
        break;
    case 1:
        *(int*)ptr[tag-1] = value;
        break;
    ...
}

Alternatively you can have a map which stores the type of the value

void *ptr[11] = {&profiles.param1, &profiles.param2,  &profiles.param3,,,,,, &profiles.param11};
char typearr[11] = {0,1,0,0....1};  // 0 for char, 1 for int etc

type = typearr[tag-1];

switch (type)
{
   ...
}

Accessing a struct field using an int or enum value

2 Answers