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Structure vs. Union: What's the Difference?

Edited by Aimie Carlson || By Harlon Moss || Updated on October 24, 2023
A structure allows multiple data types to be stored separately; a union uses shared memory for different data types.

Key Differences

A structure, in programming contexts, is a composite data type that groups together variables of different data types under a single type name. In contrast, a union is also a composite data type but it allows different variables to share the same memory location.
While a structure allocates separate memory for each of its members, a union allocates the largest memory required by its members and allows them to overlap. Thus, a union's size is determined by its largest member, whereas a structure's size is the sum of the sizes of all its members.
In terms of functionality, a structure allows for the simultaneous storage of multiple data values. In a union, since memory is shared, only one of its members can hold a value at any given time.
The primary use of structures is to represent records. For example, a structure could represent a student with a name, age, and grade. Unions, on the other hand, are used to save memory when only one variable out of several is in use at any moment.
It's crucial for programmers to note that while structures are widely used in various applications, unions are specialized tools with limited use-cases, mainly revolving around memory optimization.

Comparison Chart

Memory Allocation

Allocates memory for all members
Allocates memory for the largest member only

Member Usage

All members can hold values simultaneously
Only one member can hold a value at a time


Sum of sizes of all members
Size of the largest member

Common Use

Represent records or grouped data
Memory optimization when only one variable is used


Student: name, age, grade
Value: integer or float, but not both at once

Structure and Union Definitions


A framework or arrangement of interconnected elements.
The building's structure was complex yet sturdy.


The act or state of joining or being joined.
The union of the two companies was surprising.


A composite data type in programming that groups variables.
The structure Student holds name, age, and grade.


A political unit formed from previously separate units.
The European Union comprises multiple countries.


An organized system or method.
The company's hierarchical structure was clear.


A state of harmony or agreement.
Their marriage was a union of love and respect.


The arrangement and relationship of parts in a whole.
The narrative structure of the novel was unique.


A composite data type in programming allowing shared memory usage.
The union Value can be an integer or float.


A building or constructed object.
The Eiffel Tower is a renowned structure in Paris.


An organization of workers formed for mutual benefit.
The worker's union advocated for better wages.


Something made up of a number of parts that are held or put together in a particular way
Hierarchical social structure.


The act of uniting or the state of being united.


What is the primary use of a union?

The primary use of a union is memory optimization when only one variable is used at a time.

What is a structure in programming?

A structure is a composite data type grouping variables of different data types.

How does memory allocation differ in structures and unions?

Structures allocate memory for all members, while unions allocate memory only for the largest member.

Is the size of a structure always fixed?

Typically, yes, the size of a structure is fixed once defined, but can vary depending on compiler and alignment considerations.

What is a "union" in mathematics?

In mathematics, a "union" refers to the combination of two sets, containing all elements from both sets.

What is a "data structure"?

A data structure is a way of organizing and storing data for efficient access and modification.

Can all members of a structure hold values at the same time?

Yes, all members of a structure can simultaneously hold values.

What is a union in the context of organizations?

A union, in organizations, is a group of workers formed for mutual benefit, often for bargaining rights.

Does the order of declaration matter in structures?

In structures, the order of declaration doesn't affect memory allocation.

Can all members of a union hold values simultaneously?

No, only one member of a union can hold a value at any given time.

What's a common real-world example of a structure?

A common example is a "Student" structure with name, age, and grade fields.

Can unions and structures be nested?

Yes, both unions and structures can be nested within one another in programming.

Does the order of declaration matter in unions?

In unions, order doesn't affect memory allocation, but may affect usage.

What is meant by "structure" in literature?

In literature, "structure" refers to the arrangement and relationship of parts in a narrative or composition.

How is the term "union" used politically?

Politically, "union" can refer to a political unit formed from previously separate entities, like the European Union.

Can structures be self-referential?

Yes, structures can be self-referential, often using pointers in languages like C.

How does the size of a structure compare to a union?

A structure's size is the sum of its members, while a union's size is that of its largest member.

Are unions common in object-oriented programming?

No, unions are less common in object-oriented programming compared to structures.

Are unions and structures exclusive to C and C++?

No, while prominent in C and C++, similar concepts exist in other languages, albeit with different implementations or names.

Which is more commonly used, structures or unions?

Structures are more commonly used than unions in most applications.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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