CODEWITHSUNDEEP
c++
Alexander Mihailov
Alexander Mihailov

Reputation: 1152

Language design: default method parameters inheritance

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?

Upvotes: 2

Views: 118

Answers (2)

Crowman
Crowman

Reputation: 25936

"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.

Upvotes: 0

jxh
jxh

Reputation: 70492

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);
    // ...
}

Upvotes: 5

Related Questions