floating point literal

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

C++ language

 

Expressions

General

value categories (lvalue, rvalue, xvalue)

order of evaluation (sequence points)

constant expressions

unevaluated expressions

primary expressions

lambda-expression(C++11)

Literals

integer literals

floating-point literals

boolean literals

character literals including escape sequences

string literals

null pointer literal(C++11)

user-defined literal(C++11)

Operators

Assignment operators: a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b

Increment and decrement: ++a, --a, a++, a--

Arithmetic operators: +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b

Logical operators: a||b, a&&b, !a

Comparison operators: a==b, a!=b, a<b, a>b, a<=b, a>=b

Member access operators: a[b], *a, &a, a->b, a.b, a->*b, a.*b

Other operators: a(...), a,b, a?b:c

Alternative representations of operators

Precedence and associativity

Fold expression(C++17)

new-expression

delete-expression

throw-expression

alignof

sizeof

sizeof...(C++11)

typeid

noexcept(C++11)

Operator overloading

Conversions

Implicit conversions

const_cast

static_cast

reinterpret_cast

dynamic_cast

Explicit conversions (T)a, T(a)

User-defined conversion

 

Floating point literal defines a compile-time constant whose value is specified in the source file.

Syntax

significand exponent(optional) suffix(optional)

Where the significand has one of the following forms

digit-sequence	(1)
digit-sequence .	(2)
digit-sequence(optional) . digit-sequence	(3)
0x | 0X hex-digit-sequence	(4)	(since C++17)
0x | 0X hex-digit-sequence .	(5)	(since C++17)
0x | 0X hex-digit-sequence(optional) . hex-digit-sequence	(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

The exponent has the form

e | E exponent-sign(optional) digit-sequence	(1)
p | P exponent-sign(optional) digit-sequence	(2)	(since C++17)

1) The exponent syntax for a decimal floating-point literal

2) The exponent syntax for hexadecimal floating-point literal

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 F defines float
• l L defines long double

Optional single quotes(') can be inserted between the digits as a separator, they are ignored when compiling.

(since C++14)

Explanation

Decimal scientific notation is used, meaning that the value of the floating-point literal is the significand mutiplied by the number 10 raised to the power of exponent. The mathematical meaning of 123e4 is 123×10⁴

If the significand begins with the character sequence 0x or 0X, the floating constant is a hexadecimal floating constant. Otherwise, it is a decimal floating constant. For a hexadecimal floating constant, 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 = 0x1.2p3; // hex fraction 1.2 (decimal 1.125) scaled by 2^3, that is 9.0

(since C++17)

Example

Run this code

#include <iostream>
int main()
{
  std::cout << 123.456e-67 << '\n'
            << .1E4f       << '\n'
            << 58.         << '\n'
            << 4e2         << '\n';
}

Output:

1.23456e-65
1000
58
400

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

See also

C documentation for floating point constant