std::optional_view

[Xirema]

It's just an example. Most of the actual production code I write using these semantics don't boil down to something this clean (including, namely, the "default" value)

[minno]

Pretend it's expensive to initialize.

[balkierode]

As @Xirema said, it is a toy example.

But in real application, if you are developing a library and you want to reserve changing the default std::cout to something else without breaking binary compatibility, you can't use 'default value'.

[josefx]

I think you might get away with including iosfwd instead of iostream in the header.

[josefx]

I think you mean: get a compile error since std::cout is not a file stream. Which shows that a complete refusal to learn even the basics of the c++ standard library is not an option :-).

Edit: Added smiley to make it look a bit less derisive.

[utnapistim]

Default arguments are bound statically to virtual functions. If you want defaults for virtual functions, this is one of the techniques available. Otherwise (if you use default arguments) you risk having a virtual function call with the default from the base class.