Functional dependent attribute in c++ class/struct

Question

I want to make a class/struct where one of the attributes is functionally dependent on other attributes. How can this be achieved?

struct Numbers {
  int a;
  int b;
  int c; // c == a+b
}


Numbers n1 {1, 2, 3}; // Ok.
Numbers n2 {1, 2, 4}; // Error!

In my use case, a, b, c are constant, if that matters (so const int may be used).

All attributes will appear many times in class/struct methods, so the goal is to cache the value a+b. Addition is used as an example, the dependency function may be more complex.

cdhowie · Accepted Answer · 2020-05-23T14:48:56.740

5

If a and b are mutable then you can't enforce that c is kept in sync; all three would have to be const for you to enforce this invariant.

The simplest approach would be to make c a function:

struct Numbers {
  int a;
  int b;
  int c() const { return a + b; }
}

If you want the value of c to be cached instead of computed when needed then you need to hide a and b behind accessors as well so that you can update c when they are updated.

class Numbers {
public:
  Numbers(int a, int b) : ma{a}, mb{b} { updateC(); }

  int a() const { return ma; }
  int b() const { return mb; }
  int c() const { return mc; }

  void a(int v) { ma = v; updateC(); }
  void b(int v) { mb = v; updateC(); }
  // No setter for c

private:
  void updateC() { mc = ma + mb; }

  int ma;
  int mb;
  int mc;
};

edited May 23 '20 at 14:48

answered May 23 '20 at 14:43

cdhowie

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OP said the members don't need to be mutable, so no need for the setters - but if you have them they shouldn't have the same names as getters. – Asteroids With Wings May 23 '20 at 14:56
*"but if you have them they shouldn't have the same names as getters"* -- I disagree on this point. They have an obviously-different call signature. I don't know anyone who would be confused by this interface. – cdhowie May 23 '20 at 15:06
In fact, this approach is [_explicitly recommended_](https://stackoverflow.com/a/760877/501250) by some. – cdhowie May 23 '20 at 15:26
Some, but not many. The name of a function should describe what the function does. You can't see the signature at the callsite. – Asteroids With Wings May 23 '20 at 16:38
If the name is descriptive then this isn't a problem. Is `foo.height(42);` really that opaque? – cdhowie May 23 '20 at 17:20
Yes, it is. Use verbs. Describing words. This is wisdom that's been available for decades – Asteroids With Wings May 23 '20 at 17:23
1

I guess we'll have to agree to disagree. This isn't really the right venue for an extended discussion of style. – cdhowie May 23 '20 at 18:25
1

Indeed, no problem :) – Asteroids With Wings May 23 '20 at 18:37

Coral Kashri · Answer 2 · 2020-05-23T16:07:29.453

2

You can do something like that:

struct Numbers {
    Numbers(int a, int b) : a(a), b(b), c(a + b) {}

private:
    int a;
    int b;
    int c; // c == a+b
};

Edit:
To keep the values of a and b updated, and to get the values of those variables, you'll have to use get & set methods, like in @cdhowie response.

edited May 23 '20 at 16:07

answered May 23 '20 at 14:42

Coral Kashri

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Don't forget accessors. – Asteroids With Wings May 23 '20 at 14:56
@AsteroidsWithWings I took them as obvious, and cdhowiealready mentioned them in his response, but you are right. – Coral Kashri May 23 '20 at 16:05

Functional dependent attribute in c++ class/struct

2 Answers2