Question

I’m working on the so called Hotspot open source project, and looking at the implementation I found a nasty nested union in struct looking like that:

typedef struct RC_model_t_st
{
    union
    {
        struct block_model_t_st *block;
        struct grid_model_t_st *grid;
    };
    /* block model or grid model    */
    int type;
    thermal_config_t *config;
}RC_model_t;

As far as I’m aware in C/C++ that union is unaccesible. So how someone can make use of union declared in such manner and for what purpose?

Thanks!

Answer 1

This is an anonymous union. In C++, as per [class.union], paragraph 5:

For the purpose of name lookup, after the anonymous union definition,
the members of the anonymous union are considered to have been defined
in the scope in which the anonymous union is declared

This means you can access its members as if they were members of RC_model_t_st.