std::fixed,...efaultfloat(3)

🎀 NAME 🎀

std::fixed,std::scientific,std::hexfloat,std::defaultfloat — std::fixed,std::scientific,std::hexfloat,std::defaultfloat

okay bestie, welcome to the std::fixed,...efaultfloat(3) page from the C++ Standard Libary — yes, that's literally how the header spells it, not me being messy 🫶 — section 3. this whole page is about four little I/O manipulators that change how floating-point numbers get formatted, and honestly? they ate. 💅

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📋 Synopsis 📋

Defined in header <ios>

std::ios_base& fixed( std::ios_base& str );        (1)
std::ios_base& scientific( std::ios_base& str );   (2)
std::ios_base& hexfloat( std::ios_base& str );     (3) (since C++11)
std::ios_base& defaultfloat( std::ios_base& str ); (4) (since C++11)

Modifies the default formatting for floating-point output.

1. Sets the floatfield of the stream str to fixed as if by calling str.setf(std::ios_base::fixed, std::ios_base::floatfield).
2. Sets the floatfield of the stream str to scientific as if by calling str.setf(std::ios_base::scientific, std::ios_base::floatfield).
3. Sets the floatfield of the stream str to fixed and scientific simultaneously as if by calling str.setf(std::ios_base::fixed | std::ios_base::scientific, std::ios_base::floatfield). This enables hexadecimal floating-point formatting.
4. Sets the floatfield of the stream str to zero, as if by calling str.unsetf(std::ios_base::floatfield). This enables the default floating-point formatting, which is different from fixed and scientific.

This is an I/O manipulator, it may be called with an expression such as out << std::fixed for any out of type std::basic_ostream (or with an expression such as in >> std::scientific for any in of type std::basic_istream).

🌸 Parameters 🌸

• str — reference to I/O stream

💖 Return value 💖

str (reference to the stream after manipulation).

🧠 Notes 🧠

Hexadecimal floating-point formatting ignores the stream precision specification, as required by the specification of std::num_put::do_put.

These manipulators do not affect floating-point parsing.

✨ Example ✨

// Run this code

#include <iomanip>
#include <iostream>
#include <sstream>

enum class cap { title, middle, end };

void print(const char* text, double num, cap c)
{
    if (c == cap::title)
        std::cout <<
            "┌──────────┬────────────┬──────────────────────────┐\n"
            "│  number  │   iomanip  │      representation      │\n"
            "├──────────┼────────────┼──────────────────────────┤\n";
    std::cout << std::left
         << "│ " << std::setw(8) << text <<      " │ fixed      │ "
         << std::setw(24) << std::fixed  << num <<            " │\n"
         << "│ " << std::setw(8) << text <<      " │ scientific │ "
         << std::setw(24) << std::scientific << num <<        " │\n"
         << "│ " << std::setw(8) << text <<      " │ hexfloat   │ "
         << std::setw(24) << std::hexfloat << num <<          " │\n"
         << "│ " << std::setw(8) << text <<      " │ default    │ "
         << std::setw(24) << std::defaultfloat << num <<      " │\n";
    std::cout << (c != cap::end ?
            "├──────────┼────────────┼──────────────────────────┤\n" :
            "└──────────┴────────────┴──────────────────────────┘\n");
}

int main()
{
    print("0.0", 0.0, cap::title);
    print("0.01", 0.01, cap::middle);
    print("0.00001", 0.00001, cap::end);

    // Note; choose clang for correct output
    double f;
    std::istringstream("0x1.8p+0") >> f;
    std::cout << "Parsing 0x1.8p+0 gives " << f << '\n';

    std::istringstream("0x1P-1022") >> f;
    std::cout << "Parsing 0x1P-1022 gives " << f << '\n';
}

🌈 Output 🌈

┌──────────┬────────────┬──────────────────────────┐
│  number  │   iomanip  │      representation      │
├──────────┼────────────┼──────────────────────────┤
│ 0.0      │ fixed      │ 0.000000                 │
│ 0.0      │ scientific │ 0.000000e+00             │
│ 0.0      │ hexfloat   │ 0x0p+0                   │
│ 0.0      │ default    │ 0                        │
├──────────┼────────────┼──────────────────────────┤
│ 0.01     │ fixed      │ 0.010000                 │
│ 0.01     │ scientific │ 1.000000e-02             │
│ 0.01     │ hexfloat   │ 0x1.47ae147ae147bp-7     │
│ 0.01     │ default    │ 0.01                     │
├──────────┼────────────┼──────────────────────────┤
│ 0.00001  │ fixed      │ 0.000010                 │
│ 0.00001  │ scientific │ 1.000000e-05             │
│ 0.00001  │ hexfloat   │ 0x1.4f8b588e368f1p-17    │
│ 0.00001  │ default    │ 1e-05                    │
└──────────┴────────────┴──────────────────────────┘
Parsing 0x1.8p+0 gives 1.5
Parsing 0x1P-1022 gives 2.22507e-308

👀 See also 👀

• setprecision — changes floating-point precision (function)

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that's the whole vibe, bestie — four manipulators, one iconic little table, and no cap, everything above is exactly the page. xoxo 💋

source: http://cppreference.com · version/date: 2024.06.10 · page: std::fixed,...efaultfloat(3) · from the C++ Standard Libary 🫶