What Is a Union Data Structure?


Scott Campbell

A union data structure is a powerful tool in programming that allows you to store different types of data within the same memory location. It is similar to a struct, but with one key difference – only one member of a union can be used at a time.

Why Use a Union?

Unions are particularly useful when you need to optimize memory usage or work with data that can be represented in different ways. By sharing memory space, unions enable you to save memory by using the same space for different types of data.

Defining a Union

To define a union in C++, you use the union keyword followed by the name of the union and its members enclosed in curly braces. For example:

    union MyUnion {
        int intValue;
        float floatValue;
        char charValue;

In this example, we have defined a union named MyUnion, which contains three members: an integer (intValue), a float (floatValue), and a character (charValue). These members share the same memory space.

Accessing Union Members

To access the members of a union, you use the dot (.) operator.

However, it’s important to note that only one member should be accessed at any given time. Attempting to access multiple members simultaneously can lead to unpredictable results.

    MyUnion myUnion;
    myUnion.intValue = 42; // Accessing intValue
    std::cout << "Value: " << myUnion.intValue << std::endl;

    myUnion.floatValue = 3.14; // Accessing floatValue
    std::cout << "Value: " << myUnion.floatValue << std::endl;

    myUnion.charValue = 'A'; // Accessing charValue
    std::cout << "Value: " << myUnion.charValue << std::endl;

In this example, we create an instance of the MyUnion union called myUnion. We then assign values to its members and access them using the dot (.

Memory Layout

The memory layout of a union is determined by its largest member. The size of a union is equal to the size of its largest member. For example, if the float member in our MyUnion example above requires 4 bytes and the other members require less, the size of the entire union will be 4 bytes.

Common Use Cases

Unions are commonly used in scenarios where you need to represent data in different ways but want to save memory. Some common use cases for unions include:

  • Parsing binary data: Unions can be used to interpret raw binary data in different formats such as integers, floats, or characters.
  • Variants: Unions can be used to implement variant types where only one type is active at any given time.
  • Data type conversions: Unions can facilitate conversions between different data types by providing a shared memory space.

Cautions and Considerations

While unions offer flexibility and memory optimization, they also require careful handling. Here are some considerations when using unions:

  • Data integrity: Since only one member is active at a time, it is essential to keep track of the currently active member to avoid accessing the wrong data.
  • Alignment: Depending on the platform and compiler, unions may have alignment requirements, which can affect their memory layout and size.
  • Type safety: Unions bypass type checking, so it's crucial to ensure that the accessed member matches the expected type.


A union data structure provides a powerful mechanism for storing different types of data within the same memory space. By allowing you to optimize memory usage and work with data in multiple representations, unions are an important tool in programming. However, caution must be exercised when working with unions to ensure proper data access and maintain type safety.

Now that you have a solid understanding of what a union is and how it can be used effectively, you can start incorporating this powerful tool into your programming projects.

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