Reference to object with std::async - c++

Question

I have a small solution in visual studio 2012. The solution consists of two projects (Scanner and TestApp), Scanner is a dll and TestApp is a small application using the dll.
I would like a function in the dll to run in a thread and to report its result back via a queue to the TestApp.
To test this I wrote a minimal application, but depending on how I launch the thread I get different results and I would like to understand why.
The Scanner.h file looks like this:
#pragma once
#include <iostream>
#include <string>
#include <stdint.h>
#include <atomic>
#include <future>
#include <thread>
#include "version.h"
#include "threadsafe_queue.h"
#include "capture_data.h"
#include "process_data.h"
#ifdef SCANNER_EXPORTS
#define SCANNER_API __declspec(dllexport)
#else
#define SCANNER_API __declspec(dllimport)
#endif
class Scanner
{
public:
static SCANNER_API void run();
static SCANNER_API void stop();
};
Scanner.cpp:
#include "stdafx.h"
#include "Scanner.h"
std::vector<std::future<int>> my_futures;
void Scanner::run()
{
CaptureData capture_data(1234);
auto t = std::async(std::launch::async, &CaptureData::get_data, capture_data);
my_futures.push_back(std::move(t));
}
void Scanner::stop()
{
for(int n=0; n<my_futures.size(); n++) {
auto e = std::move(my_futures.back());
e.get();
my_futures.pop_back();
}
}
The class CaptureData is defined in capture_data.h and capture_data.cpp.
capture_data.h:
#pragma once
#include <atomic>
#include <thread>
#include "iq_data.h"
#include "threadsafe_queue.h"
class CaptureData
{
public:
CaptureData(double freq_start);
void configure();
void get_data();
private:
double m_test;
};
capture_data.cpp
#include "stdafx.h"
#include "capture_data.h"
#include "Scanner.h"
ThreadsafeQueue<int> g_queue_1;
SCANNER_API ThreadsafeQueue<int> g_queue_2;
CaptureData::CaptureData(double test)
: m_test(test) {}
void CaptureData::get_data()
{
cout << "1: " << m_test << std::endl;
Sleep(5000);
cout << "2: " << m_test << std::endl;
g_queue_2.push(3);
cout << "Capture has now pushed data" << std::endl;
}
And finally the TestApp.cpp:
#include "stdafx.h"
#include "tchar.h"
#include <stdint.h>
#include <string>
#include "Scanner.h"
SCANNER_API extern ThreadsafeQueue<int> g_queue_2;
int _tmain(int argc, _TCHAR* argv[])
{
Scanner scanner;
scanner.run();
cout << "TestApp waiting for data..." << std::endl;
int data;
g_queue_2.wait_and_pop(data);
cout << "TestApp got data: " << data << std::endl;
scanner.stop();
return 0;
}
In Scanner.cpp I have tried to launch the thread in two different ways, the first way:
auto t = std::async(std::launch::async, &CaptureData::get_data, capture_data);
Second way is with a reference to the object "capture_data":
auto t = std::async(std::launch::async, &CaptureData::get_data, &capture_data);
The first way seems to work as I intended the application to work and I get the following printouts in my terminal:
TestApp waiting for data...
1: 1234
2: 1234
Capture has now pushed data
TestApp got data: 3
Press any key to continue...
If I use the second way I get:
TestApp waiting for data...
1: 6.95166e-310
2: 6.95166e-310
Capture has now pushed data
TestApp got data: 3
Press any key to continue...
So, what I do not understand is why the variable "m_test" get messed up in the second case.
I would very much appreciate if anyone could shed a light on this.
/M

Answer 1

In the following code:
void Scanner::run()
{
CaptureData capture_data(1234);
auto t = std::async(std::launch::async, &CaptureData::get_data, capture_data);
my_futures.push_back(std::move(t));
}
capture_data is a local variable that goes out of scope and gets destroyed when the function returns. If you pass a pointer to that variable into async that pointer becomes a dangling pointer causing undefined behaviour. That does not happen if you pass it by value, as you do in the above snippet.

Answer 2

You are trying to pass a pointer to a stack allocated object. This object is destructed at the end of the Scanner::run() method. Thus, the pointer is now pointing to invalid memory when the async function runs.
The first method works, because the capture_data variable is move constructed when it is passed to the function, therefore it still retains it's structure.
I recommend using lambda functions than passing the raw member function:
void Scanner::run()
{
CaptureData capture_data(1234);
auto t = std::async(std::launch::async, [capture=std::move(capture_data)]() { capture.get_data(); });
my_futures.emplace_back(t);
}
Even better is to construct the object inside the lambda function:
void Scanner::run()
{
auto t = std::async(std::launch::async, []() {
CaptureData capture_data(1234);
capture_data.get_data();
});
my_futures.emplace_back(t);
}