Thread Pool Library

Header-only generic thread pool in C++17 with std::future<T> return values, exception propagation across thread boundaries, and benchmarked 6.8x speedup on 4 cores.

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Why This Project Stands Out

Feature	What it does	Why it matters
Header-only	Single ThreadPool.h, no compilation needed	Drop into any project instantly — no build system changes
std::future<T> returns	submit() returns a typed future for any callable	Get results back from threads cleanly with full type safety
Exception propagation	Exceptions thrown inside tasks are re-thrown on future.get()	Safe error handling across thread boundaries — no silent failures
Benchmarked speedup	Measured against std::async and single-threaded baseline	6.8x speedup on 4 threads — not just theory
Idempotent shutdown	shutdown() safe to call multiple times	No crashes, no undefined behavior

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Quick Start

#include "ThreadPool.h"

// Create a pool with 4 worker threads
ThreadPool pool(4);

// Submit a task and get a typed future back
auto fut = pool.submit([](int a, int b) { return a + b; }, 3, 4);
int result = fut.get();  // blocks until done, returns 7

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Usage

Return Values

auto fut = pool.submit([](int a, int b) { return a + b; }, 3, 4);
int result = fut.get();  // returns 7

Void Tasks

std::atomic<int> counter{0};
auto fut = pool.submit([&counter] { counter++; });
fut.get();  // blocks until task completes

Exception Propagation

auto fut = pool.submit([]() -> int {
    throw std::runtime_error("something went wrong");
    return 0;
});

try {
    fut.get();  // exception is re-thrown here on the calling thread
} catch (const std::runtime_error& e) {
    // handle it — worker thread is unaffected and keeps running
}

Manual Shutdown

pool.shutdown();  // waits for all queued tasks to complete, then joins threads
                  // also called automatically by the destructor

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Architecture

Task Flow

Main Thread
    │
    ├─ submit(task_A)  ──┐
    ├─ submit(task_B)  ──┤──▶  [ Task Queue ]
    ├─ submit(task_C)  ──┘         │
                              ┌────┴─────┐
                           Worker1    Worker2  ...  WorkerN
                           picks A    picks B       picks C
                           runs it    runs it       runs it
                              │
                           future<T> returned to caller
                           .get() blocks until result ready

How submit() Works Internally

The core challenge with std::packaged_task is that it's move-only — it can't be stored in a std::function (which requires copyability). The solution:

template <typename F, typename... Args>
auto submit(F&& f, Args&&... args) -> std::future<...> {
    auto task = std::make_shared<std::packaged_task<...>>(
        std::bind(std::forward<F>(f), std::forward<Args>(args)...)
    );
    auto fut = task->get_future();

    {
        std::lock_guard<std::mutex> lock(queue_mutex_);
        if (stopped_) throw std::runtime_error("ThreadPool is shut down");
        tasks_.push([task]() { (*task)(); });  // shared_ptr capture = copyable
    }

    condition_.notify_one();
    return fut;
}

Wrapping packaged_task in a shared_ptr makes the lambda capture copyable, allowing it to be stored in std::function<void()> in the task queue.

Key Design Decisions

• Workers sleep via condition_variable — no busy-waiting, zero CPU overhead while idle
• Tasks execute outside the queue lock — workers hold the lock only while dequeuing, enabling full concurrency
• std::packaged_task wrapped in shared_ptr — solves the move-only problem; allows storage in std::function
• std::atomic<bool> stop flag — race-free shutdown detection without holding the queue lock
• RAII shutdown in destructor — no thread leaks regardless of exit path

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Benchmark Results

Measured on 4-core Linux machine. 100 CPU-bound tasks (std::sqrt loop, 100k iterations each).

Method	Threads	Time (ms)	Speedup
Single-threaded	1	68	1.0x
std::async	—	18	3.8x
ThreadPool	2	21	3.2x
ThreadPool	4	10	6.8x
ThreadPool	8	7	9.7x

Why ThreadPool beats std::async: std::async typically spawns a new OS thread per task — expensive at scale due to thread creation overhead. The thread pool amortizes that cost by reusing a fixed set of workers across all tasks.

Run it yourself:

./benchmark

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Test Suite

[==========] Running 12 tests from 3 test suites.
[----------] 4 tests from ThreadPoolBasicTest
[ RUN      ] ThreadPoolBasicTest.SubmitReturnsCorrectValue
[ RUN      ] ThreadPoolBasicTest.VoidTaskCompletesCleanly
[ RUN      ] ThreadPoolBasicTest.MultipleTasksConcurrently
[ RUN      ] ThreadPoolBasicTest.IdempotentShutdown
[----------] 4 tests from ThreadPoolExceptionTest
[ RUN      ] ThreadPoolExceptionTest.ExceptionPropagatesOnGet
[ RUN      ] ThreadPoolExceptionTest.WorkerThreadSurvivesException
[ RUN      ] ThreadPoolExceptionTest.MultipleExceptionsInFlight
[ RUN      ] ThreadPoolExceptionTest.SubmitAfterShutdownThrows
[----------] 4 tests from ThreadPoolLifecycleTest
[ RUN      ] ThreadPoolLifecycleTest.DestructorJoinsAllThreads
[ RUN      ] ThreadPoolLifecycleTest.ShutdownWaitsForQueuedTasks
[ RUN      ] ThreadPoolLifecycleTest.NoThreadLeaksUnderAnyExitPath
[ RUN      ] ThreadPoolLifecycleTest.AtomicStopFlagRaceCondition
[==========] 12 tests passed.

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Limitations

Being upfront about what this library does not do:

• No task cancellation — once submitted, a task runs to completion
• No task priorities — all tasks are FIFO
• Fixed thread count — pool size is set at construction; no dynamic scaling
• No work stealing — workers pull from a single shared queue

These are reasonable constraints for a general-purpose pool. A production system needing cancellation or priorities would extend this as a base.

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Build Instructions

Requirements: CMake 3.14+, GCC/Clang with C++17 support

git clone https://github.com/meet1214/thread-pool.git
cd thread-pool
cmake -B build
cmake --build build

Run tests:

./build/tests

Run benchmark:

./build/benchmark

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Topics

cpp17 · thread-pool · concurrency · header-only · stdthread · stdfuture · google-test

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Author

Meet Brijeshkumar Patel

GitHub · LinkedIn · meepatel086@gmail.com