Does C++11, 14, 17 or 20 introduce a standard constant for pi?

Up to and including C++17 pi is not a constant introduced into the language, and it's a pain in the neck.

I'm fortunate in that I use boost 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/

---

From C++20 some good news. There is a defininition for pi. C++20 adds some mathematical constants in <numbers>. For example std::numbers::pi is a double type.

Reference: https://en.cppreference.com/w/cpp/numeric/constants

Up to C++20, no, none of the standards introduced the constant that would represent the number pi (π). You can approximate the number in your code:

constexpr double pi = 3.14159265358979323846;

Other languages such as C# have the constant declared in their libraries.

Update: Starting with the C++20, there indeed is a pi constant declared inside the <numbers> header. It is accessed via: std::numbers::pi.

As others said there is no std::pi but if you want precise PI value you can use:

constexpr double pi = std::acos(-1);

This assumes that your C++ implementation produces a correctly-rounded value of PI from acos(-1.0), which is common but not guaranteed.

It's not constexpr, but in practice optimizing compilers like gcc and clang evaluate it at compile time. Declaring it const is important for the optimizer to do a good job, though.