In my question about Analysis of float/double precision in 32 decimal digits, one answer said to take a look at __float128.
I used it and the compiler could find it, but I can not print it, since the complier can not find the header quadmath.h.
So my questions are:
__float128 is standard, correct?quadmath.h standard?These answers did not help:
The ref also did not help.
Note that I do not want to use any non standard library.
[EDIT]
It would be also useful, if that question had an answer, even if the answer was a negative one.
__float128 is a vendor extension.
In C23, _FloatN for N = 16, 32, 64, 128 or multiples of 32 greater than 128 are standardised extensions that a compiler can provide. So _Float128 would be the standard type. To print the type, you use strfromfN to turn it into a string:
void print(_Float128 fp) {
char buf[32 + sizeof(".e+99999")];
int sz = strfromf128(buf, sizeof buf, "%.32g", fp);
fwrite(buf, 1, sz, stdout);
}
https://godbolt.org/z/E7W6a4r8d
In C++23, as well as _FloatN probably being provided as well by your compilers as an extension, std::float128_t is provided as the standard 128-bit float type.
It is implementation defined if there is an overload for operator<< so you can simply use std::cout, but there will be a std::formatter specialization, so std::format can be used to create a string:
void print(std::float128_t fp) {
std::println("{:.32g}", fp); // std::println is C++26
// C++23: `std::cout << std::format("{:.32g}\n", fp);`
}
https://godbolt.org/z/5sahahn4P
Before C23/C++23, there was no standard type for 128 bit float. Sometimes long double is 128 bits, but that is rare (it is more likely to be 80 bits or the same 64 bits as a regular double). So you had to use compiler extensions or a software emulation library.