C++ idiomatic way for nullable reference

Question

I am writing a function which should accept const std::string &, but also a special nullptr value. Currently, I'm using const std::string *. However, I feel that there should be a more C++-ish way to perform this task.

My strongest counterpoint is passing by value (the string won't be really long but 64kB is not something I want copied around). That also implies that I want no object like optional<T> which would have a value field of type T. Also, I don't want to use any external libraries. My project is not using Boost or GSL, just plain C++ (C++17 to be exact).

Is there a standard library class (miracles occur in namespace std) or a widely accepted idiom for such situation?

Answer 1

If std::optional supported references, it'd be the fancy "modern" way to do this.

But:

• it doesn't, and
• sometimes being fancy and "modern" is not all that it is cracked up to be.

The age-old, tried-and-tested and clear-to-read solution is to take a pointer. This approach does absolutely everything that you need it to do.

Just do that, then move on to spend your precious time on more important matters!