c++ Threads inside for loop print wrong values

Question

I\'m trying to understand Multi-threading in c++, but I\'am stuck in this problem: if I launch threads in a for loop they print wrong values. This is the code:

#         


        

Answer 1

The [&] syntax is causing i to be captured by reference. So quite often therefore i will be further advanced when the thread runs than you might expect. More seriously, the behaviour of your code is undefined if i goes out of scope before a thread runs.

Capturing i by value - i.e. std::thread([i](){ print_id(i); }) is the fix.