Passing parameters by reference in C++ - Step-by-Step Execution

Concept Flow - Passing parameters by reference

Function call with variable

↓

Parameter receives reference

↓

Function modifies parameter

↓

Original variable changed

↓

Function returns

↓

Program continues with updated variable

When a function parameter is passed by reference, the function works directly with the original variable, so changes inside the function affect the original.

Execution Sample

C++

void addOne(int &num) {
    num = num + 1;
}

int main() {
    int a = 5;
    addOne(a);
}

This code increases the value of 'a' by 1 using a function that takes 'a' by reference.

Execution Table

Step	Variable	Value Before	Action	Value After
1	a	5	Call addOne(a), num references a	5
2	num	5	Add 1 to num (num = num + 1)	6
3	a	5	num is reference to a, so a changes too	6
4	Function	Running	Return from addOne	End
5	a	6	Program continues with updated a	6

💡 Function ends, original variable 'a' is updated to 6 because 'num' was a reference.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
a	5	5	6	6
num	5	6	6	6

Key Moments - 2 Insights

Why does changing 'num' inside the function change 'a' outside?

Because 'num' is a reference to 'a' (see Step 1 and Step 3 in execution_table), both names point to the same memory, so changing 'num' changes 'a'.

What if we passed 'num' by value instead of reference?

If passed by value, 'num' would be a copy of 'a', so changes to 'num' inside the function would not affect 'a' (not shown in this table).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'a' after Step 3?

A 6

B 5

C 7

D Undefined

💡 Hint

Check the 'Value After' column for 'a' at Step 3 in execution_table.

At which step does the function add 1 to the parameter?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column describing 'Add 1 to num' in execution_table.

If 'num' was passed by value, how would 'a' change after the function call?

A 'a' would become 6

B 'a' would remain 5

C 'a' would become 0

D 'a' would be undefined

💡 Hint

Refer to key_moments explanation about passing by value vs reference.

Concept Snapshot

Passing parameters by reference in C++ uses & in the function parameter.
The function works with the original variable, not a copy.
Changes inside the function affect the original variable.
Syntax example: void func(int &x) { x = x + 1; }
Call with variable: func(a);
Useful to modify variables directly.

Full Transcript

This visual trace shows how passing parameters by reference works in C++. When the function addOne is called with variable 'a', the parameter 'num' becomes a reference to 'a'. Inside the function, adding 1 to 'num' changes 'a' directly because they share the same memory. The execution table tracks each step, showing 'a' changing from 5 to 6. Key moments clarify why changes affect the original variable and what would happen if passed by value instead. The quiz tests understanding of variable values at different steps and the difference between passing by reference and by value.