Quick Answer

Thread synchronization and data sharing.

Understanding the Issue

C++11+ provides <thread>, <mutex>, and <atomic> for safe concurrency. Prefer higher-level abstractions over raw threads.

The Problem

This code demonstrates the issue:

Cpp Error 
// Problem: Data race
int counter = 0;
auto incr = [&]{ counter++; };
thread t1(incr), t2(incr); // Undefined

The Solution

Here's the corrected code:

Cpp Fixed 
// Solution: Atomic operations
atomic<int> counter{0};
auto safe_incr = [&]{ counter++; };

// Alternative: Mutex
mutex m;
auto locked_incr = [&]{
    lock_guard<mutex> lock(m);
    counter++;
};

Key Takeaways

Minimize shared mutable state.