member as default argument

**MarkZWEERS** · 03-23-2009

Hi All!!

My constructor's argument has a default value which is a 'static const' in a .h file. This argument sets a data member in the object to be constructed.

The argument of one of the member functions now has that data member as default argument.
But then it complains that that default member is not static. However, I do not want non-const static variables.

Here's an example of what I mean:

Code:

class FilterClass { .. };
static const FilterClass myFilter1(..);

class A {
 public:
 A(FilterClass const& p_filter = myFilter1)
   : filter(p_filter) { }
  void action(FilterClass const& p_filter = filter)
  {
    ...
  }
 protected:
  FilterClass filter;
};

I could not give the static const variables as default arguments in the constructor, but temporaries instead. However, for the member function "action" it becomes problematic:
the compiler raises an error telling that the argument's default is not static.

Making the protected data member static resolves the problem, however I do not want non-const static variables.
Just overloading the function without argument will do, but I am curious to how to resolve the problem.

Thanks a lot in advance!!

Mark

**laserlight** · 03-23-2009

Instead of trying to use a static const global or a member variable to provide the default argument, just use the appropriate constructor of the type:

Code:

class FilterClass { .. };

class A {
 public:
 A(FilterClass const& p_filter = FilterClass(..))
   : filter(p_filter) { }
  void action(FilterClass const& p_filter = FilterClass(..))
  {
    ...
  }
 protected:
  FilterClass filter;
};

Now, it may be your intention to use the member variable filter if no argument is provided to A::action(). In that case, overload A::action() such that the version with no arguments calls the version that takes a FilterClass by const reference. The version of A::action() that takes an argument would then not provide a default argument.

EDIT:
Better yet, define a free function named action in the same namespace as A:

Code:

void action(A& a, FilterClass const& filter)
{
    a.action(filter);
}

EDIT:
Oh wait, I am not thinking correctly: the free function version is not applicable here. I was thinking of having a free function version that only takes an A by reference as the argument, but then you would not be able to provide the member variable as an argument.

**MarkZWEERS** · 03-23-2009

Yep, indeed these two suggestions I've implemented.

I definitely wanted to use the static constant variables for clarity of the code, but it was a bad idea after all.

Thanks!

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