Language design: default method parameters inheritance

Question

I think, there is a common situation, when one function (a) is being called from within another one (b), while 'a' have some default parameters and 'b' is needed to support them. For example:

void a(int v1, int v2=0, int v3=1);
void b(int m1, int m2, int v1, int v2=0, int v3=1) {
    // ...
    a(v1, v2, v3);
    // ...
}

But this is the violation of the DRY principle. It can leads a subtile bug, when default parameter of 'a' was changed, but not changed in 'b':

void a(int v1, int v2, int v3=0);
void b(int m1, int m2, int v1, int v2=0, int v3=1) {
    // ...
    a(v1, v2, v3);
    // ...
}

Why there is no mechanism in C++ to inherit default parameter values? It might look like:

void a(int v1, int v2=0, int v3=1);
void b(int m1, int m2, int v1, int v2=default(a::v2, 0), int v3=default(a::v3, 1)) {
    // ...
    a(v1, v2, v3);
    // ...
}

Whether there are languages, that have such syntax?

It might be an offtopic on this board?

This wouldn't really be on-topic here, no. We tend more toward practical "this code isn't working" sorts of questions. Your question might be OK on Programmers.SE, but I can't be sure. The topic is definitely a better fit for them. — Andrew Barber
– Andrew Barber, Commented Aug 21, 2013 at 3:13
I removed the C tag because C does not have default parameters. — Keith Thompson
– Keith Thompson, Commented Aug 21, 2013 at 3:16
Well in C++ you could solve this issue by overloading b (with 3, 4 and 5 parameters). If you don't want to write the overloads yourself, you could use a variadic template / parameter pack. Alternatively, you could use something like a boost::optional as parameter (for b) with a default argument. — dyp
– dyp, Commented Aug 21, 2013 at 3:36

jxh · Accepted Answer · 2013-08-21 03:34:46Z

5

The issue is actually the use of magic numbers. If you remove the magic numbers, the problem is resolved nicely.

enum DefaultA { DefaultA_v2 = 0, DefaultA_v3 = 1 };

void a(int v1, int v2=DefaultA_v2, int v3=DefaultA_v3);

void b(int m1, int m2, int v1, int v2=DefaultA_v2, int v3=DefaultA_v3) {
    // ...
    a(v1, v2, v3);
    // ...
}

answered Aug 21, 2013 at 3:34

jxh

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1 Comment

Qaz Over a year ago

And once again, another level of indirection solves the problem.

Crowman · Accepted Answer · 2013-08-21 04:25:46Z

0

"But this is the violation of the DRY principle. It can leads a subtile bug, when default parameter of 'a' was changed, but not changed in 'b'"

Frankly, I'd say that if you get this "bug", then your functions are too tightly-coupled. Standalone functions are supposed to, well, stand alone. If changing a default parameter for one function causes another function to break, I think you have a design problem there.

answered Aug 21, 2013 at 4:25

Crowman

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