C++ Default Parameters

🔧 C++ Default Parameters

Default parameters allow a function to use predefined values for parameters if no argument is provided during the function call.
They make functions more flexible and easier to use.

🔹 1. Basic Example

void greet(string name = "Guest") {
cout << "Hello " << name;
}greet(); // Hello Guest
greet(“Sanjit”); // Hello Sanjit

🔹 2. Default Parameters with Return Value

int add(int a, int b = 10) {
return a + b;
}cout << add(5); // 15
cout << add(5, 20); // 25

🔹 3. Multiple Default Parameters

int calculate(int a, int b = 5, int c = 2) {
return a * b + c;
}cout << calculate(10); // 52
cout << calculate(10, 3); // 32
cout << calculate(10, 3, 4); // 34

🔹 4. Important Rule (Very Important ⚠️)

➡ Default parameters must be declared from right to left

❌ Wrong:

void func(int a = 5, int b);

✔ Correct:

void func(int a, int b = 5);

🔹 5. Default Parameters in Function Declaration

Default values are usually placed in the function declaration (prototype), not in the definition.

int multiply(int a, int b = 2); // declaration

int multiply(int a, int b) { // definition
return a * b;
}

🔹 6. Default Parameters with const

void print(const string &msg = "Hello") {
cout << msg;
}

✔ Efficient
✔ Safe

🔹 7. Default Parameters with Pointers

void show(int *p = nullptr) {
if (p != nullptr)
cout << *p;
else
cout << "No value";
}

🔹 8. Default Parameters vs Function Overloading

Using Default Parameters

void log(string msg, int level = 1);

Using Overloading

void log(string msg);
void log(string msg, int level);

✔ Default parameters reduce extra functions

❌ Common Mistakes

void fun(int a = 10, int b = 20); // ❌ wrong placement

Default values must be last parameters.

📌 Summary

  • Default parameters provide automatic values

  • Used when arguments are optional

  • Must be defined from right to left

  • Best placed in function declarations

  • Reduce function overloading

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