使用TBB在向量中运行函数会产生错误的输出

Question

Here is my code which is supposed to write the numbers 0 to 19 on the screen (in random order).

vector<std::function<void(void)>> tasks;
for(int i=0; i<20;i++)
     { 
        tasks.push_back( [&](){cout<< i<<endl;} );  
     }

tbb::parallel_for(size_t(0), size_t(tasks.size()), [&](int K) {(tasks[K])();});

The out out is:

20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20

What am I doing wrong? How can I make it to write out the numbers 0 to 19?

Answer 1

You passed a reference to i when adding the new task:

  tasks.push_back( [&](){cout<< i<<endl;} ); // ^^^ here 

When the task is run, i has reached 20, so that's what each thread will print.

Instead, you probably want to capture a copy of i :

    tasks.push_back([=i]{ std::cout << i << '\n'; });

(The = is optional there; I included it to make the point clearer)