Union basics in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a union named Data.

C++

union [1] {
    int i;
    float f;
};

Drag options to blanks, or click blank then click option'

AMyUnion

Bdata

CData

DValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or different names like 'data' or 'MyUnion'.

Forgetting to write the union name after the keyword.

The union is declared with the name Data as required.

2fill in blank

medium

Complete the code to create a variable named d of the union Data.

C++

union Data d[1];

Drag options to blanks, or click blank then click option'

A;

B()

C{}

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or braces instead of a semicolon.

Omitting the semicolon at the end.

In C++, variables are declared with a semicolon ; at the end.

3fill in blank

hard

Fix the error in assigning the integer 10 to the union variable d.

C++

d.[1] = 10;

Drag options to blanks, or click blank then click option'

Af

Bi

Cvalue

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning to the float member f instead of i.

Using names not declared in the union.

The union has an integer member named i. Assigning to d.i stores the integer 10.

4fill in blank

hard

Fill both blanks to declare a union named Value with a char and a double member.

C++

union [1] {
    [2] c;
    double d;
};

Drag options to blanks, or click blank then click option'

AValue

Bint

Cchar

Dfloat

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong union names like 'value' or 'Val'.

Using wrong types like 'int' or 'float' for the char member.

The union is named Value. It has a char member named c and a double member named d.

5fill in blank

hard

Fill all three blanks to create a union named Data with an int member i, a float member f, and assign 3.14 to f.

C++

union [1] {
    int [2];
    float f;
};

[1] d;
d.[3] = 3.14f;

Drag options to blanks, or click blank then click option'

AData

Bi

Cf

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing member names or union names.

Assigning a float value to the int member.

The union is named Data. It has an int member i and a float member f. The variable d is of type Data. The assignment d.f = 3.14f; sets the float member.

Practice

1. What is the main characteristic of a union in C++?

easy

A. All members share the same memory location

B. Each member has its own separate memory

C. Unions can only store integers

D. Unions automatically initialize all members

Solution

1. Step 1: Understand union memory sharing

   A union allocates one shared memory block for all its members, so they overlap.

2. Step 2: Compare with other data structures

   Unlike structs, unions do not allocate separate memory for each member.

3. Final Answer:

   All members share the same memory location -> Option A

4. Quick Check:

   Union memory sharing = All members share the same memory location [OK]

Hint: Remember: union members overlap in memory [OK]

Common Mistakes:

• Thinking each member has separate memory
• Assuming unions initialize all members
• Believing unions only store one data type

2. Which of the following is the correct syntax to define a union named Data with an int and a float member?

easy

A. union Data = { int i; float f; };

B. struct Data { int i; float f; };

C. union Data ( int i; float f; );

D. union Data { int i; float f; };

Solution

1. Step 1: Recall union declaration syntax

   Unions are declared using the keyword union, followed by the name and braces with members inside.

2. Step 2: Check each option

   union Data { int i; float f; }; uses correct syntax with braces and semicolons. Options B, C, and D have syntax errors or wrong keywords.

3. Final Answer:

   union Data { int i; float f; }; -> Option D

4. Quick Check:

   Correct union syntax = union Data { int i; float f; }; [OK]

Hint: Use 'union Name { members; };' syntax [OK]

Common Mistakes:

• Using struct keyword instead of union
• Using parentheses instead of braces
• Assigning values in declaration

3. What will be the output of the following code?

union Number {
  int i;
  float f;
};

Number n;
n.i = 65;
std::cout << n.f << std::endl;

medium

A. A floating-point number representing the bits of 65

B. 65.0

C. 0

D. Compilation error

Solution

1. Step 1: Understand union memory sharing and type punning

   Assigning to n.i sets the bits of the union. Reading n.f interprets those bits as a float.

2. Step 2: Predict output

   The bits for integer 65 do not represent 65.0 as a float, so output is a float number corresponding to those bits, not 65.0.

3. Final Answer:

   A floating-point number representing the bits of 65 -> Option A

4. Quick Check:

   Union type punning = A floating-point number representing the bits of 65 [OK]

Hint: Union shares bits; int bits read as float show strange value [OK]

Common Mistakes:

• Expecting output 65.0
• Assuming union members hold independent values
• Thinking code causes compile error

4. Identify the error in this union usage:

union Data {
  int i;
  char c;
};

Data d;
d.i = 100;
d.c = 'A';
std::cout << d.i << std::endl;

medium

A. Output will be 100

B. Error: Cannot assign to union members separately

C. No error; output will be 65

D. Compilation error due to missing semicolon

Solution

1. Step 1: Understand union member assignment

   Assigning to d.i then to d.c overwrites the same memory because union members share memory.

2. Step 2: Determine final value of d.i

   After d.c = 'A', the first byte of d.i is set to ASCII 65, so printing d.i shows 65.

3. Final Answer:

   No error; output will be 65 -> Option C

4. Quick Check:

   Union last write wins = No error; output will be 65 [OK]

Hint: Last assigned union member overwrites memory [OK]

Common Mistakes:

• Thinking union members hold values independently
• Expecting output 100 after assigning 'A'
• Assuming syntax error

5. You want to store either an int, a float, or a char in the same memory space efficiently. Which union definition and usage correctly allows this, and how do you safely read the stored value?

hard

A. union Value { int i; float f; char c; }; Value v; v.i = 10; std::cout << v.c << std::endl;

B. union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl;

C. union Value { int i; float f; char c; }; Value v; v.c = 'Z'; std::cout << v.i << std::endl;

D. union Value { int i; float f; char c; }; Value v; std::cout << v.i << std::endl;

Solution

1. Step 1: Check union definition and assignment

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; defines a union with int, float, char and assigns a float value correctly.

2. Step 2: Verify safe reading of stored value

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; reads the same member v.f that was assigned, ensuring correct interpretation.

3. Step 3: Analyze other options

   Other options either assign to one member and read a different one, leading to unexpected values, or read uninitialized memory.

4. Final Answer:

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; -> Option B

5. Quick Check:

   Read the same union member you wrote [OK]

Hint: Always read the union member you last wrote [OK]

Common Mistakes:

• Reading a different member than assigned
• Using uninitialized union members
• Assuming union members hold independent values