Hexadecimal floating point literals in c++

The Technical Specification P0245 Hexadecimal floating literals for C++ has been voted into C++17 at the ISO C++ Standards Committee in Jacksonville, Florida on February 2016.

The language C99 also has this feature, and the C++ feature is compatible.

However, as pointed by the Lưu Vĩnh Phúc's comment, the syntax 0x011.1 is not part of the standard. The binary exponent is mandatory for hexadecimal floating-point literals. One reason is to avoid ambiguity of the trailing F within 0x011.1F. Is it the hex digit F of the fractional part or the floating-suffix meaning float?
Therefore append p followed by a positive or negative decimal number, for example: 0x011.1p0.

See the more readable page floating literal page on cppreference.com.

0x | 0X hex-digit-sequence
0x | 0X hex-digit-sequence .
0x | 0X hex-digit-sequence(optional) . hex-digit-sequence

Hexadecimal digit-sequence representing a whole number without a radix separator. The exponent is never optional for hexadecimal floating-point literals: 0x1ffp10, 0X0p-1, 0x1.p0, 0xf.p-1, 0x0.123p-1, 0xa.bp10l

The exponent syntax for hexadecimal floating-point literal has the form
p | P exponent-sign(optional) digit-sequence

exponent-sign, if present, is either + or -

suffix, if present, is one of f, F, l, or L. The suffix determines the type of the floating-point literal:

• (no suffix) defines double
• f F defines float
• l L defines long double

See also the current working draft C++17, chapter § 2.13.4 Floating literals on GitHub: https://github.com/cplusplus/draft/raw/master/papers/n4604.pdf

floating-literal:
  decimal-floating-literal
  hexadecimal-floating-literal
decimal-floating-literal:
  fractional-constant exponent-part_opt floating-suffix_opt
  digit-sequence exponent-part floating-suffix_opt
hexadecimal-floating-literal:
  hexadecimal-prefix hexadecimal-fractional-constant binary-exponent-part floating-suffix_opt
  hexadecimal-prefix hexadecimal-digit-sequence binary-exponent-part floating-suffix_opt
fractional-constant:
  digit-sequence_opt . digit-sequence
  digit-sequence .
hexadecimal-fractional-constant:
  hexadecimal-digit-sequence_opt . hexadecimal-digit-sequence
  hexadecimal-digit-sequence .
exponent-part:
  e sign_opt digit-sequence
  E sign_opt digit-sequence
binary-exponent-part:
  p sign_opt digit-sequence
  P sign_opt digit-sequence
sign: one of
  + -
digit-sequence:
  digit
  digit-sequence ’_opt digit
floating-suffix: one of
  f l F L

¹ A floating literal consists of an optional prefix specifying a base, an integer part, a radix point, a fraction part, an e, E, p or P, an optionally signed integer exponent, and an optional type suffix. The integer and fraction parts both consist of a sequence of decimal (base ten) digits if there is no prefix, or hexadecimal (base sixteen) digits if the prefix is 0x or 0X. The literal is a decimal floating literal in the former case and a hexadecimal floating literal in the latter case. Optional separating single quotes in a digit-sequence or hexadecimal- digit-sequence are ignored when determining its value. [ Example: The literals 1.602’176’565e-19 and 1.602176565e-19 have the same value. — end example ] Either the integer part or the fraction part (not both) can be omitted. Either the radix point or the letter e or E and the exponent (not both) can be omitted from a decimal floating literal. The radix point (but not the exponent) can be omitted from a hexadecimal floating literal. The integer part, the optional radix point, and the optional fraction part, form the significand of the floating literal. In a decimal floating literal, the exponent, if present, indicates the power of 10 by which the significand is to be scaled. In a hexadecimal floating literal, the exponent indicates the power of 2 by which the significand is to be scaled. [ Example: The literals 49.625 and 0xC.68p+2 have the same value. — end example ] If the scaled value is in the range of representable values for its type, the result is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner. The type of a floating literal is double unless explicitly specified by a suffix. The suffixes f and F specify float, the suffixes l and L specify long double. If the scaled value is not in the range of representable values for its type, the program is ill-formed.

As unwind has advised, you can use strtof(). The following snippet decodes Hexadecimal floating literals (without C++17):

#include <iostream>
#include <cstdlib>
#include <cstdio>

int main(int argc, char *argv[])
{
  if (argc != 2)
  {
    std::cout <<"Usage: "<< argv[0] <<" 0xA.Bp-1  => Decode hexfloat" "\n";
    return 1;
  }

  long double l;
  double      d;
  float       f;

  std::cout <<"Decode floating point hexadecimal = "<< argv[1];
  //std::istringstream(argv[1]) >> std::hexfloat >> d;
  l = std::strtold(argv[1],NULL); if(errno == ERANGE) std::cout << "\n" "std::strtold() range error";
  d = std::strtod (argv[1],NULL); if(errno == ERANGE) std::cout << "\n" "std::strtod() range error";
  f = std::strtof (argv[1],NULL); if(errno == ERANGE) std::cout << "\n" "std::strtod() range error";

  std::cout <<"\n"  "long double = "<< std::defaultfloat << l <<'\t'<< std::hexfloat << l
            <<"\n"  "double      = "<< std::defaultfloat << d <<'\t'<< std::hexfloat << d
            <<"\n"  "float       = "<< std::defaultfloat << f <<'\t'<< std::hexfloat << f <<'\n';
}