CODEWITHSUNDEEP
c++multithreadingc++11
Eduard Rostomyan
Eduard Rostomyan

Reputation: 6546

Difference of passing argument to std::thread, C++

A quote from Nikolai Josuttis - Standard Library C++11:

Detached threads can easily become a problem if they use nonlocal resources. The problem is that you lose control of a detached thread and have no easy way to find out whether and how long it runs. Thus, make sure that a detached thread does not access any objects after their lifetime has ended. For this reason, passing variables and objects to a thread by reference is always a risk. Passing arguments by value is strongly recommended.

So further the author explains, that even if you pass a reference as a function argument to a thread, it still passes by value, so you must indicate the reference with std::ref.

I have these questions, see the code below:

void f(std::vector<int> V){...}
void g(std::vector<int>& V){...}

std::vector<int> V;

std::thread t1(f, V);
std::thread t2(f, std::ref(V));
std::thread t3(g, V);
std::thread t4(g, std::ref(V));

What are the differences in these 4 lines? Which lines are equivalent?
I am not joining or detaching thread, it's not about that, it's about the ways of passing the function argument.

Upvotes: 9

Views: 4412

Answers (1)

user4442671
user4442671

Reputation:

t1:

This simply passes a copy of V to the thread.

t2:

Similarly to t1, a copy of V is passed to the thread, but the actual copy is made in the called thread instead of the caller thread. This is an important distinction because should V be altered or cease to exist by the time the thread begins, you will end up with either a different vector or Undefined Behavior.

t3:

This should fail to compile as the thread will move the vector into the LValue reference, which is supposed to be illegal.

t4:

This passes the vector by reference to the thread. Any modifications to the passed reference will be applied to V, provided that proper synchronisation is performed, of course.

Upvotes: 9

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