How should I use thread_pool in this case?

Question

for example: when I write this code I recieve right result.

boost::asio::thread_pool t(3);
std::vector<int> vec = {10,20,30};
boost::asio::post(t, [&]{ foo(vec[0]);});
boost::asio::post(t, [&]{ foo(vec[1]);});
boost::asio::post(t, [&]{ foo(vec[2]);});
t.join();

but when i want use boost::asio::post in for-cicle i recieve error Microsoft Visual C++ Runtime Library: "Expression: vector subscript out of range"

boost::asio::thread_pool t(3);
std::vector<int> vec = {10,20,30};
for (int i = 0; i < 3; ++i) {
    boost::asio::post(t, [&]{ foo(vec[i]);});
}
t.join();

what can i do to make my last way return the correct answer?

i don't want to write boost::asio::post() multiple times and want to use it in for(){}...i do something crazy?) — Robert Klevtsov, Jul 26 '22 at 16:10
You need to change your lambda capture to `[&,i]` (or more explicitly: `[&vec,i]`) - see explanation in @DrewDormann's comment above. — wohlstad, Jul 26 '22 at 16:23

score 5 · Accepted Answer · edited Jul 26 '22 at 17:49

5

You are capturing i by reference for a lambda that is used asynchronously. When t.join(); is reached, i is a dangling reference.

Capture it by value so it doesn't change or expire.

boost::asio::post(t, [&,i]{ foo(vec[i]);});

edited Jul 26 '22 at 17:49

sehe

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answered Jul 26 '22 at 16:24

Drew Dormann

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How should I use thread_pool in this case?

1 Answers1