Initializing union in a structure - c++ - c++

Question

my code:
#include <iostream>
using namespace std;
struct widget
{
char brand[20];
int type;
union id
{
long id_num;
char id_char[20];
}id_val;
};
int main()
{
widget prize =
{"Rolls", 0, "A2X"};
return 0;
}
The problem is with initialization "A2X" when initializing a union in a structure. Compiler doesn't know I want to choose second option with array of chars when I am passing "A2X", it's requiring long type. When I put
char id_char[20]
before
long id_num
everything is ok. But I want to know how to enforce compiler to accept "A2X" with char as the second option in union. Thank for your help.

Answer 1

But I want to know how to enforce compiler to accept "A2X" with char as the second option in union.
You can use a constructor:
id(char const *id_char) {
std::strcpy(this->id_char, id_char);
}
Alternatively you could use a widget constructor.
A drawback is that the compiler probably won't be able to warn you if you use a too large input string for initialization. The shown trivial constructor can be expanded with strlen to check overflow at runtime. I suggest throwing an exception if you choose to check.

Answer 2

This works with -std=c++11:
#include <cstring>
#include <stdexcept>
struct widget
{
char brand[20];
int type;
union id
{
long id_num;
char id_char[20];
}id_val;
widget(char const*Str, int Type, char const *Id);
};
widget::widget(char const*Str, int Type, char const *Id)
{
if (strlen(Str)+1 > sizeof brand)
throw std::length_error{"brand too large"};
memcpy(brand,Str,strlen(Str)+1);
type = Type;
if (strlen(Id)+1 > sizeof id_val.id_char)
throw std::length_error{"id too large"};
memcpy(id_val.id_char,Id,strlen(Id)+1);
}
int main()
{
widget prize = {"Rolls", 0, "A2X"};
return 0;
}