Random Number Generation | C++

Intuition

Random number generation in C++ separates the source of randomness (engine) from the statistical properties (distribution). The Mersenne Twister engine produces a long, high-quality sequence of uniform random bits, and distributions reshape those bits into the bell curve, dice rolls, or other patterns you need. std::random_device provides true entropy from the operating system for seeding. The key insight is that the same engine can produce different distributions by changing only the distribution object, making the system modular and reusable.

Random Number Generation

The <random> header provides a modular random number generation system consisting of engines (stateful objects producing random number sequences), distributions (transforming engine output into Desired statistical distributions), and seed sequences (producing initial state from entropy Sources). This section covers the engine hierarchy, distribution types, seeding strategies, and Practical usage patterns.

Overview

The <random> header [N4950 §29.6] provides a modular random number generation system consisting Of:

Engines: Stateful objects that produce a sequence of random numbers.
Distributions: Objects that transform the engine”s output into a desired statistical distribution.
Seed sequences: Objects that produce seed values for engines from a small amount of entropy.

The design separates the source of randomness (engine) from the statistical properties (distribution), allowing any engine to be paired with any distribution.

┌───────────────┐    unsigned int sequence    ┌──────────────────┐    T
│   Engine      │─────────────────────────────>│  Distribution    │─────────>
│  (e.g. mt19937)│                              │ (e.g. normal)    │
└───────────────┘                              └──────────────────┘

Random Number Engines

An engine satisfies the UniformRandomBitGenerator requirement [N4950 §29.6.3]: it provides operator() that returns a UIntType value uniformly distributed over [min(), max()].

Engine	Period	State Size	Quality	Use Case
`std::linear_congruential_engine`	~2^32	Small	Low	Legacy (`rand()` equivalent)
`std::mersenne_twister_engine` (`mt19937`)	2^19937 - 1	2500 bytes	High	General purpose
`std::mt19937_64`	2^19937 - 1	2500 bytes	High	64-bit general purpose
`std::subtract_with_carry_engine`	~2^63	Moderate	Moderate	Historical
`std::random_device`	N/A	N/A	OS-dependent	True entropy (non-deterministic)

std::mt19937 (Mersenne Twister) is the standard workhorse engine. It has a period of 2^19937 - 1 (a Mersenne prime), which means the sequence does not repeat for all practical purposes. Its state Is 2500 bytes (624 × 32-bit words). It passes most statistical tests but is not Cryptographically secure [N4950 §29.6.3.4].

std::random_device is a non-deterministic uniform random bit generator that obtains entropy From the operating system (/dev/urandom on Linux, BCryptGenRandom on Windows) [N4950 §29.6.5.3].

Distributions

Distributions transform the engine’s raw output into values drawn from a specific statistical Distribution [N4950 §29.6.4]:

Distribution	Header	Output Type	Description
`std::uniform_int_distribution<IntT>`	`<random>`	`IntT`	Uniform over [a, b]
`std::uniform_real_distribution<RealT>`	`<random>`	`RealT`	Uniform over [a, b)
`std::normal_distribution<RealT>`	`<random>`	`RealT`	Normal (Gaussian) with mean μ, stddev σ
`std::bernoulli_distribution`	`<random>`	`bool`	Bernoulli trial with probability p
`std::binomial_distribution<IntT>`	`<random>`	`IntT`	Binomial(n, p)
`std::poisson_distribution<IntT>`	`<random>`	`IntT`	Poisson(λ)
`std::exponential_distribution<RealT>`	`<random>`	`RealT`	Exponential(λ)
`std::discrete_distribution<IntT>`	`<random>`	`IntT`	Arbitrary discrete distribution

Seeded Random Number Generation

#include <cstdint>
#include <iostream>
#include <random>
#include <vector>

struct Rng {
    std::mt19937 engine;

    explicit Rng(std::uint32_t seed)
        : engine(seed) {}

    std::uint32_t next_u32() {
        return engine();
    }

    std::uint32_t range(std::uint32_t lo, std::uint32_t hi) {
        std::uniform_int_distribution<std::uint32_t> dist(lo, hi);
        return dist(engine);
    }

    double uniform01() {
        std::uniform_real_distribution<double> dist(0.0, 1.0);
        return dist(engine);
    }

    bool coin_flip(double p = 0.5) {
        std::bernoulli_distribution dist(p);
        return dist(engine);
    }
};

void seeded_rng_demo() {
    // ── Seeded with a fixed seed (reproducible) ──────────────────
    Rng rng(42);

    std::cout << "Reproducible sequence:\n";
    for (int i = 0; i < 5; ++i) {
        std::cout << "  " << rng.range(1, 100);
    }
    std::cout << "\n";
    // Always produces the same 5 numbers

    // ── Seeded with random_device (non-deterministic) ───────────
    std::random_device rd;
    Rng rng2(rd());

    std::cout << "Random sequence:\n";
    for (int i = 0; i < 5; ++i) {
        std::cout << "  " << rng2.range(1, 100);
    }
    std::cout << "\n";

    // ── Seeded with seed_seq (from multiple entropy sources) ─────
    std::uint32_t seed_data[5] = {
        static_cast<std::uint32_t>(std::random_device{}()),
        static_cast<std::uint32_t>(std::random_device{}()),
        0xDEADBEEF,
        static_cast<std::uint32_t>(__LINE__),
        42
    };
    std::seed_seq seq(std::begin(seed_data), std::end(seed_data));

    std::vector<std::uint32_t> seeds(1);
    seq.generate(seeds.begin(), seeds.end());

    Rng rng3(seeds[0]);
    std::cout << "seed_seq seeded: " << rng3.range(1, 100) << "\n";
}

Sampling from Normal Distribution

#include <algorithm>
#include <cmath>
#include <cstdint>
#include <iostream>
#include <numeric>
#include <random>
#include <vector>

struct Histogram {
    std::vector<std::pair<double, double>> bins;
    std::vector<int> counts;

    explicit Histogram(double lo, double hi, int num_bins)
        : bins(num_bins)
        , counts(num_bins, 0) {
        double step = (hi - lo) / num_bins;
        for (int i = 0; i < num_bins; ++i) {
            bins[i] = {lo + i * step, lo + (i + 1) * step};
        }
    }

    void add(double value) {
        for (std::size_t i = 0; i < bins.size(); ++i) {
            if (value >= bins[i].first && value < bins[i].second) {
                ++counts[i];
                return;
            }
        }
        if (!bins.empty()) {
            ++counts.back();
        }
    }

    void print() const {
        int max_count = *std::max_element(counts.begin(), counts.end());
        for (std::size_t i = 0; i < bins.size(); ++i) {
            double mid = (bins[i].first + bins[i].second) / 2.0;
            int bar_len = max_count > 0
                ? static_cast<int>(40.0 * counts[i] / max_count)
                : 0;
            std::cout << std::fixed << std::setprecision(2)
                      << std::setw(6) << mid << " |"
                      << std::string(bar_len, '*')
                      << " " << counts[i] << "\n";
        }
    }
};

void normal_distribution_demo() {
    std::uint32_t seed = 12345;
    std::mt19937 engine(seed);

    const double mean = 0.0;
    const double stddev = 1.0;
    std::normal_distribution<double> dist(mean, stddev);

    const int n = 100'000;
    std::vector<double> samples(n);
    for (int i = 0; i < n; ++i) {
        samples[i] = dist(engine);
    }

    // Compute statistics
    double sample_mean = std::accumulate(samples.begin(), samples.end(), 0.0) / n;
    double sq_sum = 0.0;
    for (double x : samples) {
        sq_sum += (x - sample_mean) * (x - sample_mean);
    }
    double sample_stddev = std::sqrt(sq_sum / n);

    std::cout << "Sample mean:   " << sample_mean << "\n";
    std::cout << "Sample stddev: " << sample_stddev << "\n";
    std::cout << "Expected mean: " << mean << "\n";
    std::cout << "Expected stdd: " << stddev << "\n\n";

    // Print histogram
    Histogram hist(-4.0, 4.0, 16);
    for (double x : samples) {
        hist.add(x);
    }
    hist.print();
}

Output (example):

Sample mean:   -0.00129
Sample stddev: 0.99897
Expected mean: 0
Expected stdd: 1

 -3.75 |                                      2
 -3.25 |*                                     52
 -2.75 |******                               298
 -2.25 |*************                        689
 -1.75 |********************                 1312
 -1.25 |*****************************        2158
 -0.75 |*********************************    2632
 -0.25 |***********************************  2659
  0.25 |***********************************  2641
  0.75 |*********************************    2637
  1.25 |*****************************        2172
  1.75 |********************                 1336
  2.25 |*************                        693
  2.75 |******                               309
  3.25 |*                                     59
  3.75 |                                      1

#include <iostream>
#include <random>
#include <sstream>

void engine_serialization_demo() {
    std::mt19937 engine(42);

    std::cout << "Before save: " << engine() << " " << engine() << "\n";

    // Save state
    std::ostringstream oss;
    oss << engine;

    // Generate more values
    std::cout << "More values: " << engine() << " " << engine() << "\n";

    // Restore state — subsequent values will match the saved point
    std::istringstream iss(oss.str());
    iss >> engine;

    std::cout << "After restore: " << engine() << " " << engine() << "\n";
    // After restore prints the same values as "More values"
}

This serialization is essential for:

Reproducible simulations: Save the engine state at checkpoints and replay from any point.
Networked games: Synchronize the RNG state across clients for deterministic behavior.
Fuzz testing: Record the RNG state that triggered a crash and replay it.

`std::random_device` Implementation Details

std::random_device is the standard library’s interface to OS-provided entropy [N4950 §29.6.5.3]:

Platform	Implementation (Typical)	Entropy Source
Linux	Reads from `/dev/urandom`	Kernel CSPRNG (ChaCha20)
macOS	`arc4random_buf` or `/dev/urandom`	Kernel CSPRNG
Windows	`BCryptGenRandom` or `RtlGenRandom`	OS cryptographic RNG
MinGW	Historically broken (fixed GCC 9+)	Was PRNG, now OS entropy

#include <iostream>
#include <random>

void random_device_props() {
    std::random_device rd;

    std::cout << "Entropy: " << rd.entropy() << "\n";
    // On Linux: in standard practice 32.0 (full 32-bit entropy)
    // On some implementations: 0.0 (entropy estimate not available)

    std::cout << "Min: " << rd.min() << "\n";
    std::cout << "Max: " << rd.max() << "\n";
    // Min: 0, Max: 4294967295 (UINT_MAX) on most platforms
}

`std::seed_seq` and Initialization Quality

The Mersenne Twister’s standard initialization (mt19937(seed)) takes a single 32-bit seed and Expands it into the 624-word state. This expansion has known weaknesses: the lower bits of the Initial state have lower entropy [Matsumoto & Nishimura, 1998]. std::seed_seq addresses this by Producing a well-distributed initial state from multiple entropy sources [N4950 §29.6.3.8]:

#include <cstdint>
#include <iostream>
#include <random>

void seed_seq_quality_demo() {
    // Weak initialization: single 32-bit seed
    // Only 2^32 possible initial states — not enough for the 2^19937-1 period
    std::mt19937 weak(42);

    // Strong initialization: seed_seq with multiple entropy sources
    std::random_device rd;
    std::seed_seq seq{
        rd(), rd(), rd(), rd(),
        static_cast<std::uint32_t>(0xDEADBEEF),
        static_cast<std::uint32_t>(0xCAFEBABE)
    };

    std::mt19937 strong(seq);

    // seed_seq can also produce a sequence of seed values
    std::vector<std::uint32_t> seeds(10);
    seq.generate(seeds.begin(), seeds.end());

    std::cout << "Generated seeds: ";
    for (auto s : seeds) {
        std::cout << s << " ";
    }
    std::cout << "\n";
}

The std::seed_seq::generate algorithm uses a warm-up process based on the initialization algorithm From the Mersenne Twister paper. It performs multiple mixing passes to ensure all bits of the Initial state have high entropy.

`std::uniform_int_distribution` Modulo Bias

A naive way to generate a random integer in [0, n) is engine() % n. This introduces modulo Bias when n does not evenly divide the engine’s range [N4950 §29.6.4.1]:

$P(\mathrm{outcome k) = \begin{cases} \lceil R / n \rceil / R & \mathrm{if k \lt R \bmod n \\ \lfloor R / n \rfloor / R & \mathrm{otherwise \end{cases}$

Where $R = \mathrm{max - \mathrm{min + 1$ is the engine’s range.

For example, with a 16-bit engine ( $R = 65536$ ) and $n = 3$ :

$65536 = 21845 \times 3 + 1$
Outcomes 0 and 1 have probability $21846 / 65536$
Outcome 2 has probability $21845 / 65536$

This bias is small for large ranges, but std::uniform_int_distribution eliminates it entirely by Using rejection sampling internally.

Discrete Distribution: Weighted Random Selection

std::discrete_distribution allows sampling from an arbitrary discrete probability distribution Defined by a set of weights [N4950 §29.6.4.5]:

#include <iostream>
#include <random>
#include <string>
#include <vector>

void discrete_distribution_demo() {
    std::mt19937 engine(12345);

    // Define a custom distribution over actions
    std::vector<double> weights = {0.50, 0.30, 0.15, 0.05};
    std::vector<std::string> actions = {"idle", "walk", "run", "jump"};

    std::discrete_distribution<int> dist(weights.begin(), weights.end());

    std::vector<int> counts(4, 0);
    constexpr int n = 100'000;

    for (int i = 0; i < n; ++i) {
        int action = dist(engine);
        ++counts[action];
    }

    for (std::size_t i = 0; i < actions.size(); ++i) {
        double freq = static_cast<double>(counts[i]) / n;
        std::cout << actions[i] << ": " << freq
                  << " (expected " << weights[i] << ")\n";
    }
    // idle: ~0.50, walk: ~0.30, run: ~0.15, jump: ~0.05
}

Poisson and Exponential Distributions

These distributions model event arrival processes and are essential for simulation:

#include <cmath>
#include <iostream>
#include <random>
#include <vector>

void arrival_process_demo() {
    std::mt19937 engine(42);

    // Poisson distribution: number of events in a fixed interval
    // Parameter lambda = average rate per interval
    std::poisson_distribution<int> events_per_hour(5.0);

    std::cout << "Events per hour (Poisson, lambda=5):\n";
    for (int i = 0; i < 20; ++i) {
        std::cout << "  " << events_per_hour(engine);
    }
    std::cout << "\n";

    // Exponential distribution: time between events
    // Parameter lambda = rate (events per unit time)
    std::exponential_distribution<double> interarrival(5.0);

    std::cout << "Inter-arrival times (Exponential, lambda=5):\n";
    double total = 0.0;
    for (int i = 0; i < 10; ++i) {
        double t = interarrival(engine);
        total += t;
        std::cout << "  " << t << "s";
    }
    std::cout << "\n";
    std::cout << "Total simulated time: " << total << "s\n";
}

Performance Considerations

Engine	State Size	`operator()` Speed	Memory Footprint
`minstd_rand`	4 bytes	Very fast (LCG)	Minimal
`mt19937`	2500 bytes	Fast (~5 ns/call)	Large
`mt19937_64`	2500 bytes	Fast (~5 ns/call)	Large
`ranlux48`	96 bytes	Slow (discarding)	Moderate
`random_device`	N/A	Very slow (OS call)	None

#include <chrono>
#include <iostream>
#include <random>

void engine_benchmark() {
    using namespace std::chrono;
    constexpr int iterations = 10'000'000;

    // mt19937
    {
        std::mt19937 engine(42);
        auto start = steady_clock::now();
        volatile std::uint32_t sink = 0;
        for (int i = 0; i < iterations; ++i) {
            sink = engine();
        }
        auto elapsed = duration_cast<milliseconds>(steady_clock::now() - start);
        std::cout << "mt19937: " << elapsed.count() << " ms for "
                  << iterations << " values\n";
    }

    // random_device (MUCH slower — OS syscall per call)
    {
        std::random_device rd;
        auto start = steady_clock::now();
        volatile unsigned int sink = 0;
        for (int i = 0; i < 100'000; ++i) {
            sink = rd();
        }
        auto elapsed = duration_cast<milliseconds>(steady_clock::now() - start);
        std::cout << "random_device: " << elapsed.count() << " ms for "
                  << 100'000 << " values\n";
    }
}

Common Pitfalls

Seeding mt19937 with a single 32-bit value: The engine has 2496 bytes of state. A single 32-bit seed can only produce 2^32 distinct initial states — a tiny fraction of the engine’s 2^19937-1 period. Use std::seed_seq with multiple entropy sources.
Using mt19937 for cryptography: The Mersenne Twister is not cryptographically secure. Given 624 consecutive outputs, the full internal state can be recovered. Use a CSPRNG (e.g., std::random_device backed by /dev/urandomOr a library like libsodium) for security-sensitive applications.
Creating distributions inside the loop: std::uniform_int_distribution is lightweight, but some distributions (like std::discrete_distribution) have expensive constructors. Create the distribution once and reuse it.
Ignoring thread safety: None of the standard engine or distribution types are thread-safe. Each thread should have its own engine, or access must be protected by a mutex.
Assuming random_device is always non-deterministic: On some older MinGW implementations, random_device was implemented as a fixed-seed PRNG. Verify on your target platform, or read from /dev/urandom directly on POSIX systems.
Floating-point distribution bounds: std::uniform_real_distribution<double>(0.0, 1.0) produces values in $[0.0, 1.0)$ — the upper bound is exclusive. If you need a closed interval $[0.0, 1.0]$ Use std::uniform_real_distribution<double>(0.0, std::nextafter(1.0, 2.0)).

Common Pitfalls

Not practising with past papers or exercises under timed conditions.
Not making connections between different topics within the subject to build a coherent understanding.
Memorising content without understanding the underlying principles. This leads to poor application in unfamiliar contexts.
Ignoring feedback from marked work and failing to address recurring weaknesses.

flowchart TD
    A[3_Random_Numbers] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.

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Random Number Generation | C++

Intuition