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Struct and union

  • C course
  • Intermediate
  • Lesson 38/116
  • 5 min

Learn to use union inside a struct.

Struct and union

By putting a union inside a struct, you create a tagged union. The tag tells which union member is active. This is the standard pattern for safe union use.

Example: enum Type { INT_T, FLOAT_T };, union Value { int i; float f; }; and struct TaggedValue { enum Type type; union Value value; };. You set the tag with tv.type = INT_T; and the value with tv.value.i = 42;. When reading, you first check type and read the right member. This pattern is common for data formats, protocol messages and AST nodes.

Type selection with switch

To read a tagged union safely, you check the tag with a switch. Each case reads the matching member. This prevents reading the wrong member.

Example: switch (tv.type) { case INT_T: printf("%d\n", tv.value.i); break; case FLOAT_T: printf("%f\n", tv.value.f); break; }. Without break you get fall-through; that's usually a bug. A default branch catches unexpected tags. This pattern is the standard read form for tagged unions in real-world code; interpreters and serializers use it.

Practice Print with a tagged union

Define enum Type { INT_T, FLOAT_T };, union Value { int i; float f; }; and struct TaggedValue { enum Type type; union Value value; };. In main, write struct TaggedValue tv;; set tv.type = INT_T; and tv.value.i = 42;. Add switch (tv.type) with case INT_T: printf("%d\n", tv.value.i); break;.

Starter code

main.c
#include <stdio.h>

// Define enum Type, union Value and struct TaggedValue
// In main declare tv, tag as INT_T, set i to 42
// Select type with switch and print

int main(void) {
    return 0;
}

Sample solution

main.c
#include <stdio.h>

enum Type { INT_T, FLOAT_T };
union Value { int i; float f; };
struct TaggedValue { enum Type type; union Value value; };

int main(void) {
    struct TaggedValue tv;
    tv.type = INT_T;
    tv.value.i = 42;
    switch (tv.type) {
        case INT_T: printf("%d\n", tv.value.i); break;
    }
    return 0;
}

Expected output

42

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Tagged union member access

Question 
struct TaggedValue tv;
tv.type = INT_T;
tv.value____ = 42;
Answer 
struct TaggedValue tv;
tv.type = INT_T;
tv.value.i = 42;

Explanation: tv.value is a value; access with .i.

2 Match the tagged union parts

Term Meaning
enum Type type Tag field
union Value value Value field
switch (tv.type) Type selection
tv.value.i int member access

3 Why is a tag used in a tagged union?

  • For a compiler error
  • For memory savings
  • For faster access
  • To know which member is active Correct answer

Explanation: The tag tells which union member is active.

All lessons in Intermediate

1 Advanced pointers

  1. Pointer to pointer
  2. Array of pointers
  3. Function pointers
  4. Function pointer parameter
  5. void pointer
  6. Summary - Advanced pointers

2 Memory management

  1. Dynamic array
  2. Resize with realloc
  3. Memory leaks
  4. Dangling pointers
  5. Allocation patterns
  6. Summary - Memory management

3 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Array helpers
  5. Putting it together

4 Advanced string handling

  1. Dynamic strings
  2. String concatenation
  3. String search
  4. String conversion
  5. String safety
  6. Summary - Advanced strings

5 Function pointers

  1. Function pointer definition
  2. Calling function pointers
  3. qsort
  4. Callback patterns
  5. Function pointer array
  6. Summary - Function pointers

6 Advanced structs

  1. Structs and pointers
  2. Dynamic struct
  3. Struct and function
  4. Nested structs
  5. Array of structs
  6. Summary - Advanced structs

7 Unions and enums

  1. union
  2. enum
  3. Struct and union
  4. typedef enum
  5. Bit fields
  6. Summary - Unions and enums

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 File I/O

  1. FILE and fopen
  2. fprintf and fscanf
  3. fgets and fputs
  4. fclose
  5. fread and fwrite
  6. Summary - File I/O

10 Error handling

  1. errno
  2. perror
  3. strerror
  4. assert
  5. Error return values
  6. Summary - Error handling

11 Multi-file projects

  1. Headers and sources
  2. extern
  3. static
  4. Makefile basics
  5. Multiple modules
  6. Summary - Multi-file

12 Project: Library app

  1. Model design
  2. Library module
  3. Checking out
  4. Search and filter
  5. Putting it together

13 Dynamic data structures

  1. Linked list
  2. Adding to list
  3. Traversing list
  4. Deleting from list
  5. Stack and queue
  6. Summary - Dynamic data

14 Variable arguments

  1. stdarg.h
  2. va_start and va_end
  3. printf-like function
  4. vsnprintf
  5. Variadic patterns
  6. Summary - Variable arguments

15 Standard library

  1. stdlib.h
  2. string.h
  3. math.h
  4. time.h
  5. ctype.h
  6. Summary - Standard library

16 Command-line arguments

  1. argc and argv
  2. Argument processing
  3. getopt
  4. Argument validation
  5. Exit codes
  6. Summary - Command line

17 Debugging

  1. gdb basics
  2. Breakpoints
  3. Watch and print
  4. valgrind
  5. printf debugging
  6. Summary - Debugging

18 Advanced bitwise

  1. Bit masking
  2. Set and clear bits
  3. Bit shifting
  4. Bit hacks
  5. Pack and unpack
  6. Summary - Advanced bitwise

19 Advanced typedef

  1. typedef and struct
  2. typedef and function pointer
  3. typedef and array
  4. Opaque types
  5. typedef and enum
  6. Summary - Advanced typedef

20 Intermediate final

  1. Project design
  2. Data model
  3. Functional logic
  4. File I/O
  5. Putting it together