Default construct an optional function parameter

Question

I want to design a function so that the parameter is optional and default constructed if left out. The parameter is a struct that I've defined in the same header file. The tried to use the following function declaration:

void publish(MarkerStyle style=MarkerStyle());

When I try to compile I get the following error message:

error: no matching function for call to ‘bob::MarkerStyle::MarkerStyle(bob::MarkerStyle)’
  void publish(MarkerStyle style=MarkerStyle());

Is there some way to invoke the default constructor of MarkerStyle as an optional parameter?

EDIT:

Here is my header file.

struct MarkerStyle()
{
    double alpha;
    std::auto_ptr<Color> color;

    MarkerStyle() :
    alpha(100) 
    {}
};

class Publisher
{
   void publish(MakerStyle style=MarkerStyle());
};

I understand Bo Perreson's answer. Now my question is why doesn't my code generate a copy constructor for MarkerStyle?

EDIT:

I figured out the problem. auto_ptr prevented copy constructor generation.

Please **[edit]** your question with a [mcve] or [SSCCE (Short, Self Contained, Correct Example)](http://sscce.org) — NathanOliver, Oct 22 '15 at 19:49
Does your class have a well-formed, non-explicit copy-constructor? — Piotr Skotnicki, Oct 22 '15 at 19:52
No, there is no copy constructor. I have defined a constructor, though. Does that prevent the automatic copy constructor generation? — user2445507, Oct 22 '15 at 19:53
it looks like you just didnt declare the definition properly... — R Nar, Oct 22 '15 at 19:53
@user2445507 you need to provide a declaration of class `MarkerStyle` — Piotr Skotnicki, Oct 22 '15 at 19:53
You have typos and invalid syntax. Are you posting real code? — juanchopanza, Oct 22 '15 at 20:07

score 2 · Answer 1 · answered Oct 22 '15 at 19:56

2

If you pass MarkerStyle by value, it will get copied and you need a copy constructor.

If you don't want or need a copy, pass it by const reference, like

void publish(MarkerStyle const& style=MarkerStyle());

answered Oct 22 '15 at 19:56

Bo Persson

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2

Although there isn't anything to suggest the class is not copy-constructable. – juanchopanza Oct 22 '15 at 19:59
Thanks. I didn't realize the copy constructor would be invoked. I made an edit. If you look at my header, can you see why there would not be an auto-generated copy-constructor? I may go with const reference anyways. – user2445507 Oct 22 '15 at 20:03
I realize now that auto_ptr prevented copy constructor generation. Or at least I think that's it. Thanks – user2445507 Oct 22 '15 at 20:06

Default construct an optional function parameter

1 Answers1