Lambda Expression Follow-up

• Homework to use a lambda expression with a standard algorithm or container

set<int, ???> s; // How do you use a lambda for a comparison type?

• You cannot
  • Best you can do is use a pointer to function or std::function and pass the lambda to the constructor as comparison

{
    set<int, bool (*)(int a, int b)> s{[](int a, int b) { return a > b; }};
}

• Disadvantage is the expression cannot be inlined
• Recommendation - continue to use function objects

{
    set<int, greater<>> s;
}

Memory Order Follow-up

• It wasn't clear in my slides that C++11 allows you to specify the memory order on many operations

atomic<int> x;
x.store(10, memory_order_relaxed);

• You can specify the memory order for
  • operations on atomics
  • explicitly with std::atomic_thread_fence, std::atomic_signal_fence
  • and can explicitly control dependencies with std::kill_dependency

• The memory order controls
  • How the compiler can reorder operations on possibly shared memory around the atomic operations
  • Which instructions the compiler emits to control processor memory ordering around the atomic operations