Algorithms & Execution Policies

• Most functions in <algorithm> were respecified in C++17

  • Add an optional argument for the execution policy

• Some algorithms were naturally sequential

  • e.g. equal_range()

• These were left unchanged

• Some algorithms in <numeric> now have two versions

  • The C++14 version
  • A new version with policy support

• The new versions have new names

  • accumulate() -> reduce()
  • partial_sum() -> inclusive_scan(), exclusive_scan()

• There is also a new "fused" algorithm with policy support

  • transform() + inner_product() -> transform_reduce()

Algorithms and Exceptions in C++14

• Algorithms can throw exceptions

  • e.g. an algorithm call which applies a function to every element
  • The function throws an exception

• The exception will be handled by other code

  • If there is no handler, execution ends

std::vector<int> vec{3, 1, 4, 1, 5, 9};

try {
  // Predicate throws an exception
  std::sort(vec.begin(), vec.end(), [](int a, int b) {
    throw std::out_of_range("oops");
    return true;
  });
} catch (std::exception& e) {
  std::cout << "Caught exception: " << e.what() << '\n';
}

Algorithms and Exceptions in C++17

• This approach does not work with execution policies
  • May be multiple threads
  • Each thread has its own execution stack
• If an exception is thrown, std::terminate() is called