Pointer returning void working in C but not in C++

Question

I am trying to make a Native C/C++ app for Tizen platform and this a part of the code of a template Tizen Studio gives : (Note this is C code)

static char*
gl_text_get(void *data, Evas_Object *obj, const char *part)
{
    char buf[1024];
    item_data *id = data;
    int index = id->index;

    if (index == 0)
        snprintf(buf, 1023, "%s", "Email Inbox");
    else if (index == 1)
        snprintf(buf, 1023, "%s", "circle@tizen.com");
    else if (!strcmp(part, "elm.text"))
        snprintf(buf, 1023, "%s", genlist_demo_names[index - 2]);
    else if (!strcmp(part, "elm.text.1"))
        snprintf(buf, 1023, "%s", "Re: Long time no see");
    else
        snprintf(buf, 1023, "%s", "Hello~! how have you been?");

    return strdup(buf);
}

static void
gl_del(void *data, Evas_Object *obj)
{
    item_data *id = data;
    if (id) free(id);
}

static void
gl_selected_cb(void *data, Evas_Object *obj, void *event_info)
{
    Evas_Object *label;
    Elm_Object_Item *it = event_info;
    appdata_s *ad = data;
    elm_genlist_item_selected_set(it, EINA_FALSE);

    label = create_label(ad, it);
    //View changed to text detail view.
    elm_naviframe_item_push(ad->nf, elm_object_item_part_text_get(it, "elm.text"), NULL, NULL, label, NULL);

    return;
}

And this templates successfully compiles and runs on my Tizen device. However, when I copy this same code in a C++ file (because I need some cpp-only features in my project). Compilation fails :

And this is seems quite obvious to me since a pointer cannot pick-up the value of void. So why is this working in C code ? Has void the same meaning in C and C++ ?

C and C++ are different languages ... kinda like you asking for a ["biscuit" in UK/USA](https://www.eurocentres.com/blog/11-foods-different-names-uk-us) — pmg, Mar 14 '21 at 20:36
Well, C and C++ differ about the treatment of `void*` ... among many other things — pmg, Mar 14 '21 at 20:42

score 3 · Accepted Answer · answered Mar 14 '21 at 20:31

3

In both C and C++, any pointer type is implicitly convertible to void*.

But in C only, void* is implicitly convertible to any other pointer type, whereas in C++ that is not the case, you need an explicit type-cast instead, eg:

Elm_Object_Item *it = (Elm_Object_Item *) event_info;
appdata_s *ad = (appdata_s *) data;
item_data *id = (item_data *) calloc(sizeof(item_data), 1);

Or, more preferably, using static_cast instead of a C-style cast:

Elm_Object_Item *it = static_cast<Elm_Object_Item*>(event_info);
appdata_s *ad = static_cast<appdata_s*>(data);
item_data *id = static_cast<item_data*>(calloc(sizeof(item_data), 1));

answered Mar 14 '21 at 20:31

Remy Lebeau

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Thank you ! It worked perfectly. I code in Java so to me ```Elm_Object_Item *it = (Elm_Object_Item *) event_info;```is a much clearer syntax. Are there any real advantage of using ```static_cast(event_info);```? – ScarySneer Mar 14 '21 at 20:45
You guarantee that you understand what you're casting. A C-style cast can mean any combination of `static_cast`, `const_cast` and `reinterpret_cast`. – Blindy Mar 14 '21 at 20:51

Pointer returning void working in C but not in C++

1 Answers1