Full precision display of floating point numbers in C++?

Question

I have read several topics about the display of floating point numbers display in C++ and I couldn\'t find a satisfying answer.

My question is: how

Answer 1

In C++20 you'll be able to use std::format to do this:

std::cout << std::format("{}", M_PI);

Output (assuming IEEE754 double):

3.141592653589793

The default floating-point format is the shortest decimal representation with a round-trip guarantee. The advantage of this method compared to using the precision of max_digits10 from std::numeric_limits is that it doesn't print unnecessary digits. For example:

std::cout << std::setprecision(
  std::numeric_limits::max_digits10) << 0.3;

prints 0.29999999999999999 while

std::cout << std::format("{}", 0.3);

prints 0.3 (godbolt).

In the meantime you can use the {fmt} library, std::format is based on. {fmt} also provides the print function that makes this even easier and more efficient (godbolt):

fmt::print("{}", M_PI);

Disclaimer: I'm the author of {fmt} and C++20 std::format.