Scope of variables in C++ - Step-by-Step Execution

Concept Flow - Scope of variables

Start Program

↓

Declare Global Variable

↓

Enter main()

↓

Declare Local Variable

↓

Use Variables

↓

Enter Block (if/for)

↓

Declare Block Variable

↓

Use Variables in Block

↓

Exit Block

↓

Use Variables in main()

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End Program

This flow shows how variables declared in different places have different visibility and lifetime during program execution.

Execution Sample

C++

int x = 10; // global

int main() {
  int x = 20; // local
  if (true) {
    int x = 30; // block
    // print x
  }
  // print x
}

This code shows three variables named x in different scopes: global, local in main, and block inside if.

Execution Table

Step	Scope	Variable x Value	Action	Output
1	Global	10	Declare global x=10
2	main() local	20	Declare local x=20
3	if block	30	Declare block x=30
4	if block	30	Print x inside block	30
5	Exit block	20	Block x destroyed, back to main local x
6	main() local	20	Print x inside main	20
7	End	10	Global x still exists but not used here

💡 Program ends after main() finishes; block variable x is destroyed after block ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
Global x	10	10	10	10	10
main() local x	N/A	20	20	20	20
block x	N/A	N/A	30	N/A	N/A

Key Moments - 3 Insights

Why does the print inside the block show 30, not 20 or 10?

Because inside the block, the block variable x=30 shadows the local and global x. See execution_table step 4.

After the block ends, which x is used when printing?

The block variable x is destroyed after the block ends, so the local x=20 in main() is used again. See execution_table step 6.

Is the global x accessible inside main()?

Yes, but it is shadowed by the local x=20, so using x refers to the local one inside main(). See variable_tracker for values.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x printed inside the if block (step 4)?

A 20

B 30

C 10

D Undefined

💡 Hint

Check execution_table row with step 4 under 'Output' column.

At which step does the block variable x get destroyed?

A Step 5

B Step 3

C Step 6

D Step 7

💡 Hint

Look at execution_table step 5 description about exiting the block.

If we remove the local x=20 in main(), what value would be printed at step 6?

A 30

B 20

C 10

D Error

💡 Hint

Without local x, main() uses global x=10. See variable_tracker for global x value.

Concept Snapshot

Scope of variables in C++:
- Global variables declared outside functions are accessible everywhere unless shadowed.
- Local variables declared inside functions hide global variables with same name.
- Block variables declared inside blocks (if, for) hide outer variables and exist only inside block.
- When block ends, block variables are destroyed and outer variables become visible again.
- Variable name resolution uses closest scope first.

Full Transcript

This visual execution shows how variables named x declared in different scopes behave in C++. The global x=10 exists throughout the program. Inside main(), a local x=20 hides the global x. Inside the if block, a block x=30 hides both local and global x. Printing inside the block shows 30. After the block ends, the block x is destroyed, so printing in main() shows 20. The global x remains but is shadowed by local variables. This demonstrates variable scope and shadowing clearly.