C/C++ NaN constant (literal)?

Question

Is this possible to assign a NaN to a double or float in C/C++? Like in JavaScript you do: a = NaN. So later you can chec

Answer 1

Is this possible to assign a NaN to a double or float in C ...?

Yes, since C99, (C++11) offers the below functions:

#include 
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);

which are like their strtod("NAN(n-char-sequence)",0) counterparts and NAN for assignments.

// Sample C code
uint64_t u64;
double x;
x = nan("0x12345");
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);
x = -strtod("NAN(6789A)",0);
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);
x = NAN;
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);

Sample output: (Implementation dependent)

(7ff8000000012345)
(fff000000006789a)
(7ff8000000000000)