Giving default values to reference member variables

Question

so I've made a class in c++ which has 2 reference type members:

class Edge{
private:
const Node& base;
const Node& target;

public:
    Edge(const Node& new1, const Node& new2);

I want to give default values to base and target in the C'tor. Which means that:

Edge()

will not be an error, but will do create an Edge object. How do I do that?

edit: I'm also trying to do:

Edge::Edge(const Node& newBase, const Node& newTarg)
{
    m_base=newBase;
    m_target=newTarg;
}

But it won't let me, it says no operator "=" matches this operators. But I did make a "=" operator for Nodes and checked it worked.......

Answer 1

You can give new1 and new2 defaults like any other parameter. The trick is that since they're passed by reference and (presumably) you're using them to set base and target they need to live long enough for this to make sense. You can do that by making a static "dummy" Node to use as the default for where one hasn't been specified, e.g.:

class Node {};

class Edge{
private:
static const Node none;
const Node& base;
const Node& target;

public:
    Edge(const Node& new1=none, const Node& new2=none) : base(new1), target(new2) {}
};

const Node Edge::none;

int main() {
  Edge e;
}

If that's good design or not is another issue though. There might well be a smarter solution to the underlying problem that you're trying to solve.

Answer 2

Here's a version that works for non-constant references, too:

struct Bar { /* ... */ }:

struct Foo
{
    Foo(Bar & br) : b(br)          { }
    Foo()         : b(default_bar) { }
private:
    static Bar default_bar;
    Bar & b;
};
Bar Foo::default_bar;

This way, all default-constructed Foo instances contain a reference to the common, static default_bar object.

Answer 3

Another solution would be to use a wrapper that is copyable

struct Reference
    {
    T* r_noncopyable_object;
    };