In general, who should do more work: popFront or front?

[surlymoor]

On Tuesday, 15 June 2021 at 05:17:06 UTC, mw wrote:
> I think there is another convention (although it's not formally 
> enforced, but should be) is:
>
> -- `obj.front() [should be] const`, i.e. it shouldn't modify 
> `obj`, so can be called multiple times at any given state, and 
> produce the same result
> -- `obj.popFront()` perform the actual mutation of the internal 
> state the `obj`, each invocation will change the object's state 
> once.

That's a separate matter. To use as an example once more, 
MapResult's front method produces the range's elements, and its 
invocation results in the identical element given that popFront 
isn't called; it doesn't affect the range's source. It satisfies 
those expectations, but it leaves wanting to know whether one 
should strive for front being inexpensive to call.
My original question is stupid, and I think I'm just going to 
have to experiment.