Constexpr Math Functions

Actually, because of old and annoying legacy, almost none of the math functions can be constexpr, since they all have the side-effect of setting errno on various error conditions, usually domain errors.

From "The C++ Programming Language (4th Edition)", by B. Stroustrup, describing C++11:

"To be evaluated at compile time, a function must be suitably simple: a constexpr function must consist of a single return-statement; no loops, and no local variables are allowed. Also, a constexpr function may not have side effects."

Which means that it must be inline, without for, while and if statements and local variables. Side effects are also forbidden (ex: changing of errno). Another problem is that most of math functions are FPU instructions which are not represented in pure c/c++ (they are written in assembler code). That's why non of cmath function is declared as constexpr.

So noticed from this page that none of the math functions in c++11 seems to make use of constexpr, whereas I believe all of them could be. So that leaves me with two questions, one is why did they choose not to make the functions constexpr.

This part is very well answered by Sebastian Redl and Adam Szaj so won't be adding anything to it.

And two for a function like sqrt I could probably write my own constexpr, but something like sin or cos would be trickier so is there away around it.

Yes, you can write your own version of constexpr sin, cos by using the taylor series expansions of these functions. Have a look at this super cool github repo which implements several mathematical functions as constexpr functions Morwenn/static_math