How to start a variable number of threads in C++?

Question

I am searching for a way to start multiple threads whose exact number can only be determined at runtime. The threads are not dependent on each other, so it's a fire-and-forget kind of problem.

The threads do need some context which is stored as internal variables of a class (Foo). Some of these variables are references. The class also holds a method that should be executed as the thread function (bar).

#include <iostream>
#include <string>
#include <vector>
#include <thread>

class Foo
{
public:
    Foo(int a){
        std::cout << "Created" << std::endl;
        m_a = new int(a);
    }

    ~Foo(){
        std::cout << "Destroyed" << std::endl;
        delete m_a;
    }

    void bar() {
        std::cout << "Internal var: " << *m_a << std::endl;
    }

private:
    int* m_a;
};

int main() {

    for(int i = 0; i < 5; i++) {
        std::thread t(&Foo::bar, std::ref(Foo(i)));
        // the threads will be joined at a later point, this is for demo purposes
    }

    return 0;
}

I get a compile error at this point:

error: use of deleted function ‘void std::ref(const _Tp&&) [with _Tp = Foo]’

I get it that this error is caused because of the temporary nature of the object created in the for-loop. But if I remove the std::ref function, I get a segfault: double free or corruption (fasttop)

I am sure that there must be a way of doing this, but I am unaware of that. I would expect some output like (probably in this order, but not guaranteed):

Created
Internal var: 0
Destroyed

Created
Internal var: 1
Destroyed

...

Thanks!

Foo violates the [rule of three](https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three) — Mooing Duck, Jul 01 '21 at 15:01
Why not just `std::thread t(&Foo::bar, Foo(i));`? What is your purpose for using `std::ref`? — Drew Dormann, Jul 01 '21 at 15:09
This looks like a variation of the example [here](https://en.cppreference.com/w/cpp/thread/thread/thread). What exactly are you trying to do with this code? — Daniel Dearlove, Jul 01 '21 at 15:10
in general you need object with reference counting. addref on object, before pass it to thread, release, after stop use it in thread. delete when ref count became 0 — RbMm, Jul 01 '21 at 15:47

score 1 · Answer 1 · answered Jul 01 '21 at 15:33

Problem 1: Foo is missing a copy/move constructor. See The rule of three/five/zero.

Add a copy constructor:

    Foo(Foo const& that) : m_a(new int(*that.m_a)) {}

And/or a move constructor:

    Foo(Foo && that) : m_a(that.m_a) { that.m_a = nullptr; }

Problem 2: Foo(i) is a temporary instance of Foo, it lives until the end of the full-expression (the ;).

std::thread t(&Foo::bar, std::ref(Foo(i)));
//                                        ^
//                                        Foo(i) is dead at this point while the thread is starting!

You want it to live longer than that, in order to be usable inside the thread.

For example, like this (also answers your question about creating threads in a loop):

int main() {
    std::vector<Foo> inputs;
    std::vector<std::thread> threads;

    for(int i = 0; i < 5; i++) {
        inputs.emplace_back(i);
        threads.emplace_back(&Foo::bar, &inputs.back());
    }
    
    for (auto& t : threads) {
        t.join();
    }
}

Note: std::ref(Foo(i)) doesn't compile because it has protection against returning references to temporaries (precisely to prevent issues like these).

score 1 · Answer 2 · answered Jul 01 '21 at 15:41

Here is a minimaly fixed version of your code:

it includes the move ctor for Foo class (and explicitely deletes copy ctor)
it moves the threads into a vector
it joins the threads

Code:

#include <string>
#include <vector>
#include <thread>
#include <iostream>

class Foo
{
public:
    Foo(int a) {
        std::cout << "Created" << std::endl;
        m_a = new int(a);
    }

    ~Foo() {
        if (m_a != NULL) {
            std::cout << "Destroyed" << std::endl;
            delete m_a;
        }
    }

    Foo(const Foo& other) = delete; //not used here

    Foo(Foo&& other) {
        std::cout << "Move ctor" << '\n';
        m_a = other.m_a;
        other.m_a = nullptr;
    }

    void bar() {
        std::cout << "Internal var: " << *m_a << std::endl;
    }

private:
    int* m_a;
};

int main() {
    std::vector<std::thread> vec;
    for (int i = 0; i < 5; i++) {
        std::thread t(&Foo::bar, Foo(i));
        vec.push_back(std::move(t));
    }
    for (auto& t : vec) {
        t.join();
    }
    return 0;
}

score 0 · Answer 3 · answered Jul 01 '21 at 15:12

The chief design failure seems that t is a variable inside the loop. That means it's destroyed at the end of each iteration - you never have 5 std::thread instances at the same time. Also, you fail to call join on those threads.

The std::ref apparently hides this problem and replaces it with another problem, but your original thread creation was correct: std::thread t(&Foo::bar, Foo(i)).

You probably want a std::list<std::thread>, and use std::list::emplace_back to create a variable amount. std::list<std::thread> allows you to remove threads in any order from the list.

How to start a variable number of threads in C++?

3 Answers3