Get type of instantiated unnamed struct

Question

I'm writing an app in C++ that interfaces with some code written in C.

The following is a simplified version of a struct defined in C code that I can't modify.

struct event{
    uint8_t type;
    union {
        struct /* WishThisHadAName */ {
            // ...
        } connect;
        struct {
            // ...
        } disconnect;
    };
};

I'm trying to be able to define functions that take a pointer to the different unnamed structs, connect, disconnect, and all the others not listed above. To look something like the following.

void onConnect( struct WishThisHadAName *evt );

Is it possible to create this prototype without modifying the original C struct? Is there an easy way to create a wrapper for the unnamed structs to give them a name? Am I forced to create named structs that mirror the unnamed ones I'm trying to use?

I know you can get the type of an instantiated variable with decltype but I can't figure out how I could use that or declval in order to create the function prototype I'm looking for.

Note that, if you weren't using C++, with its lovely `decltype` operator`, there would be no solution. [This is not possible in C](https://stackoverflow.com/questions/16725502/anonymous-c-struct-with-declaration). — Cody Gray - on strike, Jun 08 '22 at 09:53

score 18 · Accepted Answer · answered Jun 07 '22 at 15:01

Simple as using event_connect_t = decltype(event::connect);.

Then you can use it as void onConnect( event_connect_t *evt );.

You can't declare a compatible type, but you can just extract the existing type declaration from the definition. decltype can resolve static member references just fine.

score 6 · Answer 2 · edited Jun 07 '22 at 17:36

6

If you're using a compiler such as gcc/g++, you can use typeof to create a typedef for the anonymous type:

typedef __typeof__(((struct event){0}).connect) conn_type;
typedef __typeof__(((struct event){0}).disconnect) disconn_type;

Or without compound literals:

struct event e;
typedef __typeof__(e.connect) t1;
typedef __typeof__(e.disconnect) t2;

edited Jun 07 '22 at 17:36

ShadowRanger

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answered Jun 07 '22 at 15:13

dbush

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Get type of instantiated unnamed struct

2 Answers2