constexpr function contains a const - will it ever be evaluated at compile time?

Question

Please can someone clarify (I'm using Visual Studio 15.9.2):

In the following code given that Pi_cnst is evaluated at run time (because defining Pi in this way requires a run time calculation), will RAD2DEG_cnst ever be evaluated at compile time or is the use of "constexpr" always reverting to "const" ?

edit - add: If it is always reverting to const then should I expect a warning, or is it bad in some other way i.e. it seems odd to be able to so easily declare a constexpr for the body to be accepted but ALWAYS be such that it is never actually a constexpr. What have I missed?

constexpr double Pi_error = std::acos(-1.0); //error function call must have a constant value in a constant expression (does not compile)
const double Pi_cnst = std::acos(-1.0); //ok evaluated at run time

constexpr double Pi_expr = 3.1415926; //ok valid constexpr

constexpr double RAD2DEG_cnst(double rad) { return rad * 180.0 / Pi_cnst; } //no error or warning BUT is this ever evaluated at compile time?
constexpr double RAD2DEG_expr(double rad) { return rad * 180.0 / Pi_expr; } //ok can be evaluated at compile time

const double result1 = RAD2DEG_cnst(0.1); //evaluated at run time?
const double result2 = RAD2DEG_expr(0.2); //evaluated at compile time?

double myVariable = 0.3;
const double result3 = RAD2DEG_expr(myVariable); //ok - but can only be evaluated at run time

const double myOtherVariable = 0.4;
const double result4 = RAD2DEG_expr(myOtherVariable); //ok - evaluated at compile time because parameter is CONST?

I found also constexpr function and its parameter and constexpr vs const vs constexpr const .

What's the question? You saw that acosdidn't work within a constexpr. — Matthieu Brucher
@MatthieuBrucher: Pi_cnst cannot be constexpr because of acos but RAD2DEG_cnst is constexpr although it relies on Pi_cnst. I believe the question is: how can it be declared as constexpr while performing unconstexprable operations, and what does it mean in practice about compile-time evaluation? — papagaga
Oh, OK. I think constexpr doesn't always been a build time... — Matthieu Brucher
I get an error for RAD2DEG_cnst with gcc9 though: "In function 'constexpr double RAD2DEG_cnst(double)': error: the value of 'Pi_cnst' is not usable in a constant expression" — papagaga
Possible duplicate of When does a constexpr function get evaluated at compile time? — Matthieu Brucher

NathanOliver NathanOliver · Accepted Answer · 2018-11-21T15:23:51

Your code is ill-formed, no diagnostic required. In

constexpr double RAD2DEG_cnst(double rad) { return rad * 180.0 / Pi_cnst; }

There is no input which will ever make the function a core constant expression which is specified by [dcl.constexpr]/5

For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, a constant initializer for some object ([basic.start.static]), the program is ill-formed, no diagnostic required.

constexpr function contains a const - will it ever be evaluated at compile time?

1 Answers