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

Answer 1

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" }}; 

Answer 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"}};