Union initialisation in a struct

Question

I've been browsing stackoverflow concerning the problem of initialising a union in a struct but I didn't manage to get it right.

Here is my struct

typedef struct dc_netif_filter {
    unsigned char filter_type;  
    union {
        uint32_t itf_nb;
        char * buf; 
    } value;
} dc_netif_filter_t;

In my code, i have tried to use :

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME,{{0, "tun"}}};

which gives error: braces around scalar initializer for type ‘uint32_t’

and

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME,{0, "tun"}};

which gives error: too many initializers for ‘dc_netif_filter::< anonymous union>’

How do i declare such a dc_netif_filter_t ?

I'm using g++ on ubuntu. Note that the dc_netif_filter_t isn't a struct that I can modify as it comes from a third party project.

**EDIT : as I've been explained, i can only initialise one field. The problem is that with

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME,"tun0"};

I get a conversion error : invalid conversion from ‘const char*’ to ‘uint32_t

Thanks

You don't want to initialize all the members of the union; how about: dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME, "tun"}; — trojanfoe
– trojanfoe, Commented Apr 16, 2012 at 13:24
You don't. Unions aren't structures. BTW, your code looks like C, not C++. — David Hammen
– David Hammen, Commented Apr 16, 2012 at 13:26
@trojanfoe i get " error: invalid conversion from ‘const char*’ to ‘uint32_t’ " — djfoxmccloud
– djfoxmccloud, Commented Apr 16, 2012 at 13:26

user703016 · Accepted Answer · 2012-04-16 13:39:11Z

7

As the compiler says, too many initializers for ‘dc_netif_filter::< anonymous union>’.

Initialize only one field, not both.

Use the name of the field to initialize it properly:

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME, { buf: "tun0" }};

edited Apr 16, 2012 at 13:39

answered Apr 16, 2012 at 13:24

user703016

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4 Comments

djfoxmccloud Over a year ago

If I initialize only the field i'm interested in with what is my edit (see the OP edited) I get a conversion error

Nick Over a year ago

I believe that this is a non-standard g++ extension.

user703016 Over a year ago

@Nick I have no idea. It's called Designated Initializers in ISO C99. Probably the same in C++ but I'm not certain.

djfoxmccloud Over a year ago

@Soohjun Thank you, the edited version works ! You sir, deserve some love ;)

Sergey Kalinichenko · Accepted Answer · 2012-04-16 13:34:40Z

2

It looks like you are trying to initialize your structure to indicate that the buf member is to be used, and that the value of that buf should be "tun". Since C++ before C++11 lacks designated initializers, you cannot do it with an initializer: only the first field of the union can be initialized, so you need to do your assignment in code:

static get_dc_netif_filter_t() {
    static c_netif_filter_t netif = {DC_NETIF_SELECT_NAME, {0}};
    if (netif.value.itf_nb == 0) {
        netif.value.buf = "tun";
    }
    return netif;
}

C++11 lets you do it like this:

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME, { .buf = "tun"}};

answered Apr 16, 2012 at 13:34

Sergey Kalinichenko

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3 Comments

djfoxmccloud Over a year ago

I get : expected primary-expression before ‘.’ token Is there a compiler option I am missing ?

Sergey Kalinichenko Over a year ago

@djfoxmccloud try -std=c++11

minexew Over a year ago

This is incorrect. Designated Initializers are not in C++11. They are currently proposed for C++20 (see open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0329r4.pdf). However, they do seem to currently work in GCC, Clang as well as MSVC.

trojanfoe · Accepted Answer · 2012-04-16 13:42:56Z

1

This works under g++ 4.7.0:

dc_netif_filter_t netif = {DC_NETIF_SELECT_NAME, {.buf="tun"}};

Although designated initializers are supposed to be C only, not C++! Maybe it's a GNU extension?

I guess the best (most compatible) way is to assign the members after initialization:

dc_netif_filter_t netif;
netif.itf_nb = DC_NETIF_SELECT_NAME;
netif.value.buf = "TUN";

edited Apr 16, 2012 at 13:42

answered Apr 16, 2012 at 13:34

trojanfoe

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3 Comments

djfoxmccloud Over a year ago

I get : expected primary-expression before ‘.’ token Is there a compiler option I am missing ?

trojanfoe Over a year ago

No, I used no command line options. I guess it must be dependent on compiler version; as I say it's not supposed to work in C++ anyway.

djfoxmccloud Over a year ago

I understand that it is not "supposed to work in C++", the thing is that I need to compile this code "as is" ;) With g++ 4.4.3 it doesn't work, i'll try upgrading

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