extend scope of pass by ref object for std::thread

Question

To demonstrate the problem, let me present a short code -

void someMethod() {

  // CustomType obj;
  const auto obj = getCustomTypeObj();

  std::thread([](customType &obj) {
    // some delay
    obj.doSomething();
    obj.close();
    // can now be destructed
  }).detach();

  // similarly for std::async
  std::async(std::launch::async, [](customType &obj){
    obj.doSomething();
    obj.close();
  }

  // there might not be any use of obj here

  // should not be destructed here because std::thread might not get it.

}

In above code, an CustomType type object is constructed for which copy constructor is deleted. So I must pass it by reference everywhere, or create it from scratch in relevant scope. However for 1 scenario I'm currently dealing with, it is not quite possible to create it in relevant scope which is inside std::thread 's execution method.

What I'm afraid of is obj might be destructed before std::thread even completes its job and then I've no idea what's going to happen. So how should I solve this problem of extending it's scope to std::thread's lambda.

Answer 1

Btw your code is incorrect, you do not pass your object, so your code should be instead:

  auto obj = getCustomTypeObj();

  std::thread([](customType &obj) {
    // some delay
    obj.doSomething();
    obj.close();
    // can now be destructed
  }, std::ref( obj ) ).detach();

To avoid issue with object lifetime pass your object to the lambda or function by value and move your object there:

  auto obj = getCustomTypeObj();

  std::thread([](customType arg) { // note by value, not reference
    // some delay
    arg.doSomething();
    arg.close();
    // arg will be destroyed here
  }, std::move( obj ) ).detach(); // object moved

now lambda or function owns that object and it will be destroyed at the end of the function. Here is the live example , I just used std::unique_ptr there instead of customType as type that has copying disabled to validate that moving works.