C++: What is std::mutex and how do you prevent deadlocks?

1#include <mutex>
2
3std::mutex mtx;
4int shared_data = 0;
5
6void increment() {
7    std::lock_guard<std::mutex> lock(mtx);  // RAII lock
8    shared_data++;  // Protected
9}  // Automatically unlocked
10
11// Deadlock prevention: lock multiple mutexes
12std::mutex mtx1, mtx2;
13
14// BAD — potential deadlock
15void bad() {
16    std::lock_guard<std::mutex> l1(mtx1);
17    std::lock_guard<std::mutex> l2(mtx2);  // May deadlock!
18}
19
20// GOOD — atomic lock
21void good() {
22    std::scoped_lock lock(mtx1, mtx2);  // C++17
23    // Both locked atomically
24}
25
26// Or std::lock + lock_guard
27void also_good() {
28    std::lock(mtx1, mtx2);
29    std::lock_guard<std::mutex> l1(mtx1, std::adopt_lock);
30    std::lock_guard<std::mutex> l2(mtx2, std::adopt_lock);
31}