Share object between two lambdas - c++

Question

I am trying to share the same object (in my case, a string) between to lambdas using shared_ptr:
auto fileToLoad = make_shared<string>();
StartAsync(
[=]()
{
QDir dir(dirPath.c_str());
QString fileExtension(fileExt.c_str());
for (int i = 0; i < 150; i++)
{
auto files = dir.entryList(QStringList() << fileExtension << fileExtension.toUpper(), QDir::Files);
if (!files.empty())
{
//fileToLoad.reset(new string(files.first().toUtf8().constData()));
fileToLoad = make_shared<string>(files.first().toUtf8().constData());
break;
}
this_thread::sleep_for(chrono::milliseconds(200));
}
},
[=]()
{
if (fileToLoad == nullptr)
{
DisplayMessage("Timeout reached");
}
else
{
doFunc(*fileToLoad);
}
});
After many attempts I have still not been able to make it work as I want: to store fileToLoad in the first lambda and use it in the second. Or it fails to compile due to qualifiers (const), or it compiles but 'fileToLoad' remains empty.
As you can see I am trying to notify if a file has appeared in a folder, using StartAsync function, which takes 2 lambdas (this function basically creates QObjects and move them to background thread and then makes some connections between signals and slots).
Edit:
StartAsync: takes a lambda of a task (lengthy work), and a lambda of post task (UI updates)
void MainWidget::StartAsync(function<void()> func, function<void()> postTask)
{
AsyncTask* task = new AsyncTask(func, [=]() { if (postTask != nullptr) { postTask(); HideProgressBar(); }});
QThread* thread = new QThread();
task->moveToThread(thread);
connect(thread, &QThread::started, this, &MainWidget::ShowProgressBar);
connect(thread, &QThread::started, task, &AsyncTask::RunTask);
connect(task, &AsyncTask::TaskFinished, task, &AsyncTask::RunPostTask);
connect(thread, &QThread::finished, task, &QObject::deleteLater);
connect(thread, &QThread::finished, thread, &QObject::deleteLater);
thread->start();
}
Any suggestions are appreciated.

Answer 1

When you assign a new value to a std::shared_ptr - you create a new shared value, that doesn't have any connection with the previous one (you can have a look at counters of both shared_ptrs). You should change
fileToLoad = make_shared<string>(files.first().toUtf8().constData());
to
*fileToLoad = files.first().toUtf8().constData()
(of course checking beforehand that fileToLoad isn't null)

Answer 2

You code won't work, because in first lambda you overwrite the temporary pointer inside lambda. What you can do is overwrite std::string behind the pointer and it should work fine:
auto fileToLoad = make_shared<string>();
StartAsync(
[=]()
{
QDir dir(dirPath.c_str());
QString fileExtension(fileExt.c_str());
for (int i = 0; i < 150; i++)
{
auto files = dir.entryList(QStringList() << fileExtension << fileExtension.toUpper(), QDir::Files);
if (!files.empty())
{
//fileToLoad.reset(new string(files.first().toUtf8().constData()));
*fileToLoad = files.first().toStdString();
break;
}
this_thread::sleep_for(chrono::milliseconds(200));
}
},
[=]()
{
if (fileToLoad == nullptr)
{
DisplayMessage("Timeout reached");
}
else
{
doFunc(*fileToLoad);
}
});