Question

I have a question about the following code in C++:

typedef struct {
    int id;
    int age;
} Group1;


typedef struct {
    int id;
    char name;
    float time;  
} Group2;


typedef union {
    Group1 group1;
    Group2 group2;
} ServiceData;

typedef struct {
    ServiceData data;
} Time;

Then I have a variable:

Group1 * group1;
group1 = new Group1;

group1->id = 10;
group1->age = 20;

Then there are two methods defined like this:

void method1(ServiceData * data) {
    //inside the method call method hello
    hello(data);
};

void hello(Group1 *group1) {
    printf("%d",group1->id);
}

I call method1 like this:

method1((ServiceData *)group1);

But inside method1, when the parameter group1 is passed to method hello(), I want to get the value of id inside of group1. Do I need to do any cast in hello method? Or inside of method1, do I need to cast it to (group*) before I pass it to hello()?

Answer 1

You don’t need a cast, just to access the correct field in the union:

void method1(ServiceData * data) {
    //inside the method call method hello
    hello(&data->group1);
};