Inverted std::optional's

notty.cpp

// Utility that makes it a little bit nicer to
// invert the condition on assignment from optional:
// if ((Not{foo} = maybeMakeFoo()) {
//   std::cerr << "can't make foo\n";
// }

#include <optional>

template<typename T>
struct Not {
  explicit Not(T &V) : V(V), HasVal(false) {}
  Not<T> &operator=(const std::optional<T> &RHS) {
    if (RHS.has_value()) { HasVal = true; V = *RHS; }
    return *this;
  }
  explicit operator bool() const { return !HasVal; }
  T &V;
  mutable bool HasVal;
};

int if_then() {
    int foo;
    if ((Not{foo} = std::nullopt)) {
      return 42;
    }
    return 0;
}

int if_else() {
    int foo;
    if ((Not{foo} = 42)) {
      return 42;
    }
    return 0;
}