gecko-dev/xpcom/string/nsTPromiseFlatString.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef nsTPromiseFlatString_h
#define nsTPromiseFlatString_h

#include "mozilla/Attributes.h"
#include "nsTString.h"

/**
 * NOTE:
 *
 * Try to avoid flat strings.  |PromiseFlat[C]String| will help you as a last
 * resort, and this may be necessary when dealing with legacy or OS calls,
 * but in general, requiring a null-terminated array of characters kills many
 * of the performance wins the string classes offer.  Write your own code to
 * use |nsA[C]String&|s for parameters.  Write your string proccessing
 * algorithms to exploit iterators.  If you do this, you will benefit from
 * being able to chain operations without copying or allocating and your code
 * will be significantly more efficient.  Remember, a function that takes an
 * |const nsA[C]String&| can always be passed a raw character pointer by
 * wrapping it (for free) in a |nsDependent[C]String|.  But a function that
 * takes a character pointer always has the potential to force allocation and
 * copying.
 *
 *
 * How to use it:
 *
 * A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
 * promises.  You must never use it to promise characters out of a string
 * with a shorter lifespan.  The typical use will be something like this:
 *
 *   SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
 *
 * Here's a BAD use:
 *
 *  const char* buffer = PromiseFlatCString(aCSubstring).get();
 *  SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
 *
 * The only way to make one is with the function |PromiseFlat[C]String|,
 * which produce a |const| instance.  ``What if I need to keep a promise
 * around for a little while?'' you might ask.  In that case, you can keep a
 * reference, like so:
 *
 *   const nsCString& flat = PromiseFlatString(aCSubstring);
 *     // Temporaries usually die after the full expression containing the
 *     // expression that created the temporary is evaluated.  But when a
 *     // temporary is assigned to a local reference, the temporary's lifetime
 *     // is extended to the reference's lifetime (C++11 [class.temporary]p5).
 *     //
 *     // This reference holds the anonymous temporary alive.  But remember: it
 *     // must _still_ have a lifetime shorter than that of |aCSubstring|, and
 *     // |aCSubstring| must not be changed while the PromiseFlatString lives.
 *
 *  SomeOSFunction(flat.get());
 *  SomeOtherOSFunction(flat.get());
 *
 *
 * How does it work?
 *
 * A |nsPromiseFlat[C]String| is just a wrapper for another string.  If you
 * apply it to a string that happens to be flat, your promise is just a
 * dependent reference to the string's data.  If you apply it to a non-flat
 * string, then a temporary flat string is created for you, by allocating and
 * copying.  In the event that you end up assigning the result into a sharing
 * string (e.g., |nsTString|), the right thing happens.
 */

template <typename T>
class MOZ_STACK_CLASS nsTPromiseFlatString : public nsTString<T> {
 public:
  typedef nsTPromiseFlatString<T> self_type;
  typedef nsTString<T> base_string_type;
  typedef typename base_string_type::substring_type substring_type;
  typedef typename base_string_type::string_type string_type;
  typedef typename base_string_type::substring_tuple_type substring_tuple_type;
  typedef typename base_string_type::char_type char_type;
  typedef typename base_string_type::size_type size_type;

  // These are only for internal use within the string classes:
  typedef typename base_string_type::DataFlags DataFlags;
  typedef typename base_string_type::ClassFlags ClassFlags;

 private:
  void Init(const substring_type&);

  // NOT TO BE IMPLEMENTED
  void operator=(const self_type&) = delete;

  // NOT TO BE IMPLEMENTED
  nsTPromiseFlatString(const self_type&) = delete;

  // NOT TO BE IMPLEMENTED
  nsTPromiseFlatString() = delete;

  // NOT TO BE IMPLEMENTED
  nsTPromiseFlatString(const string_type& aStr) = delete;

 public:
  explicit nsTPromiseFlatString(const substring_type& aStr) : string_type() {
    Init(aStr);
  }

  explicit nsTPromiseFlatString(const substring_tuple_type& aTuple)
      : string_type() {
    // nothing else to do here except assign the value of the tuple
    // into ourselves.
    this->Assign(aTuple);
  }
};

extern template class nsTPromiseFlatString<char>;
extern template class nsTPromiseFlatString<char16_t>;

// We template this so that the constructor is chosen based on the type of the
// parameter. This allows us to reject attempts to promise a flat flat string.
template <class T>
const nsTPromiseFlatString<T> TPromiseFlatString(
    const typename nsTPromiseFlatString<T>::substring_type& aString) {
  return nsTPromiseFlatString<T>(aString);
}

template <class T>
const nsTPromiseFlatString<T> TPromiseFlatString(
    const typename nsTPromiseFlatString<T>::substring_tuple_type& aString) {
  return nsTPromiseFlatString<T>(aString);
}

#ifndef PromiseFlatCString
#  define PromiseFlatCString TPromiseFlatString<char>
#endif

#ifndef PromiseFlatString
#  define PromiseFlatString TPromiseFlatString<char16_t>
#endif

#endif