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Pass by Value vs Reference

Last Updated: January 3, 2026

6 min read

Understanding how C++ handles function arguments is essential for writing efficient and effective code. One of the key concepts in this area is the difference between pass by value and pass by reference.

These two techniques determine how arguments are passed to functions and can significantly impact both performance and behavior.

Let’s dive into the details of each approach, understand their implications, and look at how you can leverage them in your C++ functions.

Pass by Value

When you pass arguments by value, you are sending a copy of the actual data to the function. This means that any changes made to the parameter within the function do not affect the original argument. Here’s a simple illustration:

In this example, originalValue remains unchanged after calling modifyValue. The function works with a copy of the value, so modifications have no side effects on the original variable.

Pros:

  • Safety: Since the original data is not modified, this method is less error-prone.
  • Simplicity: It’s straightforward to understand and implement.

Cons:

  • Performance: For large objects, the overhead of copying data can be significant, leading to slower execution times.
  • Memory Usage: Each call creates a new copy of the data, which can consume more memory.

Practical Use Cases

Pass by value is appropriate when:

  • You’re working with primitive data types (like int, char, etc.), where the size is small.
  • The function should not modify the original data.
  • You want to ensure that the original variable remains unchanged during the function execution.

Pass by Reference

In contrast, passing by reference sends a reference (or address) to the actual variable, rather than copying its value. This means that modifications made to the parameter inside the function will affect the original argument. Here’s how it looks in code:

Here, originalValue is modified because modifyValue works with a reference to it. Consequently, any changes made to num directly affect originalValue.

Pros:

  • Efficiency: No need to copy large objects, which saves time and memory.
  • Direct Modification: Allows functions to modify the original variables, which can be beneficial in many scenarios.

Cons:

  • Unintended Side Effects: Since the original data can be modified, it can lead to unexpected behavior if not carefully managed.
  • Complexity: Understanding references can be more challenging for beginners.

Practical Use Cases

Pass by reference is ideal when:

  • You need to modify the original variable within the function.
  • You’re dealing with large objects, like classes or structures, where copying would be inefficient.
  • You want to return multiple values from a function without using a return statement.

Comparing Pass by Value and Reference

Now that we understand both methods, let’s compare them more directly. Here’s a quick summary of the differences:

Scroll
Feature
Pass by Value
Pass by Reference

Data Modification

Original data remains unchanged

Original data can be modified

Performance

Can be slow for large objects

Generally faster for large objects

Memory Usage

Uses more memory for copies

More memory efficient

Safety

Safer, less prone to side effects

Risk of unintended modifications

Code Example: Use Case Comparison

Let’s see a code example that illustrates both methods in a single scenario:

In this example, updateVectorByValue does not affect originalVec, while updateVectorByReference changes it. This demonstrates how the choice between the two can lead to different behaviors in your programs.

When to Use Each Method

Choosing between pass by value and pass by reference generally depends on the context and requirements of your function. Here are some guidelines to help you decide:

Use Pass by Value When:

  • Working with small data types where performance is not a concern.
  • You want to ensure that the function does not alter the input.
  • You prefer safety and simplicity in function design.

Use Pass by Reference When:

  • You are handling large objects or data structures to avoid costly copies.
  • You want the function to modify the input data.
  • You need to return multiple values to the caller.

Edge Cases and Nuances

Be cautious when using pass by reference with temporary objects, as they can lead to dangling references. Consider this code:

However, if you try to modify a temporary object, you might inadvertently create undefined behavior. Therefore, always ensure that the referenced object outlives the function call.

Conclusion and Best Practices

Understanding pass by value and pass by reference is crucial for writing efficient, effective C++ code. Here are some best practices to keep in mind:

  • Always consider performance implications when passing large data structures.
  • Be mindful of potential side effects when passing by reference, especially in larger codebases.
  • Use const references when you don’t intend to modify the input but still want to avoid copying.

By applying these concepts and practices, you’ll write cleaner, more efficient functions that behave exactly as you intend.