Why default argument for the last parameter is mandatory ?

**techie_san778** · 12-01-2014

Dear Friends,

I faced a compilation error in the following code :

Code:

#include <iostream>

using namespace std;

void addition(int a, int b = 2, int c);

int main()
{   
    int a = 10, b = 20, c = 30;
    addition(10,20,30);
    return 0;
}

void addition(int a, int b, int c)
{  
   cout << "\n Sum  = "<< a + b + c;
   return;
}

Error : default argument missing for parameter 3 of ‘void addition(int, int, int)’

My question is that when i have called addition() with the 3rd argument, then what is the necessity of having the default argument for the 3rd parameter ?

**jimblumberg** · 12-01-2014

The default argument for the third parameter is necessary because you have a default argument for the second parameter. Once you have one default parameter the rest of the parameters to the right of that one must also have default parameters as well.

Jim

**grumpy** · 12-01-2014

Because that's the language rules. Default values, if provided must be for rightmost arguments. That means, if there is only one argument with a default value, it must be the last one. If there are two arguments with default values, they must be the last two. And so on.

**techie_san778** · 12-01-2014

Thanks friends, surprisingly though, i could not find this important point in any text so far. Thanks again for the timely replies.. may i ask why such rule is there ? any specific reason or it's just a rule ??

**grumpy** · 12-01-2014

Eliminates ambiguity in some circumstances.

**Elysia** · 12-01-2014

Well, if you have the 2nd parameter as default, then how does the compiler know what value to put for the last? Consider:

addition(0, 1, 2); // Fine
addition(0, 2); // Wait: does "2" refer to the 2nd or 3rd parameter?

Technically, while a parameter is optional, you can still provide an argument for it. So how does the compiler know you skip it or not?
It would have to be made explicit somehow, like:

addition(0, [skip], 2);
or
addition(0, [c]=2);

Where I simply put "[skip]" and "[c]" as some placeholds for some syntax. Such syntax does not exist in the language.

**brewbuck** · 12-01-2014

Originally Posted by techie_san778

Thanks friends, surprisingly though, i could not find this important point in any text so far. Thanks again for the timely replies.. may i ask why such rule is there ? any specific reason or it's just a rule ??

Are you kidding? It is hopefully obvious why it must be this way...

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