Floating-point literal defines a compile-time constant whose value is specified in the source file.
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| digit-sequencedecimal-exponentsuffix (optional) | (1) | |
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digit-sequence.decimal-exponent (optional)suffix (optional) | (2) | |
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digit-sequence (optional).digit-sequencedecimal-exponent (optional)suffix (optional) | (3) | |
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0x |0Xhex-digit-sequencehex-exponentsuffix (optional) | (4) | (since C++17) |
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0x |0Xhex-digit-sequence.hex-exponentsuffix (optional) | (5) | (since C++17) |
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0x |0Xhex-digit-sequence (optional).hex-digit-sequencehex-exponentsuffix (optional) | (6) | (since C++17) |
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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) Hexadecimaldigit-sequence representing a whole number without a radix separator. The exponent is never optional for hexadecimal floating-point literals:0x1ffp10,0X0p-1.
5) Hexadecimaldigit-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) Hexadecimaldigit-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
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e |Eexponent-sign (optional)digit-sequence | | |
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hex-exponent has the form
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p |Pexponent-sign (optional)digit-sequence | | (since C++17) |
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exponent-sign, if present, is either+ or-
suffix, if present, is one off,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) definesdouble
f F definesfloatl L defineslongdouble
f16 F16 definesstd::float16_tf32 F32 definesstd::float32_tf64 F64 definesstd::float64_tf128 F128 definesstd::float128_tbf16 BF16 definesstd::bfloat16_t
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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) |
[edit]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 ofdecimal-exponent. E.g. the mathematical meaning of123e4 is123×104.
If the floating literal begins with the character sequence0x or0X, the floating literal is ahexadecimal floating literal. Otherwise, it is adecimal floating literal. For ahexadecimal floating literal, the significand is interpreted as a hexadecimal rational number, and thedigit-sequence of the exponent is interpreted as the (decimal) integer power of 2 by which the significand has to be scaled. double d= 0x1.4p3;// hex fraction 1.4 (decimal 1.25) scaled by 23, that is 10.0 | (since C++17) |
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 whenstd::hexfloat is enabled and the C I/O streams:std::printf,std::scanf, etc. Seestd::strtof for the format description.
[edit]Example
#include <iomanip>#include <iostream>#include <limits>#include <typeinfo> #define OUT(x) '\n' << std::setw(16) << #x << x int main(){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 4 3.4028235e38f);// ends with 5 static_assert(3.4028234e38!=// ends with 43.4028235e38);// ends with 5 // Both floating-point constants below are 3.4028234e38 static_assert(3.4028234e38f!=// a float (then promoted to double)3.4028234e38);// a double}Possible output:
Literal Printed value58. 584e2 400123.456e-67 1.23456e-65123.456e-67f 0.1E4f 10000x10.1p0 16.06250x1p5 320x1e5 4853.14'15'92 3.141591.18e-4932l 1.18e-49323.4028234e38f 3402823466385288598117041834845169254403.4028234e38 3402823399999999923958539968431909765123.4028234e38l 340282339999999999995912555211526242304
[edit]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]
[edit]See also
