Why creating a thread inside an unique_ptr makes the thread destruct? - c++

Question

I wrote a simple function that flushes to a file in a thread, so it does not block the main thread:
void MultiChannelDiskRecordingWav::flush() {
size_t amountToWrite = mWriteCursorFrames*mChannelCount;
for (size_t i = 0; i < amountToWrite; i++) {
tempWriteBuffer[i] = writeBuffer[i];
}
auto flush = [this, capturedWriteCursorFrames = mWriteCursorFrames, capturedChannelCount = mChannelCount](){
{
std::unique_lock<std::mutex>lk{recordingFileMutex};
std::vector<float> floats(tempWriteBuffer, tempWriteBuffer + mWriteCursorFrames*mChannelCount);
recordingFile.Write(floats);
bytesWritten += capturedWriteCursorFrames*capturedChannelCount*sizeof(float);
}
};
flushThread = std::make_unique<std::thread>(flush);
}
I'm getting a crash with these functions in the backtrace:
(_ZSt9terminatev+52)
(_ZNSt6__ndk16threadD1Ev+24)
(_ZN28MultiChannelDiskRecordingWav5flushEv+268)
which translate to
(std::terminate() 52)
(std::__ndk1::thread::~thread() 24)
(MultiChannelDiskRecordingWav::flush() 268)
Why is the thread being deleted? I'm not moving it, I'm creating inside an std::unique_ptr

Answer 1

On this line:
flushThread = std::make_unique<std::thread>(flush);
You are creating a new std::unique_ptr<std::thread> and assigning it to an existing std::unique_ptr<std::thread> named flushThread. When flushThread is assigned to, it will destroy any std::thread that it already holds. It is that existing std::thread that is being destroyed, not the new std::thread that you just created.
When a std::thread object is destroyed, its destructor will call std::terminate() if the thread is joinable(). So, you will have to do something like this to ensure that the existing thread is finished before you create the new thred:
if (flushThread) flushThread->join(); // <-- add this
flushThread = std::make_unique<std::thread>(flush);