First let’s take a look at the following piece of code.
std::future<void> * ret_vals = new std::future<void>[3];
for (int i = 0; i < 3; ++i) {
ret_val[i] = std::async([&]() {cout << "I am feeling great! Getting task " << i << endl});
}
Very trial use of async tasks and lambda in C++11 huh? I naively thought the code will produce some output like
I am feeling great! Getting task 0 I am feeling great! Getting task 1 I am feeling great! Getting task 2
But what it actually outputs is
I am feeling great! Getting task 3 I am feeling great! Getting task 3 I am feeling great! Getting task 3
It turns out that i is a variable captured by reference, and before the three async tasks have a chance to print it out, its value has long undergone many changes.
