Optional argument, by reference, for polymorphic method?

Question

I have a polymorphic method hierarchy like this:

void func(double x, const std::string& s) = 0;

and I want to pass an "optional" argument which is modified within the method:

void func(double x, const std::string& s, uint64_t& i = 0) = 0;

but I receive errors that my reference was not initialised.

What is the best way to implement the above?

Answer 1

uint64_t& is an L-Value reference, and can therefore not bind to an R-Value, like the literal you provide as default argument. You must provide an L-Value as default.

One option is to define a dummy object that you pass as default, e.g.:

static uint64_t dummy_default_value = 0;
void func(double x, const std::string& s, uint64_t& i = dummy_default_value ) = 0;

Answer 2

I would avoid default parameter for virtual method (see good-practice-default-arguments-for-pure-virtual-method), so just add an extra overload

virtual void func(double x, const std::string& s, uint64_t& i) = 0;
void func(double x, const std::string& s) { int i; func(x, s, i) };

If you really want to keep default argument, you may use boost::optional:

virtual void func(double x,
                  const std::string& s,
                  boost::optional<uint64_t&> i = boost::none) = 0;