When Does async_logger Defer Destruction in spdlog?

Destruction is deferred whenever the async_logger still has messages pending in the lock-free queue, keeping the logger alive via shared_ptr until the background thread pool processes every queued entry.

The async_logger in the gabime/spdlog library provides high-performance asynchronous logging by offloading message processing to a background thread pool. When examining when does async_logger defer destruction in spdlog, the answer centers on how the library prevents message loss during shutdown by delaying the logger's actual destruction until its queue is drained.

How the Deferral Mechanism Works

In include/spdlog/async.h, the global thread pool implementation explicitly documents this behavior. When an asynchronous logger is destroyed while messages remain queued, the actual destruction is postponed until the background thread finishes processing all pending entries.

The mechanism relies on reference counting through the lock-free queue:

• Each queued message holds a std::shared_ptr to its originating logger
• This shared pointer keeps the logger alive even if the application releases its own references
• Only when the queue empties and the background thread processes the final message does the reference count drop to zero

As implemented in include/spdlog/async_logger.h (lines 14-15), the implicitly generated destructor ensures that any remaining queued messages are logged before the object is finally destroyed. According to the source comments in async.h (lines 11-15), the logger's "actual destruction will defer until all its messages are processed by the thread pool."

Code Examples

Basic Logger Destruction

The following example demonstrates how destruction is deferred when messages are still pending:

#include <spdlog/async.h>
#include <spdlog/sinks/basic_file_sink.h>

int main() {
    // Create an async logger that writes to a file.
    auto async_file = spdlog::create_async<spdlog::sinks::basic_file_sink_mt>(
        "async_file", "mylog.txt");

    async_file->info("First message");   // queued, not written immediately
    // async_file goes out of scope here …
}

When async_file leaves scope, the async_logger object is scheduled for destruction, but the actual memory release is delayed until the background thread finishes processing the queued "First message".

Demonstrating Queue-Dependent Deferral

This example uses a small queue to force pending messages and make the deferral behavior observable:

#include <spdlog/async.h>
#include <spdlog/sinks/basic_file_sink.h>
#include <thread>

int main() {
    // Create a logger with a tiny queue to force pending messages.
    spdlog::init_thread_pool(2, 1);               // 2-slot queue, 1 thread
    auto logger = spdlog::create_async<spdlog::sinks::basic_file_sink_mt>(
        "tiny", "tiny.log");

    // Flood the queue – the last messages will still be pending.
    for (int i = 0; i < 10; ++i)
        logger->info("msg {}", i);

    // The logger is destroyed *after* this block, but its real destruction
    // occurs only after the background thread empties the queue.
    logger.reset();            // explicit reset – still deferred
    std::this_thread::sleep_for(std::chrono::seconds(1)); // give the worker time
}

Even after reset() the logger remains alive until the worker thread drains the queue; the deferred destruction guarantees that no message is lost.

Key Implementation Files

Understanding when async_logger defers destruction requires examining these specific source files:

• include/spdlog/async.h (lines 11-15): Contains the global async logger factory and the explicit comment stating that the logger's "actual destruction will defer until all its messages are processed by the thread pool"
• include/spdlog/async_logger.h (lines 14-15): Implements the async_logger class whose destructor handles remaining queued messages before final destruction
• include/spdlog/details/thread_pool.h: Provides the background thread pool that processes the queued log messages

These files work together to ensure that destruction is deferred whenever the logger still has queued messages—that is, as long as the background thread has not emptied the queue.

Summary

• Destruction timing: The async_logger defers its actual destruction whenever messages remain in the lock-free queue
• Reference counting: Each queued message holds a std::shared_ptr to the logger, keeping it alive until processing completes
• Thread pool dependency: The background thread in thread_pool.h must drain all messages before the logger is finally destroyed
• Message safety: This mechanism prevents log message loss during application shutdown or logger destruction

Frequently Asked Questions

How long does the deferral last?

The deferral lasts until the background thread pool processes every queued message containing the logger's std::shared_ptr. Once the queue empties, the reference count drops to zero and the destructor runs immediately. According to the source code in async.h, this ensures that the "actual destruction will defer until all its messages are processed by the thread pool."

Can I force immediate destruction of an async_logger?

No, you cannot safely force immediate destruction if messages are queued. Calling logger.reset() or letting the logger go out of scope only removes the application's reference; the logger persists until the background thread finishes processing its queue. To minimize wait time, call spdlog::default_logger()->flush() or use the flush_every method before destruction to ensure messages are processed quickly.

Does this deferral prevent memory leaks?

The deferral mechanism itself prevents message loss but requires careful application exit handling. If the application terminates before the thread pool drains the queue, the logger objects may persist longer than expected. Always ensure the thread pool has sufficient time to process messages, or explicitly flush queues before exiting the main thread to avoid potential issues with logger lifetimes.

What happens if the thread pool is destroyed before the logger?

If the global thread pool is destroyed while loggers still have pending messages, those messages may be lost or the application may encounter undefined behavior. The deferral mechanism assumes the thread pool remains active. Always destroy loggers or allow them to go out of scope only after ensuring the thread pool will continue running until queues are empty.

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