What is the purpose of creating a struct with only one field

Question

On the project in my company I saw a couple of times people creating a struct that contains only one element. the latest one is added, in this example ipAddr is another struct( a good explanation to the case when ipAddr is an array is given by 'Frerich Raabe' but unfortunately thats not the case here)

typedef struct
{
  ipAddr ip;
} Record;

I guess if the code is changing and in its beginning this makes sense since more fields can be easily added and the code can be easily manipulated to support the new fields, but this is a project started a long time ago written according to a design so I dont think its the issue here.

Why would one create a struct of one field then?

Answer 1

I can think of a couple reasons:

1. In case more fields may be needed later. This is somewhat common.
2. To deliberately make the outer type incompatible with the inner type.

For an example of the second, imagine this:

typedef struct
{
    char postal_code[12];
} Destination;

In this case, the Destination is fully specified by the postal code, yet this will let us define functions like this:

int deliver(const char* message, const Destination* to);

This way, no user can inadvertently call the function with the two arguments reversed, which they could easily do if they were both plain strings.