argument's default value base on others

Question

i want to achieve a class like this:

class A{
    int a;
    int b;
    int c;
    A():a(),b(),c(){};
    A(int ia,int ib,int ic=ia+ib):a(ia),b(ib),c(ic){};  //this is what i need
};

I want the ic's default value is caculated based on the ia and ib, the code here will get error when being compiled.

I wonder if there's a way to get something like this.

thanks.

The reason you got an error is because the Standard says so in **8.3.6 Default arguments [dcl.fct.def]** "9 Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspeciﬁed. Consequently, parameters of a function shall not be used in a default argument, even if they are not evaluated." — TemplateRex, Sep 27 '13 at 09:47
@TemplateRex Making default parameters depend on others when it probably won't work. — Neil Kirk, Sep 27 '13 at 15:55
@NeilKirk if arguments were evaluated left-to-right, it *would* work, and not everyone knows that (I think Java does it) — TemplateRex, Sep 27 '13 at 15:57
@TemplateRex It should not let you compile with default parameters depending on each other. — Neil Kirk, Sep 27 '13 at 16:25

score 14 · Accepted Answer · answered Sep 27 '13 at 05:51

Just add another constructor overload:

A(int ia, int ib)
  : a(ia), b(ib), c(ia + ib) {}

A(int ia, int ib, int ic)
  : a(ia), b(ib), c(ic) {}

For more complex initialisation, you can avoid repetition using a C++11 delegating constructor:

A(int ia, int ib)
  : A(ia, ib, ia + ib) {}

A(int ia, int ib, int ic)
  : a(ia), b(ib), c(ic) {}

score 0 · Answer 2 · answered Sep 27 '13 at 05:55

You can simply build a default constructor with the required values or simply overload by adding another constructor. The values will be assigned after you've executed it.Something like..

A (int ia, int ib, int ic): a(ia), b(ib), c(ic) {}

Read this link if you need further help in setting up a default constructor.

argument's default value base on others

2 Answers2