CppHow-ToBeginner · 3 min read

How to Pass by Reference in C++: Syntax and Examples

In C++, you pass by reference by using & in the function parameter, like void func(int &x). This allows the function to modify the original variable directly without copying it.

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Syntax

To pass a variable by reference, add & after the type in the function parameter. This means the function receives the original variable, not a copy.

Type &name: Declares a reference parameter.
Function call: Pass the variable normally without &.

cpp

void updateValue(int &num) {
    num = 10; // changes original variable
}

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Example

This example shows how a function changes the original variable by passing it by reference.

cpp

#include <iostream>

void updateValue(int &num) {
    num = 10; // modifies original variable
}

int main() {
    int a = 5;
    std::cout << "Before: " << a << std::endl;
    updateValue(a);
    std::cout << "After: " << a << std::endl;
    return 0;
}

Output

Before: 5 After: 10

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Common Pitfalls

Common mistakes when passing by reference include:

Forgetting the & in the function parameter, which causes pass-by-value (copy) instead.
Trying to pass literals or temporary values by reference, which is not allowed.
Confusing pointers and references; references are safer and simpler to use.

cpp

/* Wrong: pass by value, original not changed */
void wrongFunc(int num) {
    num = 20;
}

/* Correct: pass by reference, original changed */
void correctFunc(int &num) {
    num = 20;
}

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Quick Reference

Concept	Syntax	Effect
Pass by value	void func(int num)	Function gets a copy, original not changed
Pass by reference	void func(int &num)	Function gets original, can modify it
Pass by const reference	void func(const int &num)	Function gets original, cannot modify it

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Key Takeaways

Use & in function parameters to pass variables by reference in C++.

Passing by reference lets functions modify the original variable directly.

Always pass variables normally when calling; no special syntax needed.

Avoid passing literals or temporary values by non-const reference.

Use const references to avoid copying without allowing modification.