xpcom/string/nsTPromiseFlatString.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7
   8 /**
   9  * NOTE:
  10  *
  11  * Try to avoid flat strings.  |PromiseFlat[C]String| will help you as a last
  12  * resort, and this may be necessary when dealing with legacy or OS calls,
  13  * but in general, requiring a null-terminated array of characters kills many
  14  * of the performance wins the string classes offer.  Write your own code to
  15  * use |nsA[C]String&|s for parameters.  Write your string proccessing
  16  * algorithms to exploit iterators.  If you do this, you will benefit from
  17  * being able to chain operations without copying or allocating and your code
  18  * will be significantly more efficient.  Remember, a function that takes an
  19  * |const nsA[C]String&| can always be passed a raw character pointer by
  20  * wrapping it (for free) in a |nsDependent[C]String|.  But a function that
  21  * takes a character pointer always has the potential to force allocation and
  22  * copying.
  23  *
  24  *
  25  * How to use it:
  26  *
  27  * A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
  28  * promises.  You must never use it to promise characters out of a string
  29  * with a shorter lifespan.  The typical use will be something like this:
  30  *
  31  *   SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
  32  *
  33  * Here's a BAD use:
  34  *
  35  *  const char* buffer = PromiseFlatCString(aCSubstring).get();
  36  *  SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
  37  *
  38  * The only way to make one is with the function |PromiseFlat[C]String|,
  39  * which produce a |const| instance.  ``What if I need to keep a promise
  40  * around for a little while?'' you might ask.  In that case, you can keep a
  41  * reference, like so
  42  *
  43  *   const nsCString& flat = PromiseFlatString(aCSubstring);
  44  *     // this reference holds the anonymous temporary alive, but remember,
  45  *     // it must _still_ have a lifetime shorter than that of |aCSubstring|
  46  *
  47  *  SomeOSFunction(flat.get());
  48  *  SomeOtherOSFunction(flat.get());
  49  *
  50  *
  51  * How does it work?
  52  *
  53  * A |nsPromiseFlat[C]String| is just a wrapper for another string.  If you
  54  * apply it to a string that happens to be flat, your promise is just a
  55  * dependent reference to the string's data.  If you apply it to a non-flat
  56  * string, then a temporary flat string is created for you, by allocating and
  57  * copying.  In the event that you end up assigning the result into a sharing
  58  * string (e.g., |nsTString|), the right thing happens.
  59  */
  60
  61 class nsTPromiseFlatString_CharT : public nsTString_CharT
  62 {
  63 public:
  64
  65   typedef nsTPromiseFlatString_CharT self_type;
  66
  67 private:
  68
  69   void Init(const substring_type&);
  70
  71   // NOT TO BE IMPLEMENTED
  72   void operator=(const self_type&) MOZ_DELETE;
  73
  74   // NOT TO BE IMPLEMENTED
  75   nsTPromiseFlatString_CharT() MOZ_DELETE;
  76
  77   // NOT TO BE IMPLEMENTED
  78   nsTPromiseFlatString_CharT(const string_type& aStr) MOZ_DELETE;
  79
  80 public:
  81
  82   explicit
  83   nsTPromiseFlatString_CharT(const substring_type& aStr)
  84     : string_type()
  85   {
  86     Init(aStr);
  87   }
  88
  89   explicit
  90   nsTPromiseFlatString_CharT(const substring_tuple_type& aTuple)
  91     : string_type()
  92   {
  93     // nothing else to do here except assign the value of the tuple
  94     // into ourselves.
  95     Assign(aTuple);
  96   }
  97 };
  98
  99 // We template this so that the constructor is chosen based on the type of the
 100 // parameter. This allows us to reject attempts to promise a flat flat string.
 101 template<class T>
 102 const nsTPromiseFlatString_CharT
 103 TPromiseFlatString_CharT(const T& aString)
 104 {
 105   return nsTPromiseFlatString_CharT(aString);
 106 }