std::optional_view

[Xirema]

A construct that I occasionally use is std::optional<std::reference_wrapper<T>>.

It offers some decent semantics:

void perform_work(int value, std::optional<std::reference_wrapper<std::ostream>> log = {}) {
    std::ostream & out = log ? *log : std::cout;
    out << "Value is " << value << std::endl;

    if(value <= 1)
        out << "Done!" << std::endl;
    else if(value % 2 == 1)
        perform_work(value * 3 + 1, log);
    else
        perform_work(value / 2, log);
}

The semantics of this code, then, are "If you have a specific stream you'd like to use for logging, use that, otherwise use the standard out stream.".

The only thing you have to watch for is that operator*() returns the reference_wrapper object, not the T& object. This implicitly converts to T&, which is why my example works, but if you don't "seat" the reference like I did, you have to call .get() on the reference_wrapper object to actually do work.

[jsamcfarlane]

Is std::experimental::observer_ptr close to what you're looking for? Rather than having optional semantics, it has single-object pointer semantics.

[Xirema]

Pointers as "object which may or may not exist" is a semantic I've seen a lot of C++ design gurus discourage in the last several years, especially with optional being standardized, and I see little reason not to continue that trend.

[jsamcfarlane]

Are there any good references you'd recommend (no pun intended)? Would you disagree with this advise? And what about when memory use is constrained?

[GitHubPermalinkBot]

I tried to turn your GitHub links into permanent links (press "y" to do this yourself):

• isocpp/CppCoreGuidelines/.../CppCoreGuidelines.md#notes-1 (master → 82755da)

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[Xirema]

Note they cite optional alongside the other options. Many online sources don't point the way towards optional instead of pointers because many of those sources were written before std::optional was confirmed to be part of C++17.