Loop for creating threads and joining with variable number of threads

Question

I am building a program that, for testing purposes can create N number of threads in C++. I am relativity new to C++ and my current attempt so far is

//Create a list of threads
std::vector<std::thread> t;
for(i=0; i < THREADS; i ++){
    std::thread th = std::thread([](){ workThreadProcess(); });
    t.push_back(th);
    printf("Thread started \n");
    }

for(std::thread th : t){
    th.join();
}

I currently an error that says call to deleted constructor of 'std::thread'. I am unusre what this means or how to fix in

Note:
I have looked at:

But I don't feel they answer my question. Most of them use pthreads or a a different constructor.

When asking about build errors, first of all try to create a Minimal, Complete, and Verifiable Example that you can show us. Then copy-paste (as text) the full and complete build log from that example into the question body. And please mark out the lines in the example where the errors are, with for example comments. Also please take some time to review how to ask good questions, as well as this question checklist. — Some programmer dude
– Some programmer dude, Commented Nov 15, 2018 at 23:04
A hint though: You loop joining the threads should probably not copy the thread objects. — Some programmer dude
– Some programmer dude, Commented Nov 15, 2018 at 23:05
The error message isn't lieing. std::thread is defined with 'thread(const thread&) = delete;' You cannot copy it. How would you write the copy constructor for a thread, if it was you? Can you identify the copy in your code and fix that somehow? — Christopher Pisz
– Christopher Pisz, Commented Nov 15, 2018 at 23:22

Christophe · Accepted Answer · 2018-11-15 23:52:52Z

13

You can't copy threads. You need to move them in order to get them into the vector. Also, you can't create temporary copies in the loop to join them: you have to use a references instead.

Here a working version

std::vector<std::thread> t;
for(int i=0; i < THREADS; i ++){
    std::thread th = std::thread([](){ workThreadProcess(); });
    t.push_back(std::move(th));  //<=== move (after, th doesn't hold it anymore 
    std::cout<<"Thread started"<<std::endl;
    }

for(auto& th : t){              //<=== range-based for uses & reference
    th.join();
}

Online demo

answered Nov 15, 2018 at 23:52

Christophe

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5 Comments

Ryan Haining Over a year ago

This is the right explanation of the error, but OP would probably be better off using emplace_back as in t.emplace_back([] { workThreadProcess(); }); rather than creating a std::thread object only to move it

tomtom1-4 Over a year ago

How would you pass an argument to the function workThreadProcess?

Christophe Over a year ago

@tomtom1-4 just pass the argument. Since it's in a lambda, you need to ensure capture. Example, here: ideone.com/78DbTf

tomtom1-4 Over a year ago

Mhh, your example runs fine but it doesn't work inside my program. Are there complications if you pass multiple arguments or arguments by reference?

Christophe Over a year ago

@tomtom1-4 yes, this works fine for value arguments, but not for reference. There are two obstacles here: the first, is that I didn't make a capture by reference (more here). The second is that thread uses a reference to a copy if you do not use std::ref() as explained here

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