加入线程时异常

Question

The following test program compiles with g++ -o test -pthread -std=c++11 test.cpp just fine:

#include <iostream>
#include <vector>
#include <thread>
#include <chrono>

using namespace std;

void foo(int);

int main()
{
        vector<thread> threads(5);
        for (int i = 0; i < 5; i++)
        {
                threads.push_back(thread(foo, i));
        }

        for (vector<thread>::iterator iter = threads.begin(); iter != threads.end(); iter++)
        {
                iter->join();
        }
}

void foo(int id)
{
        cout << "thread " << id << " started.";
        this_thread::sleep_for(chrono::seconds(10));
        cout << "thread " << id << " terminated.";
}

However, when run, it gives output similar to this:

terminate called after throwing an instance of 'std::system_error'
  what():  Invalid argument
thread thread 2 started.1 started.thread 4 started.thread 3 started.thread 0 started.Aborted (core dumped)  

I have no idea where this error is coming from. From debugging, I know that the error happens as soon as I try to join the first thread.
http://www.cplusplus.com/reference/thread/thread/join/ states that join will only throw invalid_argument if the thread is not joinable.
http://www.cplusplus.com/reference/thread/thread/joinable/ states that a thread is not joinable only if:

• it was default-constructed.
• it has been moved from (either constructing another thread object, or assigning to it).
• either of its members join or detach has been called.

As one can clearly see, the thread was not default-constructed.
I am never overwriting the thread with another thread object. The output of the program also shows that each thread is clearly running, as the "thread x started." statements are reached.
I am only calling join once for each thread.

If I put a 5-second pause between creation and joining, I get the same outout, so it's not a race issue. Although it is interesting that the output is still ordered in the same way. Any ideas?

Answer 1

You shouldn't presize the vector with 5 default-constructed threads, then ignore those while push_back -ing additional ones. Your join then tries to join the default-constructed threads and throws. Get rid of the (5) , or move it to a reserve call.