Deadlock avoidance with std::lock

locks.cpp

#include <iostream>
#include <mutex>

struct Lock
{
    Lock(int id) : id(id) {}
    void lock()     { std::cout << "lock()      from " << id << std::endl; }
    bool try_lock() { std::cout << "try_lock()  from " << id << std::endl; return true; }
    void unlock()   { std::cout << "unlock()    from " << id << std::endl; }
    int id;
};

int main(void)
{
    Lock a(101);
    Lock b(102);
    Lock c(103);

    std::cout << "--------------------" << std::endl;

    {
        std::lock_guard<Lock> lg(a);
        std::unique_lock<Lock> un(b, std::try_to_lock);
    }

    std::cout << "--------------------" << std::endl;

    std::lock(a, b, c);

    std::cout << "--------------------" << std::endl;
}