2 Lexical conventions [lex]

2.14 Literals [lex.literal]

2.14.4 Floating literals [lex.fcon]

    fractional-constant exponent-partopt floating-suffixopt
    digit-sequence exponent-part floating-suffixopt
    digit-sequenceopt . digit-sequence
    digit-sequence .
    e signopt digit-sequence
    E signopt digit-sequence
sign: one of
    +  -
    digit-sequence digit
floating-suffix: one of
    f  l  F  L

A floating literal consists of an integer part, a decimal point, a fraction part, an e or E, 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. Either the integer part or the fraction part (not both) can be omitted; either the decimal point or the letter e (or E ) and the exponent (not both) can be omitted. The integer part, the optional decimal point and the optional fraction part form the significant part of the floating literal. The exponent, if present, indicates the power of 10 by which the significant part is to be scaled. 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.