- A+

There is a rather silly problem with the number pi in C and C++. As far as I know `M_PI`

defined in `math.h`

is not required by any standard.

New C++ standards introduced a lot of complicated math in the standard library - hyperbolic functions, `std::hermite`

and `std::cyl_bessel_i`

, different random number generators and so on and so forth.

Did any of the 'new' standards bring in a constant for pi? If not - why? How does all this complicated math work without it?

I am aware of similar questions about pi in C++ (they are several years and standards old); I would like to know the current state of the problem.

I am also very interested in *why oh why* C++ still doesn't have a pi constant but has a lot of more complicated math.

UPD: I know that I can define pi myself as 4*atan(1) or acos(1) or double pi = 3.14. Sure. But why in 2018 do I still have to do it? How do standard math functions work without pi?

No, *pi* is still not a constant introduced into the language, and it's a pain in the neck.

I'm fortunate in that I use Boost (www.boost.org) and they define *pi* with a sufficiently large number of decimal places for even a 128 bit `long double`

.

If you don't use Boost then hardcode it yourself. Defining it with a trigonometric function is tempting but if you do that you can't then make it a `constexpr`

. The accuracy of the trigonometric functions is also not guaranteed by any standard I know of (cf. std::sqrt), so really you are on dangerous ground indeed relying on such a function.

There is a way of getting a `constexpr`

value for *pi* using metaprogramming: see http://timmurphy.org/2013/06/27/template-metaprogramming-in-c/