Question

The classes and structs have one difference between them (as far as I know), that the struct defaults to public and class defaults to private. And then I came to know that there is a similar kind of data type which is also used in a similar manner, that is union. The union can not be used as a base class in inheritance (i don’t know what that means, but I still accept it).

I wanted to know whether there are some particular instances, where struct/ union/ class, or they can be used interchangeably (except for the cases I enlisted)? Please do tell me if I am wrong somewhere.
Regards

Answer 1

My use of class, struct and union is the following:

• class for objects that have behaviour.
• struct for passive data.
• union for very special cases where different data requires to be accessed as different types.

I’ve read this (except the union point) in the Google C++ Style guide a long time ago and I was following it since then.

Using structs to carry passive data (objects without behaviour attached to the object) have the advantage of default publicness of the members, so they can be accessed without Getters and Setters. If some member data needs to be checked/modified before assign or some member data needs to be computed/modified before be getted, IMHO they need a Setter/Getter pair and the object is a class instead of a struct.

For the union type, I find it useful for some kind of data structures that requires some weird access to the members, or needs some members to be treated as another type in some contexts. For example a 3D vector or a IP address:

union 3DVector
{
    double x, y, z;
    double vector[3];
} v;

// Acess members with name
v.x = 6.0; v.y = 7.0; v.z = 8.0;
// Acess members as a vector
Normalize(v.vector);

union IPAddress
{
    int binary;
    char octet[4];
} ip;

// Acess the binary address
std::cout << std::hex << ip.binary << '\n';
// Print in a human-readable form
std::cout << static_cast<int>(ip.octet[0]) << '.'
          << static_cast<int>(ip.octet[1]) << '.'
          << static_cast<int>(ip.octet[2]) << '.' 
          << static_cast<int>(ip.octet[3]) << '\n';

The above functionality could be achieved overloading operators and conversion operators, but the union approach looks neat for me.

The unions can also be templated and can have constructor/destructor, this could be useful for serialization purposes (not for all kind of objects):

template <typename T> union Serializer
{
    Serializer(const T &o) : object(o) {}
    T object;
    char binary[sizeof(T)];
};

SomePODObject obj; // Only POD objects please!
Serializer s(obj);
SendBuffer(s.binary);