问题
As the title asks: Is it possible to ensure a constexpr function is called at most once at compile time?
This clearly won't be possible if the function is not constepxr; I could write a function that gets called whenever I press the space bar, so the compiler could never figure that out at compile time.
回答1:
Short answer: no, because constexpr functions cannot read/set external state. (They can have internal state, but they still need to be "pure").
Real answer: probably yes, but it's a bad idea. There is a series of blog posts by Filip Roséen which covers the implementation of stateful constexpr functions by abusing friendship and ADL:
"NON-CONSTANT CONSTANT-EXPRESSIONS IN C++" - (cached by Google)
"HOW TO IMPLEMENT A CONSTANT-EXPRESSION COUNTER IN C++" - (cached by Google)
"HOW TO IMPLEMENT A COMPILE-TIME META-CONTAINER IN C++" - (cached by Google)
The technique is very arcane and complicated. It is considered an abuse of features by CWG, which is trying to make it ill-formed with issue #2118.
来源：https://stackoverflow.com/questions/41939704/is-it-possible-to-ensure-a-constexpr-function-is-called-at-most-once-at-compile