Passing lambdas to std::thread and calling class methods

Question

I'm having a bit of trouble using std::thread together with lambdas. I have a method TheMethod where I should use std::thread to parallelize some function calls to methods in the same class.

I define a lambda function, and try to pass it as follows to the std::thread instance I create:

auto functor = [this](const Cursor& c, size_t& result) ->void {result = classMethod(c);}; size_t a; Cursor cursor = someCursor(); std::thread t1(functor, cursor, a); t1.join();

Unfortunately, the compiler gives me:

 /usr/include/c++/4.8/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<TheMethod...

I tried a lot of combinations in the lambda definition, and in the way of calling the std::thread constructor, but I get the same error always. The thread library is included, I link pthread too.

Thanks for hints!

Say std::thread t1(functor, std::ref(cursor), std::ref(a)); — Kerrek SB
– Kerrek SB, Commented Mar 11, 2014 at 17:33
you should mark the anwer as valid, so next time people will catch that this is the good answer — Gabriel
– Gabriel, Commented Mar 30, 2014 at 0:09
How can I do that? I only see the option to vote up next to Kerrek SBs answer. Sorry, I'm kind of new here. — Diego H.
– Diego H., Commented Apr 2, 2014 at 9:03

alexk7 · Accepted Answer · 2014-07-04 22:29:28Z

You can use std::ref to pass the parameters by reference:

std::thread t1(functor, std::ref(cursor), std::ref(a))

You could also capture the parameters by reference in the lambda itself:

size_t a; Cursor cursor = someCursor(); std::thread t1([&] {a = classMethod(cursor);}); t1.join();

I do want to warn that if t1 is detached and the main thread exits, calling a class method by reference without using std::ref is potentially dangerous.

Alexander Malakhov · Accepted Answer · 2021-09-29 15:17:09Z

This is because the objects cursor and a are passed by value to the constructor of thread. The functor takes a reference to the local copies of the newly created thread and not on the objects you expected them to be.

Hence, as answered by "alexk7", you should use std::ref or if you want to capture them pass by reference.

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