gecko-dev/xpcom/string/public/nsStringBuffer.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim:set ts=2 sw=2 sts=2 et cindent: */
/* 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 nsStringBuffer_h__
#define nsStringBuffer_h__

#include "mozilla/Atomics.h"
#include "mozilla/MemoryReporting.h"

template<class T> struct already_AddRefed;

/**
 * This structure precedes the string buffers "we" allocate.  It may be the
 * case that nsTAString::mData does not point to one of these special
 * buffers.  The mFlags member variable distinguishes the buffer type.
 *
 * When this header is in use, it enables reference counting, and capacity
 * tracking.  NOTE: A string buffer can be modified only if its reference
 * count is 1.
 */
class nsStringBuffer
  {
    private:
      friend class CheckStaticAtomSizes;

      mozilla::Atomic<int32_t> mRefCount;
      uint32_t mStorageSize;

    public:
      
      /**
       * Allocates a new string buffer, with given size in bytes and a
       * reference count of one.  When the string buffer is no longer needed,
       * it should be released via Release.
       *
       * It is up to the caller to set the bytes corresponding to the string
       * buffer by calling the Data method to fetch the raw data pointer.  Care
       * must be taken to properly null terminate the character array.  The
       * storage size can be greater than the length of the actual string
       * (i.e., it is not required that the null terminator appear in the last
       * storage unit of the string buffer's data).
       *
       * @return new string buffer or null if out of memory.
       */
      static already_AddRefed<nsStringBuffer> Alloc(size_t storageSize);

      /**
       * Resizes the given string buffer to the specified storage size.  This
       * method must not be called on a readonly string buffer.  Use this API
       * carefully!!
       *
       * This method behaves like the ANSI-C realloc function.  (i.e., If the
       * allocation fails, null will be returned and the given string buffer
       * will remain unmodified.)
       *
       * @see IsReadonly
       */
      static nsStringBuffer* Realloc(nsStringBuffer* buf, size_t storageSize);

      /**
       * Increment the reference count on this string buffer.
       */
      void NS_FASTCALL AddRef();

      /**
       * Decrement the reference count on this string buffer.  The string
       * buffer will be destroyed when its reference count reaches zero.
       */
      void NS_FASTCALL Release();

      /**
       * This method returns the string buffer corresponding to the given data
       * pointer.  The data pointer must have been returned previously by a
       * call to the nsStringBuffer::Data method.
       */
      static nsStringBuffer* FromData(void* data)
        {
          return reinterpret_cast<nsStringBuffer*> (data) - 1;
        }

      /**
       * This method returns the data pointer for this string buffer.
       */
      void* Data() const
        {
          return const_cast<char*> (reinterpret_cast<const char*> (this + 1));
        }

      /**
       * This function returns the storage size of a string buffer in bytes.
       * This value is the same value that was originally passed to Alloc (or
       * Realloc).
       */
      uint32_t StorageSize() const
        {
          return mStorageSize;
        }

      /**
       * If this method returns false, then the caller can be sure that their
       * reference to the string buffer is the only reference to the string
       * buffer, and therefore it has exclusive access to the string buffer and
       * associated data.  However, if this function returns true, then other
       * consumers may rely on the data in this buffer being immutable and
       * other threads may access this buffer simultaneously.
       */
      bool IsReadonly() const
        {
          return mRefCount > 1;
        }

      /**
       * The FromString methods return a string buffer for the given string 
       * object or null if the string object does not have a string buffer.
       * The reference count of the string buffer is NOT incremented by these
       * methods.  If the caller wishes to hold onto the returned value, then
       * the returned string buffer must have its reference count incremented
       * via a call to the AddRef method.
       */
      static nsStringBuffer* FromString(const nsAString &str);
      static nsStringBuffer* FromString(const nsACString &str);

      /**
       * The ToString methods assign this string buffer to a given string
       * object.  If the string object does not support sharable string
       * buffers, then its value will be set to a copy of the given string
       * buffer.  Otherwise, these methods increment the reference count of the
       * given string buffer.  It is important to specify the length (in
       * storage units) of the string contained in the string buffer since the
       * length of the string may be less than its storage size.  The string
       * must have a null terminator at the offset specified by |len|.
       *
       * NOTE: storage size is measured in bytes even for wide strings;
       *       however, string length is always measured in storage units
       *       (2-byte units for wide strings).
       */
      void ToString(uint32_t len, nsAString &str,
                           bool aMoveOwnership = false);
      void ToString(uint32_t len, nsACString &str,
                           bool aMoveOwnership = false);

      /**
       * This measures the size.  It should only be used if the StringBuffer is
       * unshared.  This is checked.
       */
      size_t SizeOfIncludingThisMustBeUnshared(mozilla::MallocSizeOf aMallocSizeOf) const;

      /**
       * This measures the size only if the StringBuffer is unshared.
       */
      size_t SizeOfIncludingThisIfUnshared(mozilla::MallocSizeOf aMallocSizeOf) const;

      /**
       * This measures the size regardless of whether the StringBuffer is
       * unshared.
       *
       * WARNING: Only use this if you really know what you are doing, because
       * it can easily lead to double-counting strings.  If you do use them,
       * please explain clearly in a comment why it's safe and won't lead to
       * double-counting.
       */
      size_t SizeOfIncludingThisEvenIfShared(mozilla::MallocSizeOf aMallocSizeOf) const;
  };

#endif /* !defined(nsStringBuffer_h__ */