5 Lexical conventions [lex]

5.13 Literals [lex.literal]

5.13.4 Floating-point literals [lex.fcon]

sign: one of
+ -
floating-point-suffix: one of
f l f16 f32 f64 f128 bf16 F L F16 F32 F64 F128 BF16
The type of a floating-point-literal ([basic.fundamental], [basic.extended.fp]) is determined by its floating-point-suffix as specified in Table 11.
[Note 1: 
The floating-point suffixes f16, f32, f64, f128, bf16, F16, F32, F64, F128, and BF16 are conditionally-supported.
— end note]
Table 11: Types of floating-point-literals[tab:lex.fcon.type]
f or F
l or L
long double
f16 or F16
f32 or F32
f64 or F64
f128 or F128
bf16 or BF16
In the significand, the sequence of digits or hexadecimal-digits and optional period are interpreted as a base N real number s, where N is 10 for a decimal-floating-point-literal and 16 for a hexadecimal-floating-point-literal.
[Note 2: 
Any optional separating single quotes are ignored when determining the value.
— end note]
If an exponent-part or binary-exponent-part is present, the exponent e of the floating-point-literal is the result of interpreting the sequence of an optional sign and the digits as a base 10 integer.
Otherwise, the exponent e is 0.
The scaled value of the literal is for a decimal-floating-point-literal and for a hexadecimal-floating-point-literal.
[Example 1: 
The floating-point-literals 49.625 and 0xC.68p+2 have the same value.
The floating-point-literals 1.602'176'565e-19 and 1.602176565e-19 have the same value.
— end example]
If the scaled value is not in the range of representable values for its type, the program is ill-formed.
Otherwise, the value of a floating-point-literal is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner.