std::setprecision(3)

💅 NAME

std::setprecision — literally just std::setprecision, bestie 💖 (and yeah, this whole thing lives in the C++ Standard Libary — that's the manual name, no cap, spelling and all, we don't fix it, we respect it 🫶)

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✨ Synopsis ✨

Okay so first things first, it's defined in the header <iomanip> 👑

/*unspecified*/ setprecision( int n );

And here's the whole vibe: when you use it in an expression like out << setprecision(n) or in >> setprecision(n), it sets the precision parameter of the stream out or in to exactly n. Exactly. Not approximately. Not ~vibes~. Exactly n 🔥

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🎀 Parameters 🎀

• n — the new value for precision. That's it, that's the parameter 💁‍♀️

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🦋 Return value 🦋

So it returns an object of unspecified type — mysterious, main character energy, we don't know what it is and honestly we don't need to 😌 — such that:

• if out is an object of type std::basic_ostream<CharT, Traits>, the expression out << setprecision(n)
  • has type std::basic_ostream<CharT, Traits>&
  • has value out
  • behaves as if it called f(out, n)
• if in is an object of type std::basic_istream<CharT, Traits>, the expression in >> setprecision(n)
  • has type std::basic_istream<CharT, Traits>&
  • has value in
  • behaves as if it called f(in, n)

where the function f is defined as ⬇️

void f(std::ios_base& str, int n)
{
    // set precision
    str.precision(n);
}

Like, that's the whole function. It's giving "short but powerful" 💅

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🌸 Example 🌸

// Run this code

#include <iomanip>
#include <iostream>
#include <limits>
#include <numbers>

int main()
{
    constexpr long double pi{std::numbers::pi_v<long double>};

    const auto default_precision{std::cout.precision()};
    constexpr auto max_precision{std::numeric_limits<long double>::digits10 + 1};

    std::cout << "default precision: " << default_precision << '\n'
              << "maximum precision: " << max_precision << "\n\n"
                 "precision: pi:\n";

    for (int p{0}; p <= max_precision; ++p)
        std::cout << std::setw(2) << p << "  " << std::setprecision(p) << pi << '\n';

    std::cout << std::setprecision(default_precision); // restore defaults
}

⭐ Output ⭐

default precision: 6
maximum precision: 19

precision: pi:
 0  3
 1  3
 2  3.1
 3  3.14
 4  3.142
 5  3.1416
 6  3.14159
 7  3.141593
 8  3.1415927
 9  3.14159265
10  3.141592654
11  3.1415926536
12  3.14159265359
13  3.14159265359
14  3.1415926535898
15  3.14159265358979
16  3.141592653589793
17  3.1415926535897932
18  3.14159265358979324
19  3.141592653589793239

Not the 13 and 14 rows both showing 3.14159265359 😭 yes bestie that's really the output, we don't edit it, we love it as it is 🫶

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👑 Defect reports 👑

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (Retroactively! Time travel! Iconic ⏳✨)

• DR LWG 183 — applied to C++98
  • behavior as published: setprecision could only be used with streams of type std::ostream or std::istream
  • correct behavior: usable with any character stream 🌈

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💜 See also 💜

• fixed, scientific, hexfloat (C++11), defaultfloat (C++11) — changes formatting used for floating-point I/O (function)
• precision — manages decimal precision of floating point operations (public member function of std::ios_base)

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ok that's the whole page, you ate that 💅 now go set some precision and slay ✨

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source: http://cppreference.com · version/date: 2024.06.10 · std::setprecision(3) — mwah, bye bestie, see you in the next manpage 🎀💖🧚