std::numeri...round_style(3)

💅 NAME 💅

std::numeric_limits::round_style — std::numeric_limits::round_style

okay bestie, welcome to the std::numeri...round_style(3) page from the C++ Standard Libary manual. This is literally the glow-up guide to how your floating-point type rounds stuff when a value isn't exactly representable. iconic behavior only ✨

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✨ Synopsis ✨

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 representable values of T is stored in an object of that type. no cap, that's the whole vibe 🎀

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🎀 Standard specializations 🎀

Here's the value of std::numeric_limits<T>::round_style for each T, bestie:

• /* 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

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🧠 Notes 🧠

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. so yeah, don't expect this constant to spill the tea about runtime rounding changes 👀

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💖 Example 💖

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. literally math with main character energy 💅

// Run this code

#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

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👀 See also 👀

• float_round_style — indicates floating-point rounding modes (enum)

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xoxo, your bestie 💖 — source: http://cppreference.com, version/date: 2024.06.10, page: std::numeri...round_style(3) 🌈👑