Why doesn't function overloading work when default parameters are specified?

Question

When default parameters are not specified, function overloading works.

But, why doesn't function overloading work when default parameters are specified?

This program takes two integers and compares them to find the larger of the two.

Then, it compares the maximum value with bigger integers obtained earlier, and the smaller value is output.

Here's the code I written

#include <iostream>
using namespace std;

int big(int a, int b);
int big(int a, int b, int max = 100);

int big(int a, int b) {
    if (a > b)
        if (a > 100)
        {
            return 100; 
        }
        else {
            return a; 
        }
    else {
        if (b > 100) {
            return 100; 
        }
        else
        {
            return b; 
        }
    }
}

int big(int a, int b, int max) {
    if (a > b) {
        if (a < max) {
            return a; 
        }
        else
        {
            return max; 
        }
    }
    else {
        if (b < max) {
            return b; 
        }
        else {
            return max; 
        }

    }
}
  
int main() {
    int x = big(3, 5); 
    int y = big(300, 60); 
    int z = big(30, 60, 50); 
    cout << x << ' ' << y << ' ' << z << endl;
    return 0;
}

When I debug the code I wrote, it says

E0308 Compilation Error: More than one instance of overloaded function matches the argument list

I learned that function overloading can work when the function parameters are different.

In my code, function names are the same, but function parameters are different:

(int x, int y) and (int x, int y, int max)

But it doesn't work.

In conclusion, why does function overloading not work when default parameters are specified?

IDE that I used:

Microsoft Visual Studio Community 2022 (64-bit) - Current version. 17.5.1

Here's the desire result:

1. Variables x and y have the return value of the first function big(int x, int y)
2. Variable z has the return value of the second function big(int x, int y, int max = 100)

This Question is related, but I don't understand how the solution is what I want.

Answer 1

You've effectively got these two functions named the same

int big(int a, int b) {}

int big(int a, int b, int max=100) {}

And then when you invoke big(3,5)....

Exactly which version of big did you expect to get called? The first implementation which explicitly takes two parameters or the second implementation that can take two or three parameters?

The compiler is doing you a favor by erroring out and telling you it's ambiguous.

As I called out in my comments, function overloading is overrated. Just name the functions explicitly such that there is zero ambiguity on which implementation is getting called.

Also, your entire implementation can be simplified to just a single function and two lines of code

int big(int a, int b, int max=100) {
    int result = (a > b) ? a : b;
    return (result > max) ? max : result;
}

The default parameter for max seems innocuously ok to me if you keep it as a single function with no overloads.

Answer 2

This part is ambiguous, since the second function declaration has a default argument:

int big(int a, int b);
int big(int a, int b, int max = 100);

So while you call, for example, big(20,30), both versions of big() could be used. So there will be 2 candidates.

It would be enough to use the second version of your function to make it work without overloading. Something like this :

#include <iostream>
using namespace std;

// declare
int big(int a, int b, int max = 100);

// define
int big(int a, int b, int max) {
    if (a > b) {
        if (a < max) {
            return a;
        }
        else
        {
            return max;
        }
    }
    else {
        if (b < max) {
            return b;
        }
        else {
            return max;
        }

    }
}

int main() {
    int x = big(3, 5);
    int y = big(300, 60);
    int z = big(30, 60, 50);
    cout << x << ' ' << y << ' ' << z << endl;
    return 0;
}

In case you still want to define 2 functions. I would suggest you remove the default argument.

#include <iostream>
using namespace std;

int big(int a, int b);
int big(int a, int b, int max);

int big(int a, int b) {
    if (a > b)
        if (a > 100)
        {
            return 100;
        }
        else {
            return a;
        }
    else {
        if (b > 100) {
            return 100;
        }
        else
        {
            return b;
        }
    }
}

int big(int a, int b, int max) {
    if (a > b) {
        if (a < max) {
            return a;
        }
        else
        {
            return max;
        }
    }
    else {
        if (b < max) {
            return b;
        }
        else {
            return max;
        }

    }
}

int main() {
    int x = big(3, 5);
    int y = big(300, 60);
    int z = big(30, 60, 50);
    cout << x << ' ' << y << ' ' << z << endl;
    return 0;
}

Extra note: About declare and define your functions. Since you define the function int the same source file and before main(), perhaps you do not need to declare and then define. So you could skip to declare it. If you move the implementation to another source file or after main(), then declare it. More info: Link