THINK++ — Engineering Visualized

Memory Overlay

C++ Unions

A union is a special data type where all members share the SAME memory location. It can hold only one of its members at a time.

DEFINITION

union Data {
    int i;
    float f;
    char str[20];
};

Data d;
d.i = 10; // Memory now holds an int

ONE AT A TIME

Setting a new member overwrites the previous one. Accessing a member that wasn't set last results in undefined data.

SIZE EFFICIENCY

The size of a union is the size of its LARGEST member. Ideal for memory-constrained systems.

Use Case: Variants

Modern C++ usually prefers std::variant over unions as it's type-safe and "knows" which member is currently active.

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