Difference between revisions of "cpp/memory/start lifetime as"

enum stream_t : char { FOO, BAR };
struct Foo {
  stream_t type;
  int data;
};
struct Bar {
  stream_t type;
  float data;
};
 
 
void printFoo(Foo const* ptr) {
  std::print("Foo({})", ptr->data);
}
void printBar(Bar const* ptr) {
  std::print("Bar({})", ptr->data);
}
 
void printStream(Stream* stream) {
  // the function read() returns a pointer to an allocated storage
  // occupied by data bits written by the external stream device.
  // and it represents either a Foo or a Bar
  std::unique_ptr<char[]> buffer = stream->read(); 
  if (buffer[0] == static_cast<char>(FOO)) {
    // printFoo(reinterpret_cast<Foo*>(buffer.get())); 
    // UB. Since the lifetime of the object is not yet started, trying to
    // access its data member is a undefined behavior.
    printFoo(std::start_lifetime_as<Foo>(buffer.get()));
    // Ok, since the lifetime are now explicitly started.
  } else {
    // printBar(reinterpret_cast<Bar*>(buffer.get()));   // UB.
    printBar(std::start_lifetime_as<Bar>(buffer.get())); // OK.
  }
}