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ISO C99 and ISO C++17 support floating-point numbers written not only in the usual decimal notation, such as 1.55e1
, but also numbers such as 0x1.fp3
written in hexadecimal format. As a GNU extension, GCC supports this in C90 mode (except in some cases when strictly conforming) and in C++98, C++11 and C++14 modes. In that format the 0x hex introducer and the p or P exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significant part is multiplied. Thus 0x1.f is 1 15/16, p3 multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1
.
Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f
. This could mean 1.0f
or 1.9375
since f is also the extension for floating-point constants of type float
.