static const std::float_round_style round_style; | (until C++11) | |

static constexpr std::float_round_style round_style; | (since C++11) |

The value of `std::numeric_limits<T>::round_style`

identifies the rounding style used by the floating-point type `T`

whenever a value that is not one of the exactly repesentable values of `T`

is stored in an object of that type.

`T` | value of `std::numeric_limits<T>::round_style` |
---|---|

/* non-specialized */ | `std::round_toward_zero` |

bool | `std::round_toward_zero` |

char | `std::round_toward_zero` |

signed char | `std::round_toward_zero` |

unsigned char | `std::round_toward_zero` |

wchar_t | `std::round_toward_zero` |

char8_t (since C++20) | `std::round_toward_zero` |

char16_t (since C++11) | `std::round_toward_zero` |

char32_t (since C++11) | `std::round_toward_zero` |

short | `std::round_toward_zero` |

unsigned short | `std::round_toward_zero` |

int | `std::round_toward_zero` |

unsigned int | `std::round_toward_zero` |

long | `std::round_toward_zero` |

unsigned long | `std::round_toward_zero` |

long long (since C++11) | `std::round_toward_zero` |

unsigned long long (since C++11) | `std::round_toward_zero` |

float | usually `std::round_to_nearest` |

double | usually `std::round_to_nearest` |

long double | usually `std::round_to_nearest` |

These values are constants, and do not reflect the changes to the rounding made by `std::fesetround`

. The changed values may be obtained from `FLT_ROUNDS`

or `std::fegetround`

.

The decimal value `0.1`

cannot be represented by a binary floating-point type. When stored in an IEEE-754 double, it falls between 0x1.9999999999999*2-4

and 0x1.999999999999a*2-4

. Rounding to nearest representable value results in 0x1.999999999999a*2-4

.

Similarly, the decimal value `0.3`

, which is between 0x1.3333333333333*2-2

and 0x1.3333333333334*2-2

is rounded to nearest and is stored as 0x1.3333333333333*2-2

.

#include <iostream> #include <limits> auto print(std::float_round_style frs) { switch (frs) { case std::round_indeterminate: return "Rounding style cannot be determined"; case std::round_toward_zero: return "Rounding toward zero"; case std::round_to_nearest: return "Rounding toward nearest representable value"; case std::round_toward_infinity: return "Rounding toward positive infinity"; case std::round_toward_neg_infinity: return "Rounding toward negative infinity"; } return "unknown round style"; } int main() { std::cout << std::hexfloat << "The decimal 0.1 is stored in a double as " << 0.1 << '\n' << "The decimal 0.3 is stored in a double as " << 0.3 << '\n' << print(std::numeric_limits<double>::round_style) << '\n'; }

Output:

The decimal 0.1 is stored in a double as 0x1.999999999999ap-4 The decimal 0.3 is stored in a double as 0x1.3333333333333p-2 Rounding toward nearest representable value

indicates floating-point rounding modes (enum) |

© cppreference.com

Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.

https://en.cppreference.com/w/cpp/types/numeric_limits/round_style