Floating-point literal
Floating-point literal defines a compile-time constant whose value is specified in the source file.
Syntax
| digit-sequence decimal-exponent suffix(optional) | (1) | |
digit-sequence . decimal-exponent(optional) suffix(optional) |
(2) | |
digit-sequence(optional) . digit-sequence decimal-exponent(optional) suffix(optional) |
(3) | |
0x | 0X hex-digit-sequence hex-exponent suffix(optional) |
(4) | (since C++17) |
0x | 0X hex-digit-sequence . hex-exponent suffix(optional) |
(5) | (since C++17) |
0x | 0X hex-digit-sequence(optional) . hex-digit-sequence hex-exponent suffix(optional) |
(6) | (since C++17) |
1e10,
1e-5L
1.,
1.e-2
3.14,
.1f,
0.1e-1L
0x1ffp10,
0X0p-1
0x1.p0,
0xf.p-1
0x0.123p-1,
0xa.bp10l
decimal-exponent has the form.
e | E exponent-sign(optional) digit-sequence |
hex-exponent has the form.
p | P exponent-sign(optional) digit-sequence |
(since C++17) |
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 Fdefinesfloat -
l Ldefineslong double
| Optional single quotes ( |
(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 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 integer power of 2 to which the significand has to be scaled. double d = 0x1.4p3; // hex fraction 1.4 (decimal 1.25) scaled by 2^3, 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 std::hexfloat is enabled and the C I/O streams: std::printf, std::scanf, etc. See std::strtof for the format description.
Example
#include <iostream>
#include <iomanip>
#include <limits>
#include <cassert>
#include <typeinfo>
int main()
{
std::cout << "Literal " << "Printed value"
<< "\n58. " << 58. // double
<< "\n4e2 " << 4e2 // double
<< "\n123.456e-67 " << 123.456e-67 // double
<< "\n123.456e-67f " << 123.456e-67f // float, truncated to zero
<< "\n.1E4f " << .1E4f // float
<< "\n0x10.1p0 " << 0x10.1p0 // double
<< "\n0x1p5 " << 0x1p5 // double
<< "\n0x1e5 " << 0x1e5 // integer literal, not floating-point
<< "\n3.14'15'92 " << 3.14'15'92 // double, single quotes ignored (C++14)
<< "\n1.18e-4932l " << 1.18e-4932l // long double
<< std::setprecision(39)
<< "\n3.4028234e38f " << 3.4028234e38f // float
<< "\n3.4028234e38 " << 3.4028234e38 // double
<< "\n3.4028234e38l " << 3.4028234e38l // long double
<< '\n';
assert(3.4028234e38f == std::numeric_limits<float>::max());
assert(3.4028234e38f == 3.402823'5e38f); // Left ends in 4, right ends in 5
assert(3.4028234e38 != 3.402823'5e38);
// Both side below are 3.4028234e38 but the right is a double and
assert(3.4028234e38f != 3.4028234e38); // the left is a float (then promoted to 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
See also
| user-defined literals(C++11) | literals with user-defined suffix |
| C documentation for Floating constant | |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/language/floating_literal
