Question

Recently I am working on windows and I found that lot of data structures are defined as struct with union as member variables. Example of this would be EVT_VARIANT in Windows.

I failed to understand what is the purpose behind this.

Answer 1

When a struct contains union members it’s generally done as a space saving mechanism. If the struct can be of certain sub-types by which only certain members are valid then a union is a good way to not waste space.

For example

enum NumberKind {
  Integer, 
  FloatingPoint 
};

struct Number {
  NumberKind kind;
  union {
    int integerValue;
    float floatValue;
  };
};

In this scenario I’ve defined a struct Number which can have type types of numeric values: floating point and integer. It’s not valid to have both at the same time so rather than waste space by having both members always defined I created a union which makes the storage of both equal to the size of the biggest.

Sample Usage of Above as requested

void PrintNumber(Number value) {
  if (value.kind == Integer) {
    printf("%d\n", value.integerValue);
  } else {
    printf("%f\n", value.floatValue);
  }
}