Question

A schematic of my problem…

class A
{
public:
    // etc.
protected:
    uint num;
};

class B : public A
{
public: 
    void foo(uint x = num); //bad
};

gives this error:

error: invalid use of non-static data member ‘A::num’
error: from this location

Why does this happen, and what can I do to work around this?

Answer 1

I suspect this happens (based on the complaint about non-staticness) because there is no this pointer for it to use to know which instance of B it should get num from.

The Microsoft compiler (at least) allows you to specify an expression, but not a non-static member. From MSDN:

The expressions used for default
arguments are often constant
expressions, but this is not a
requirement. The expression can
combine functions that are visible in
the current scope, constant
expressions, and global variables. The
expression cannot contain local
variables or non-static class-member
variables.

Work-arounds for this are numerous and others have pointed out a few. Here’s one more which you may or may not like:

void foo(uint* x = NULL) {
  uint y = (x == NULL ? num : *x);
  // use y...
}