c++11 multithreading memory error

I have the following code:

somefunc(string s, semaphore* sem) { 
     //some functionality
     sem->signal();
}

int main() {
    int num = 0;
    semaphore sem(0);
    vector<string> arr;
    for (string& s : arr) {
        ++num; 
        thread(somefunc, s, &sem).detach();
    }

    for (int i = 0; i < num; i++)
        sem.wait();
}

I am getting std::string allocation memory errors on the line where thread() is. Is there something wrong with this code? semaphore is a custom class that uses a mutex and a conditional variable.