How to pass arguments by reference vs. by value in C
In C, the method of passing parameters affects performance, security and modification of original data: use the value when passing basic types or when there is no modification, use the reference when large objects and when modifying, use the reference when reading large objects, and use const reference when reading large objects, avoid returning references to local variables to ensure efficiency and security.
In C , how you pass arguments to functions—by value or by reference—affects performance, safety, and whether the original data can be modified. Understanding the difference is key to writing efficient and correct code.
Passing by Value
When you pass an argument by value, a copy of the actual argument is made and used inside the function. Any changes to the parameter inside the function do not affect the original variable.
- The function receives a copy of the data.
- Original data is safe from modification .
- Suitable for small data types like
int,double, orbool.
void increment(int x) {
x ; // Only modify the copy
}
int main() {
int a = 5;
increment(a);
// a is still 5
}Use pass-by-value when:
- The data type is small (eg, primitives).
- You don't want the function to alter the original value.
- You don't mind the overhead of copying (minimal for small types).
Passing by Reference
Passing by reference means the function works directly with the original variable. No copy is made—the parameter acts as an alias to the original.
- Use the
&symbol in the parameter declaration. - Changes inside the function affect the original variable .
- Avoids copying large objects (eg, objects, structs, containers).
void increment(int& x) {
x ; // Modifies the original
}
int main() {
int a = 5;
increment(a);
// a is now 6
}Use pass-by-reference when:
- You need to modify the original variable.
- Working with large objects (eg,
std::vector,std::string, class instances) to avoid expensive copying.
Using const References for Safety and Efficiency
A common and recommended pattern is to pass large objects by const reference when you need to read them but not modify them.
void printVector(const std::vector<int>& vec) {
for (int x : vec) {
std::cout << x << " ";
}
}Benefits:
- No copy is made → efficient.
- Original cannot be modified → safe.
- Ideal for function parameters that accept strings, containers, or class objects.
Key Differences Summary
| Method | Syntax | Copies Data? | Can Modify Original? | Best For |
|---|---|---|---|---|
| Pass by value | void f(int x) | Yes | No | Small types, read-only |
| Pass by reference | void f(int& x) | No | Yes | Modify original, large data |
| Pass by const ref | void f(const int& x) | No | No | Large data, read-only |
One thing to watch: never return a reference to a local variable. That leads to dangling references and undefined behavior.
int& badFunction() {
int x = 10;
return x; // ❌ x is destroyed when function ends
} Basically, choose the method based on whether you need to modify the argument, how big it is, and whether performance matters. For most user-defined types used in read-only contexts, const T& is the go-to choice.
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