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constexpr and Moving Work to the Build

Templates compute with types. constexpr computes with values, using ordinary function syntax, and since C++20 it can do almost everything ordinary code can. It is the more practical half of compile-time programming and the one that appears most often in real trading code.

Can, will, and must

Three keywords, and the distinction between them is a standard interview question.

constexpr
MeansMay run at compile time, if the arguments allow
Applied toFunctions and variables
With run-time argumentsFalls back to a normal call
Use forAnything that could be constant-folded
consteval
MeansMust run at compile time, always
Applied toFunctions
With run-time argumentsCompilation error
Use forGuaranteeing no run-time fallback
constinit
MeansMust be initialised at compile time
Applied toVariables
With run-time argumentsNot applicable
Use forAvoiding static initialisation order problems

constexpr is a permission. consteval is a requirement. constinit is about when the value is ready.

The trap is assuming constexpr guarantees anything. A constexpr function called with a run-time value is an ordinary function call, silently. If the point is that the work must not happen at run time, the answer is consteval, or forcing the result into a constexpr variable so it has to be a constant expression.

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