What is the Intended Usage of Unions in C and C and How to Avoid Undefined Behavior?

The Intended Usage of Unions in C and C

Question:

Although I have used unions before, recent concerns have emerged regarding their alleged undefined behavior. In particular, can someone clarify the intended usage of unions, especially with regards to accessing non-active members?

Purpose of Unions:

Contrary to common misconceptions, the primary purpose of unions is not for "type conversion." Instead, their main objective is to conserve memory by allowing multiple data structures to share the same memory space at different times.

This memory-saving technique is analogous to a hotel where guests occupy rooms during separate intervals without ever interacting or being aware of each other's presence. Similarly, in a union, only one member can be active at any given time.

Non-Active Member Access:

accessing non-active union members is generally undefined behavior in both C and C . This practice, often referred to as "type punning," can lead to unpredictable outcomes. In C, while it may be technically permitted by the standard, accessing a non-active member can still result in undefined behavior if the retrieved value is invalid for the target data type.

Intended Usage:

The correct usage of unions involves:

• Identifying data structures with non-overlapping lifespans that can be stored in the same memory region.
• Writing to the active member to set its value, effectively making it the active member.
• Reading only the active member to obtain its current value.

By following these guidelines, programmers can harness the memory conservation benefits of unions without encountering undefined behavior.

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