C++: terminate called without an active exception — std::thread destroyed while joinable

Meaning

Every std::thread must be either joined or detached before its destructor runs. Exiting main, returning early, or an exception unwinding past a live thread object all trigger it. Use std::jthread (C++20), which joins automatically, or wrap threads in an RAII guard.

Common causes

• join() or detach() never called on a thread
• Early return or exception skipping the join()
• Assigning a new thread to a variable that still owns a running thread
• Thread stored in a container that is destroyed before the threads end

Detailed fix by platform

Code examples

#include <thread>
#include <iostream>

int main() {
    std::thread t([] { std::cout << "working\n"; });
    // forgot t.join();   -> terminate called without an active exception
    t.join();             // fix (or use std::jthread in C++20)
}

How to diagnose

1. Join — Is every thread joined or detached on all paths?
2. Exceptions — Can an exception skip the join?
3. Ownership — Is a thread variable overwritten?

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