Floating-point literal defines a compile-time constant whose value is specified in the source file.
Syntax
digit-sequencedecimal-exponentsuffix(optional)(1) digit-sequence.decimal-exponent(optional)suffix(optional)(2) digit-sequence(optional).digit-sequencedecimal-exponent(optional)suffix(optional)(3) 0x | 0Xhex-digit-sequencehex-exponentsuffix(optional)(4) (since C++17)0x | 0Xhex-digit-sequence.hex-exponentsuffix(optional)(5) (since C++17)0x | 0Xhex-digit-sequence(optional).hex-digit-sequencehex-exponentsuffix(optional)(6) (since C++17)1)digit-sequence representing a whole number without a decimal separator, in this case the exponent is not optional: 1e10, 1e-5L.
2)digit-sequence representing a whole number with a decimal separator, in this case the exponent is optional: 1., 1.e-2.
3)digit-sequence representing a fractional number. The exponent is optional: 3.14, .1f, 0.1e-1L.
4) Hexadecimal digit-sequence representing a whole number without a radix separator. The exponent is never optional for hexadecimal floating-point literals: 0x1ffp10, 0X0p-1.
5) Hexadecimal digit-sequence representing a whole number with a radix separator. The exponent is never optional for hexadecimal floating-point literals: 0x1.p0, 0xf.p-1.
6) Hexadecimal digit-sequence representing a fractional number with a radix separator. The exponent is never optional for hexadecimal floating-point literals: 0x0.123p-1, 0xa.bp10l.
decimal-exponent has the form
e | Eexponent-sign(optional)digit-sequencehex-exponent has the form
p | Pexponent-sign(optional)digit-sequence(since C++17)exponent-sign, if present, is either + or -
suffix, if present, is one of f, l, F, L, f16, f32, f64, f128, bf16, F16, F32, F64, F128, BF16(since C++23). The suffix determines the type of the floating-point literal:
(no suffix) defines double
f F defines float
l L defines longdouble
f16 F16 defines std::float16_t
f32 F32 defines std::float32_t
f64 F64 defines std::float64_t
f128 F128 defines std::float128_t
bf16 BF16 defines std::bfloat16_t
(since C++23)Optional single quotes (') may be inserted between the digits as a separator; they are ignored when determining the value of the literal.
(since C++14)Explanation
Decimal scientific notation is used, meaning that the value of the floating-point literal is the significand multiplied by the number 10 raised to the power of decimal-exponent. E.g. the mathematical meaning of 123e4 is 123×104.
If the floating literal begins with the character sequence 0x or 0X, the floating literal is a hexadecimal floating literal. Otherwise, it is a decimal floating literal.
For a hexadecimal floating literal, the significand is interpreted as a hexadecimal rational number, and the digit-sequence of the exponent is interpreted as the (decimal) integer power of 2 by which the significand has to be scaled.
doubled=0x1.4p3;// hex fraction 1.4 (decimal 1.25) scaled by 23, that is 10.0
(since C++17)Notes
The hexadecimal floating-point literals were not part of C++ until C++17, although they can be parsed and printed by the I/O functions since C++11: both C++ I/O streams when
is enabled and the C I/O streams:
,
, etc. See
for the format description.
Feature-test macroValueStdFeature
(C++17)Hexadecimal floating literals Example
Run this code
#include<iomanip>#include<iostream>#include<limits>#include<typeinfo>#define OUT(x) '\n' << std::setw(16) << #x << xintmain(){std::cout<<"Literal""\t""Printed value"<<std::left<<OUT(58.)// double<<OUT(4e2)// double<<OUT(123.456e-67)// double<<OUT(123.456e-67f)// float, truncated to zero<<OUT(.1E4f)// float<<OUT(0x10.1p0)// double<<OUT(0x1p5)// double<<OUT(0x1e5)// integer literal, not floating-point<<OUT(3.14'15'92)// double, single quotes ignored (C++14)<<OUT(1.18e-4932l)// long double<<std::setprecision(39)<<OUT(3.4028234e38f)// float<<OUT(3.4028234e38)// double<<OUT(3.4028234e38l)// long double<<'\n';static_assert(3.4028234e38f==std::numeric_limits<float>::max());static_assert(3.4028234e38f==// ends with 43.4028235e38f);// ends with 5static_assert(3.4028234e38!=// ends with 43.4028235e38);// ends with 5// Both floating-point constants below are 3.4028234e38static_assert(3.4028234e38f!=// a float (then promoted to double)3.4028234e38);// a double}Possible output:
Literal Printed value 58. 58 4e2 400 123.456e-67 1.23456e-65 123.456e-67f 0 .1E4f 1000 0x10.1p0 16.0625 0x1p5 32 0x1e5 485 3.14'15'92 3.14159 1.18e-4932l 1.18e-4932 3.4028234e38f 340282346638528859811704183484516925440 3.4028234e38 340282339999999992395853996843190976512 3.4028234e38l 340282339999999999995912555211526242304 References
C++23 standard (ISO/IEC 14882:2024):
5.13.4 Floating-point literals [lex.fcon]
C++20 standard (ISO/IEC 14882:2020):
5.13.4 Floating-point literals [lex.fcon]
C++17 standard (ISO/IEC 14882:2017):
5.13.4 Floating literals [lex.fcon]
C++14 standard (ISO/IEC 14882:2014):
2.14.4 Floating literals [lex.fcon]
C++11 standard (ISO/IEC 14882:2011):
2.14.4 Floating literals [lex.fcon]
C++98 standard (ISO/IEC 14882:1998):
2.13.3 Floating literals [lex.fcon]
See also