C++ Representing Optional Objects

Question

I have a C++ project. I work on the project to teach myself about building a semi-realistic program in C++. It downloads content recursively from a website.

Each download has a URL for the content to download, as well as a URL for the referrer (or the URL of the page that the content was extracted from).

There is always a referrer unless it's the very first URL. I had been cheating and just treating the first URL as its own referrer. I recently changed the URL class to have a blank (or empty) representation. This feels like a hack.

Is there a way to represent optional objects in C++ without:

• Using pointers?
• Wasting space for the invalid object?
• or Making a "blank" version of the object?

Declared

W\O: Using pointers? IMHO not. – Genny

Answer 1

You could use boost::optional. boost is well-respected 3rd party library; often regarded as a prototype for new stl functions: See Ralph's answer std::optional is available with new C++.

See http://www.boost.org/

Fuller

I originally posted that as a comment, but won't that waste space for the "blank" version? Not that I think it is a serious problem, but it is one of the specific requirements of the question. – Sin

No. boost::optional uses a pointer, set to NULL as appropriate, in its implementation. Don't know how std::optional works. – Fuller

@Fuller I'm just trying to avoid reinventing established conventions. I used unique_ptr but decided it made the code look lopsided. I felt it wasn't explicitly saying "this field is optional". Instead it looked like a memory management technique. – Declared

Answer 2

Use the std::optional template, if you have a C++14 compiler. If not you can use boost::optional or std::unique_ptr. You cannot avoid both using pointers and wasting space for the invalid object. std::optional will contain the optional object, std::unique_ptr will obviously point to it.

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