dom/base/nsAttrValueOrString.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 file,
   5  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * A wrapper to contain either an nsAttrValue or an nsAString. This is useful
   9  * because constructing an nsAttrValue from an nsAString can be expensive when
  10  * the buffer of the string is not shared.
  11  *
  12  * This treats nsAttrValueOrString(nullptr) as the empty string,
  13  * to help with contexts where a null pointer denotes an empty value.
  14  *
  15  * Since a raw pointer to the passed-in string is kept, this class should only
  16  * be used on the stack.
  17  */
  18
  19 #ifndef nsAttrValueOrString_h___
  20 #define nsAttrValueOrString_h___
  21
  22 #include "nsString.h"
  23 #include "nsAttrValue.h"
  24
  25 class MOZ_STACK_CLASS nsAttrValueOrString {
  26  public:
  27   explicit nsAttrValueOrString(const nsAString& aValue)
  28       : mAttrValue(nullptr), mStringPtr(&aValue), mCheapString(nullptr) {}
  29
  30   explicit nsAttrValueOrString(const nsAString* aValue)
  31       : mAttrValue(nullptr), mStringPtr(aValue), mCheapString(nullptr) {}
  32
  33   explicit nsAttrValueOrString(const nsAttrValue& aValue)
  34       : mAttrValue(&aValue), mStringPtr(nullptr), mCheapString(nullptr) {}
  35
  36   explicit nsAttrValueOrString(const nsAttrValue* aValue)
  37       : mAttrValue(aValue), mStringPtr(nullptr), mCheapString(nullptr) {}
  38
  39   void ResetToAttrValue(const nsAttrValue& aValue) {
  40     mAttrValue = &aValue;
  41     mStringPtr = nullptr;
  42     // No need to touch mCheapString here.  If we need to use it, we will reset
  43     // it to the rigthe value anyway.
  44   }
  45
  46   /**
  47    * Returns a reference to the string value of the contents of this object.
  48    *
  49    * When this object points to a string or an nsAttrValue of string or atom
  50    * type this should be fairly cheap. Other nsAttrValue types will be
  51    * serialized the first time this is called and cached from thereon.
  52    */
  53   const nsAString& String() const;
  54
  55   /**
  56    * Compares the string representation of this object with the string
  57    * representation of an nsAttrValue.
  58    */
  59   bool EqualsAsStrings(const nsAttrValue& aOther) const {
  60     if (mStringPtr) {
  61       return aOther.Equals(*mStringPtr, eCaseMatters);
  62     }
  63     return aOther.EqualsAsStrings(*mAttrValue);
  64   }
  65
  66   /*
  67    * Returns true if the value stored is empty
  68    */
  69   bool IsEmpty() const {
  70     if (mStringPtr) {
  71       return mStringPtr->IsEmpty();
  72     }
  73     if (mAttrValue) {
  74       return mAttrValue->IsEmptyString();
  75     }
  76     return true;
  77   }
  78
  79  protected:
  80   const nsAttrValue* mAttrValue;
  81   mutable const nsAString* mStringPtr;
  82   mutable nsCheapString mCheapString;
  83 };
  84
  85 #endif  // nsAttrValueOrString_h___