My platform does not have a __float128, but here’s an example showing output by precision with float, double, and long double:
#include <cmath>
#include <iomanip>
#include <iostream>
#include <limits>
int main()
{
float PI0 = std::acos(-1.0F);
double PI1 = std::acos(-1.0);
long double PI2 = std::acos(-1.0L);
constexpr auto PI0_max_digits10 = std::numeric_limits<decltype(PI0)>::max_digits10;
constexpr auto PI1_max_digits10 = std::numeric_limits<decltype(PI1)>::max_digits10;
constexpr auto PI2_max_digits10 = std::numeric_limits<decltype(PI2)>::max_digits10;
std::cout << "Float=" << std::setprecision(PI0_max_digits10) << PI0 << std::endl;
std::cout << "Double=" << std::setprecision(PI1_max_digits10) << PI1 << std::endl;
std::cout << "LongDouble=" << std::setprecision(PI2_max_digits10) << PI2 << std::endl;
}