std::mersen...ister_engine(3)

💖 NAME 💖

std::mersenne_twister_engine::mersenne_twister_engine — okay so this is the constructor of the Mersenne Twister engine, and yes the name is a whole entire mouthful, we know 💅

This page lives in the C++ Standard Library manual, and it's gonna walk you through how a std::mersenne_twister_engine gets built, seeded, and copied. No cap, it's more math than you'd expect from a constructor, but we're gonna get through it together 🫶

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✨ Synopsis ✨

mersenne_twister_engine() :                                (1) (since C++11)
mersenne_twister_engine(default_seed) {}
explicit mersenne_twister_engine( result_type value );     (2) (since C++11)
template< class SeedSeq >                                  (3) (since C++11)
explicit mersenne_twister_engine( SeedSeq& seq );
mersenne_twister_engine( const mersenne_twister_engine&    (4) (since C++11)
other );
(implicitly declared)

Constructs the pseudo-random number engine. That's the whole vibe of this page 🎀

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🌸 DESCRIPTION 🌸

So here's the thing — there are four ways to build this engine, and each one has its own little personality:

• (1) The default constructor. Just vibes, no seed argument, it does the default thing.
• (2) Constructs the engine with a seed value value. Given 2^w as p, the engine's initial state is determined as follows:
  1. Sets X_{-n} to value % p.
  2. For each integer i in [i - n, -1], sets X_i to [f·(X_{i-1} xor (X_{i-1} rshift (w-2)))+i mod n] mod p, where xor and rshift stand for built-in bitwise XOR and bitwise right-shift respectively. (Yes, it's giving math class. We're doing it though 🧚)
• (3) Constructs the engine with a seed sequence seq. Given std::size_t(w / 32) + 1 as k, the engine's initial state is determined as follows:
  1. Creates an invented array object a of length n * k.
  2. Calls seq.generate(a + 0, a + n * k).
  3. For each integer i in [-n, -1], sets X_i to (∑^{k-1}_{j=0} a_{k(i+n)+j} · 2^{32j}) mod 2^w.
  4. If the most significant w − r bits of X_{-n} are zero, and if each of the other resulting X_i is 0, changes X_{-n} to 2^{w-1}.

Bestie, listen — this overload participates in overload resolution only if SeedSeq meets the requirements of SeedSequence. That's the condition, we don't make the rules 🫠

• (4) The copy constructor. Upon construction, *this == other is true. Iconic, honestly — it just works 👑

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🦋 PARAMETERS 🦋

• value — seed value to use in the initialization of the internal state
• seq — seed sequence to use in the initialization of the internal state

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🔥 COMPLEXITY 🔥

• 1, 2) O(n).
• 3) Same as the complexity of the seq.generate call.
• 4) O(n).

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🌈 DEFECT REPORTS 🌈

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (Retroactive glow-ups, we love to see it ✨)

• P0935R0 — Applied to: C++11 — Behavior as published: the default constructor was explicit — Correct behavior: made implicit.

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⭐ SEE ALSO ⭐

• seed — sets the current state of the engine — (C++11) (public member function)

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Okay that's the whole page, you did amazing, go seed something responsibly 🎀

Source/version + date check: straight from http://cppreference.com, version 2024.06.10, filed under std::mersen...ister_engine(3). Mwah 💋