Type casting in C++ - Step-by-Step Execution

Concept Flow - Type casting

Start with a value

↓

Choose target type

↓

Apply cast syntax

↓

Convert value to target type

↓

Use converted value

↓

End

Type casting changes a value from one type to another using explicit syntax, allowing different operations or storage.

Execution Sample

C++

int main() {
  double pi = 3.14;
  int intPi = (int)pi;
  return intPi;
}

This code converts a double value 3.14 to an int by cutting off the decimal part.

Execution Table

Step	Variable	Original Value	Cast Type	Result after Cast	Explanation
1	pi	3.14 (double)	-	3.14 (double)	Initial double value assigned
2	intPi	3.14 (double)	int	3 (int)	Cast double to int truncates decimal
3	return	intPi	-	3	Return the integer value
4	-	-	-	-	Program ends

💡 Program ends after returning the casted integer value 3

Variable Tracker

Variable	Start	After Cast	Final
pi	3.14 (double)	3.14 (double)	3.14 (double)
intPi	uninitialized	3 (int)	3 (int)

Key Moments - 3 Insights

Why does casting from double to int remove the decimal part?

Casting from double to int truncates the decimal part instead of rounding, as shown in step 2 of the execution_table where 3.14 becomes 3.

Is the original variable 'pi' changed after casting?

No, 'pi' remains a double with value 3.14. The cast creates a new int value in 'intPi' as shown in variable_tracker.

What happens if you cast an int to a double?

Casting int to double converts the integer to a floating-point number with decimal zero, e.g., 3 becomes 3.0, but this example shows double to int.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'intPi' after casting?

A 3.14

B 3

C 0

D 4

💡 Hint

Check step 2 in the execution_table where 'intPi' is assigned after casting.

At which step does the decimal part get removed?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Explanation' column in execution_table for when casting happens.

If we changed the cast to (float) instead of (int), what would 'intPi' hold?

A 0

B 3.14 (float)

C 3 (int)

D Error

💡 Hint

intPi is an int variable, so even casting to float first, it truncates the decimal to 3.

Concept Snapshot

Type casting in C++ changes a value's type explicitly.
Syntax: (target_type) value or static_cast<target_type>(value).
Casting double to int truncates decimals.
Original variable stays unchanged.
Useful to convert types for operations or storage.

Full Transcript

This example shows type casting in C++. We start with a double variable pi holding 3.14. We cast it to int using (int)pi, which removes the decimal part and stores 3 in intPi. The original pi remains 3.14. The program returns intPi which is 3. This shows how casting changes the type and value representation explicitly.

Practice

1. What does static_cast<int>(3.14) do in C++?

easy

A. Leaves the value unchanged as 3.14

B. Converts the integer 3 to the floating-point number 3.14

C. Causes a runtime error

D. Converts the floating-point number 3.14 to the integer 3

Solution

1. Step 1: Understand static_cast usage

   static_cast<int> converts a value to an integer type explicitly.

2. Step 2: Apply static_cast to 3.14

   The floating-point number 3.14 is converted to integer by dropping the decimal part, resulting in 3.

3. Final Answer:

   Converts the floating-point number 3.14 to the integer 3 -> Option D

4. Quick Check:

   static_cast<int>(3.14) = 3 [OK]

Hint: static_cast truncates decimals when converting float to int [OK]

Common Mistakes:

• Thinking it rounds the number instead of truncating
• Confusing static_cast with dynamic_cast
• Expecting a runtime error from static_cast

2. Which of the following is the correct syntax for casting a double variable d to int using static_cast?

easy

A. int i = (int)static_cast(d);

B. int i = static_cast<int>(d);

C. int i = static_cast(d, int);

D. int i = static_cast<int>d;

Solution

1. Step 1: Recall static_cast syntax

   The correct syntax is static_cast<type>(expression).

2. Step 2: Match syntax with options

   int i = static_cast<int>(d); matches the correct syntax: int i = static_cast<int>(d);.

3. Final Answer:

   int i = static_cast<int>(d); -> Option B

4. Quick Check:

   Correct static_cast syntax = int i = static_cast<int>(d); [OK]

Hint: Use angle brackets for type in static_cast [OK]

Common Mistakes:

• Using parentheses instead of angle brackets for type
• Placing type after variable instead of inside <>
• Mixing C-style cast syntax with static_cast

3. What is the output of the following C++ code?

double x = 9.99;
int y = static_cast<int>(x);
std::cout << y << std::endl;

medium

A. 9.99

B. 10

C. 9

D. Compilation error

Solution

1. Step 1: Understand static_cast behavior on double to int

   static_cast truncates the decimal part, so 9.99 becomes 9.

2. Step 2: Predict output of std::cout

   The integer 9 is printed to the console.

3. Final Answer:

   9 -> Option C

4. Quick Check:

   static_cast<int>(9.99) = 9 [OK]

Hint: Casting double to int drops decimals, no rounding [OK]

Common Mistakes:

• Expecting rounding to 10 instead of truncation
• Confusing output type with original double
• Thinking static_cast causes compile error here

4. Identify the error in this code snippet:

float f = 3.5f;
int i = static_cast<int> f;
std::cout << i;

medium

A. Missing parentheses after static_cast<int>

B. Cannot cast float to int using static_cast

C. Variable i is not declared

D. Missing semicolon after float declaration

Solution

1. Step 1: Check static_cast syntax

   static_cast requires parentheses around the value to cast, like static_cast<int>(f).

2. Step 2: Identify missing parentheses

   The code misses parentheses after static_cast<int>, causing syntax error.

3. Final Answer:

   Missing parentheses after static_cast<int> -> Option A

4. Quick Check:

   static_cast<int>(f) needs parentheses [OK]

Hint: Always use parentheses after static_cast type [OK]

Common Mistakes:

• Omitting parentheses after static_cast
• Thinking static_cast can't convert float to int
• Ignoring compiler error messages

5. Given the code below, what will be the output?

int a = 65;
char c = static_cast<char>(a);
std::cout << c << std::endl;

Note: ASCII value 65 corresponds to 'A'.

hard

A. 'A'

B. Compilation error

C. 65

D. Undefined behavior

Solution

1. Step 1: Understand static_cast from int to char

   static_cast converts the integer 65 to its corresponding ASCII character 'A'.

2. Step 2: Predict output of std::cout

   Printing the char variable outputs the character 'A'.

3. Final Answer:

   'A' -> Option A

4. Quick Check:

   static_cast<char>(65) = 'A' [OK]

Hint: Casting int to char prints ASCII character [OK]

Common Mistakes:

• Expecting numeric 65 to print instead of 'A'
• Thinking casting int to char causes error
• Confusing char with string output