Default Arguments in C++

In this lesson: Give function parameters default values in C++ — rules, order and examples with output. In Hindi & English.

What are default arguments?

Default arguments let you give a parameter a value in advance, so the caller can leave it out. If they omit it, the default is used; if they pass a value, that wins. This makes some parameters effectively optional.

A working example

#include <iostream>
using namespace std;

void greet(string name, string msg = "Hello") {
    cout << msg << ", " << name << "!\n";
}

int main() {
    greet("Amit");             // uses default msg
    greet("Riya", "Welcome");  // overrides default
    return 0;
}

The trailing rule

Defaults must be on the rightmost parameters. Once one parameter has a default, all parameters to its right must have defaults too:

void f(int a, int b = 2, int c = 3);   // OK
// void g(int a = 1, int b);           // ERROR: b has no default

This is because arguments fill in left to right — you cannot skip a middle one and supply a later one.

Where to put the default

If your function has a separate prototype, put the default there (not in the definition):

int power(int base, int exp = 2);   // default in prototype

int power(int base, int exp) {      // no default here
    int r = 1;
    for (int i = 0; i < exp; i++) r *= base;
    return r;
}

Putting the same default in both places is a compile error.

With overloading

Default arguments can overlap with overloading, but be careful — if a defaulted call could match two functions, the compiler reports an ambiguity. Prefer one approach or the other for a given set of calls.

Common mistakes

• Putting a default on a middle parameter but not the ones after it.
• Repeating the default in both the declaration and the definition.
• Creating an ambiguity between a defaulted function and an overload.
• Assuming you can skip an argument in the middle of a call — you cannot.

Summary

• Default arguments give parameters fallback values, making them optional.
• A supplied value overrides the default; an omitted one uses it.
• Defaults must be on the trailing (rightmost) parameters.
• Put the default in the prototype, not repeated in the definition.
• Watch for ambiguity when mixing defaults with overloading.