Union (C++)

A union in C++ is a user-defined type that can hold different types of data, but only one type at a time.

It’s like a struct, but all members share the same memory location.

union Number {
    int intVal;
    float floatVal;
};
 
Number num;
num.intVal = 5; // num holds an integer
num.floatVal = 7.5; // now num holds a float, and the integer value is overwritten

Resources

• https://www.geeksforgeeks.org/structures-unions-and-enumerations-in-cpp/

Can different member fields are different sizes?

Yes, different members of a union can have different sizes in bits. The size of the union is determined by the size of its largest member.

An example for exception handling

optional<int*> Malloc(size_t size) {
  if (random() % 2) return (int*)malloc(sizeof(int));
  return nullopt;  // no storage
}
optional<int> rtn() {
  optional<int*> p = Malloc(sizeof(int));
  if (!p) return nullopt;  // malloc successful (true/false) ?
  **p = 7;                 // compute
  if (random() % 2) return **p;
  return nullopt;  // bad computation
}
 
int main() {
  srandom(getpid());
  optional<int> ret = rtn();
  if (ret) cout << *ret << endl;  // rtn successful?
}

Related

• Struct
• variant (C++)