gecko-dev/xpcom/docs/stringguide.rst

String Guide
============

Most of the Mozilla code uses a C++ class hierarchy to pass string data,
rather than using raw pointers. This guide documents the string classes which
are visible to code within the Mozilla codebase (code which is linked into
``libxul``).

Introduction
------------

The string classes are a library of C++ classes which are used to manage
buffers of wide (16-bit) and narrow (8-bit) character strings. The headers
and implementation are in the `xpcom/string
<https://searchfox.org/mozilla-central/source/xpcom/string>`_ directory. All
strings are stored as a single contiguous buffer of characters.

The 8-bit and 16-bit string classes have completely separate base classes,
but share the same APIs. As a result, you cannot assign a 8-bit string to a
16-bit string without some kind of conversion helper class or routine. For
the purpose of this document, we will refer to the 16-bit string classes in
class documentation. Every 16-bit class has an equivalent 8-bit class:

===================== ======================
Wide                  Narrow
===================== ======================
``nsAString``         ``nsACString``
``nsString``          ``nsCString``
``nsAutoString``      ``nsAutoCString``
``nsDependentString`` ``nsDependentCString``
===================== ======================

The string classes distinguish, as part of the type hierarchy, between
strings that must have a null-terminator at the end of their buffer
(``ns[C]String``) and strings that are not required to have a null-terminator
(``nsA[C]String``). nsA[C]String is the base of the string classes (since it
imposes fewer requirements) and ``ns[C]String`` is a class derived from it.
Functions taking strings as parameters should generally take one of these
four types.

In order to avoid unnecessary copying of string data (which can have
significant performance cost), the string classes support different ownership
models. All string classes support the following three ownership models
dynamically:

* reference counted, copy-on-write, buffers (the default)

* adopted buffers (a buffer that the string class owns, but is not reference
  counted, because it came from somewhere else)

* dependent buffers, that is, an underlying buffer that the string class does
  not own, but that the caller that constructed the string guarantees will
  outlive the string instance

Auto strings will prefer reference counting an existing reference-counted
buffer over their stack buffer, but will otherwise use their stack buffer for
anything that will fit in it.

There are a number of additional string classes:


* Classes which exist primarily as constructors for the other types,
  particularly ``nsDependent[C]String`` and ``nsDependent[C]Substring``. These
  types are really just convenient notation for constructing an
  ``nsA[C]String`` with a non-default ownership mode; they should not be
  thought of as different types.

* ``nsLiteral[C]String`` which should rarely be constructed explicitly but
  usually through the ``""_ns`` and ``u""_ns`` user-defined string literals.
  ``nsLiteral[C]String`` is trivially constructible and destructible, and
  therefore does not emit construction/destruction code when stored in statics,
  as opposed to the other string classes.

The Major String Classes
------------------------

The list below describes the main base classes. Once you are familiar with
them, see the appendix describing What Class to Use When.


* **nsAString**/**nsACString**: the abstract base class for all strings. It
  provides an API for assignment, individual character access, basic
  manipulation of characters in the string, and string comparison. This class
  corresponds to the XPIDL ``AString`` or ``ACString`` parameter types.
  ``nsA[C]String`` is not necessarily null-terminated.

* **nsString**/**nsCString**: builds on ``nsA[C]String`` by guaranteeing a
  null-terminated storage. This allows for a method (``.get()``) to access the
  underlying character buffer.

The remainder of the string classes inherit from either ``nsA[C]String`` or
``ns[C]String``. Thus, every string class is compatible with ``nsA[C]String``.

.. note::

    In code which is generic over string width, ``nsA[C]String`` is sometimes
    known as ``nsTSubstring<CharT>``. ``nsAString`` is a type alias for
    ``nsTSubstring<char16_t>``, and ``nsACString`` is a type alias for
    ``nsTSubstring<char>``.

.. note::

    The type ``nsLiteral[C]String`` technically does not inherit from
    ``nsA[C]String``, but instead inherits from ``nsStringRepr<CharT>``. This
    allows the type to not generate destructors when stored in static
    storage.

    It can be implicitly coerced to ``const ns[C]String&`` (though can never
    be accessed mutably) and generally acts as-if it was a subclass of
    ``ns[C]String`` in most cases.

Since every string derives from ``nsAString`` (or ``nsACString``), they all
share a simple API. Common read-only methods include:

* ``.Length()`` - the number of code units (bytes for 8-bit string classes and ``char16_t`` for 16-bit string classes) in the string.
* ``.IsEmpty()`` - the fastest way of determining if the string has any value. Use this instead of testing ``string.Length() == 0``
* ``.Equals(string)`` - ``true`` if the given string has the same value as the current string. Approximately the same as ``operator==``.

Common methods that modify the string:

* ``.Assign(string)`` - Assigns a new value to the string. Approximately the same as ``operator=``.
* ``.Append(string)`` - Appends a value to the string.
* ``.Insert(string, position)`` - Inserts the given string before the code unit at position.
* ``.Truncate(length)`` - shortens the string to the given length.

More complete documentation can be found in the `Class Reference`_.

As function parameters
~~~~~~~~~~~~~~~~~~~~~~

In general, use ``nsA[C]String`` references to pass strings across modules. For example:

.. code-block:: c++

    // when passing a string to a method, use const nsAString&
    nsFoo::PrintString(const nsAString& str);

    // when getting a string from a method, use nsAString&
    nsFoo::GetString(nsAString& result);

The Concrete Classes - which classes to use when
------------------------------------------------

The concrete classes are for use in code that actually needs to store string
data. The most common uses of the concrete classes are as local variables,
and members in classes or structs.

.. digraph:: concreteclasses

    node [shape=rectangle]

    "nsA[C]String" -> "ns[C]String";
    "ns[C]String" -> "nsDependent[C]String";
    "nsA[C]String" -> "nsDependent[C]Substring";
    "nsA[C]String" -> "ns[C]SubstringTuple";
    "ns[C]String" -> "nsAuto[C]StringN";
    "ns[C]String" -> "nsLiteral[C]String" [style=dashed];
    "nsAuto[C]StringN" -> "nsPromiseFlat[C]String";
    "nsAuto[C]StringN" -> "nsPrintfCString";

The following is a list of the most common concrete classes. Once you are
familiar with them, see the appendix describing What Class to Use When.

* ``ns[C]String`` - a null-terminated string whose buffer is allocated on the
  heap. Destroys its buffer when the string object goes away.

* ``nsAuto[C]String`` - derived from ``nsString``, a string which owns a 64
  code unit buffer in the same storage space as the string itself. If a string
  less than 64 code units is assigned to an ``nsAutoString``, then no extra
  storage will be allocated. For larger strings, a new buffer is allocated on
  the heap.

  If you want a number other than 64, use the templated types ``nsAutoStringN``
  / ``nsAutoCStringN``. (``nsAutoString`` and ``nsAutoCString`` are just
  typedefs for ``nsAutoStringN<64>`` and ``nsAutoCStringN<64>``, respectively.)

* ``nsDependent[C]String`` - derived from ``nsString``, this string does not
  own its buffer. It is useful for converting a raw string pointer (``const
  char16_t*`` or ``const char*``) into a class of type ``nsAString``. Note that
  you must null-terminate buffers used by to ``nsDependentString``. If you
  don't want to or can't null-terminate the buffer, use
  ``nsDependentSubstring``.

* ``nsPrintfCString`` - derived from ``nsCString``, this string behaves like an
  ``nsAutoCString``. The constructor takes parameters which allows it to
  construct a 8-bit string from a printf-style format string and parameter
  list.

There are also a number of concrete classes that are created as a side-effect
of helper routines, etc. You should avoid direct use of these classes. Let
the string library create the class for you.

* ``ns[C]SubstringTuple`` - created via string concatenation
* ``nsDependent[C]Substring`` - created through ``Substring()``
* ``nsPromiseFlat[C]String`` - created through ``PromiseFlatString()``
* ``nsLiteral[C]String`` - created through the ``""_ns`` and ``u""_ns`` user-defined literals

Of course, there are times when it is necessary to reference these string
classes in your code, but as a general rule they should be avoided.

Iterators
---------

Because Mozilla strings are always a single buffer, iteration over the
characters in the string is done using raw pointers:

.. code-block:: c++

    /**
     * Find whether there is a tab character in `data`
     */
    bool HasTab(const nsAString& data) {
      const char16_t* cur = data.BeginReading();
      const char16_t* end = data.EndReading();

      for (; cur < end; ++cur) {
        if (char16_t('\t') == *cur) {
          return true;
        }
      }
      return false;
    }

Note that ``end`` points to the character after the end of the string buffer.
It should never be dereferenced.

Writing to a mutable string is also simple:

.. code-block:: c++

    /**
    * Replace every tab character in `data` with a space.
    */
    void ReplaceTabs(nsAString& data) {
      char16_t* cur = data.BeginWriting();
      char16_t* end = data.EndWriting();

      for (; cur < end; ++cur) {
        if (char16_t('\t') == *cur) {
          *cur = char16_t(' ');
        }
      }
    }

You may change the length of a string via ``SetLength()``. Note that
Iterators become invalid after changing the length of a string. If a string
buffer becomes smaller while writing it, use ``SetLength`` to inform the
string class of the new size:

.. code-block:: c++

    /**
     * Remove every tab character from `data`
     */
    void RemoveTabs(nsAString& data) {
      int len = data.Length();
      char16_t* cur = data.BeginWriting();
      char16_t* end = data.EndWriting();

      while (cur < end) {
        if (char16_t('\t') == *cur) {
          len -= 1;
          end -= 1;
          if (cur < end)
            memmove(cur, cur + 1, (end - cur) * sizeof(char16_t));
        } else {
          cur += 1;
        }
      }

      data.SetLength(len);
    }

Note that using ``BeginWriting()`` to make a string longer is not OK.
``BeginWriting()`` must not be used to write past the logical length of the
string indicated by ``EndWriting()`` or ``Length()``. Calling
``SetCapacity()`` before ``BeginWriting()`` does not affect what the previous
sentence says. To make the string longer, call ``SetLength()`` before
``BeginWriting()`` or use the ``BulkWrite()`` API described below.

Bulk Write
----------

``BulkWrite()`` allows capacity-aware cache-friendly low-level writes to the
string's buffer.

Capacity-aware means that the caller is made aware of how the
caller-requested buffer capacity was rounded up to mozjemalloc buckets. This
is useful when initially requesting best-case buffer size without yet knowing
the true size need. If the data that actually needs to be written is larger
than the best-case estimate but still fits within the rounded-up capacity,
there is no need to reallocate despite requesting the best-case capacity.

Cache-friendly means that the zero terminator for C compatibility is written
after the new content of the string has been written, so the result is a
forward-only linear write access pattern instead of a non-linear
back-and-forth sequence resulting from using ``SetLength()`` followed by
``BeginWriting()``.

Low-level means that writing via a raw pointer is possible as with
``BeginWriting()``.

``BulkWrite()`` takes three arguments: The new capacity (which may be rounded
up), the number of code units at the beginning of the string to preserve
(typically the old logical length), and a boolean indicating whether
reallocating a smaller buffer is OK if the requested capacity would fit in a
buffer that's smaller than current one. It returns a ``mozilla::Result`` which
contains either a usable ``mozilla::BulkWriteHandle<T>`` (where ``T`` is the
string's ``char_type``) or an ``nsresult`` explaining why none can be had
(presumably OOM).

The actual writes are performed through the returned
``mozilla::BulkWriteHandle<T>``. You must not access the string except via this
handle until you call ``Finish()`` on the handle in the success case or you let
the handle go out of scope without calling ``Finish()`` in the failure case, in
which case the destructor of the handle puts the string in a mostly harmless but
consistent state (containing a single REPLACEMENT CHARACTER if a capacity
greater than 0 was requested, or in the ``char`` case if the three-byte UTF-8
representation of the REPLACEMENT CHARACTER doesn't fit, an ASCII SUBSTITUTE).

``mozilla::BulkWriteHandle<T>`` autoconverts to a writable
``mozilla::Span<T>`` and also provides explicit access to itself as ``Span``
(``AsSpan()``) or via component accessors named consistently with those on
``Span``: ``Elements()`` and ``Length()``. (The latter is not the logical
length of the string but the writable length of the buffer.) The buffer
exposed via these methods includes the prefix that you may have requested to
be preserved. It's up to you to skip past it so as to not overwrite it.

If there's a need to request a different capacity before you are ready to
call ``Finish()``, you can call ``RestartBulkWrite()`` on the handle. It
takes three arguments that match the first three arguments of
``BulkWrite()``. It returns ``mozilla::Result<mozilla::Ok, nsresult>`` to
indicate success or OOM. Calling ``RestartBulkWrite()`` invalidates
previously-obtained span, raw pointer or length.

Once you are done writing, call ``Finish()``. It takes two arguments: the new
logical length of the string (which must not exceed the capacity retuned by
the ``Length()`` method of the handle) and a boolean indicating whether it's
OK to attempt to reallocate a smaller buffer in case a smaller mozjemalloc
bucket could accommodate the new logical length.

Helper Classes and Functions
----------------------------

Converting Cocoa strings
~~~~~~~~~~~~~~~~~~~~~~~~

Use ``mozilla::CopyCocoaStringToXPCOMString()`` in
``mozilla/MacStringHelpers.h`` to convert Cocoa strings to XPCOM strings.

Searching strings - looking for substrings, characters, etc.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``nsReadableUtils.h`` header provides helper methods for searching in runnables.

.. code-block:: c++

    bool FindInReadable(const nsAString& pattern,
                        nsAString::const_iterator start, nsAString::const_iterator end,
                        nsStringComparator& aComparator = nsDefaultStringComparator());

To use this, ``start`` and ``end`` should point to the beginning and end of a
string that you would like to search. If the search string is found,
``start`` and ``end`` will be adjusted to point to the beginning and end of
the found pattern. The return value is ``true`` or ``false``, indicating
whether or not the string was found.

An example:

.. code-block:: c++

    const nsAString& str = GetSomeString();
    nsAString::const_iterator start, end;

    str.BeginReading(start);
    str.EndReading(end);

    constexpr auto valuePrefix = u"value="_ns;

    if (FindInReadable(valuePrefix, start, end)) {
        // end now points to the character after the pattern
        valueStart = end;
    }

Checking for Memory Allocation failure
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Like other types in Gecko, the string classes use infallible memory
allocation by default, so you do not need to check for success when
allocating/resizing "normal" strings.

Most functions that modify strings (``Assign()``, ``SetLength()``, etc.) also
have an overload that takes a ``mozilla::fallible_t`` parameter. These
overloads return ``false`` instead of aborting if allocation fails. Use them
when creating/allocating strings which may be very large, and which the
program could recover from if the allocation fails.

Substrings (string fragments)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is very simple to refer to a substring of an existing string without
actually allocating new space and copying the characters into that substring.
``Substring()`` is the preferred method to create a reference to such a
string.

.. code-block:: c++

    void ProcessString(const nsAString& str) {
        const nsAString& firstFive = Substring(str, 0, 5); // from index 0, length 5
        // firstFive is now a string representing the first 5 characters
    }

Unicode Conversion
------------------

Strings can be stored in two basic formats: 8-bit code unit (byte/``char``)
strings, or 16-bit code unit (``char16_t``) strings. Any string class with a
capital "C" in the classname contains 8-bit bytes. These classes include
``nsCString``, ``nsDependentCString``, and so forth. Any string class without
the "C" contains 16-bit code units.

A 8-bit string can be in one of many character encodings while a 16-bit
string is always in potentially-invalid UTF-16. (You can make a 16-bit string
guaranteed-valid UTF-16 by passing it to ``EnsureUTF16Validity()``.) The most
common encodings are:


* ASCII - 7-bit encoding for basic English-only strings. Each ASCII value
  is stored in exactly one byte in the array with the most-significant 8th bit
  set to zero.

* `UCS2 <http://www.unicode.org/glossary/#UCS_2>`_ - 16-bit encoding for a
  subset of Unicode, `BMP <http://www.unicode.org/glossary/#BMP>`_. The Unicode
  value of a character stored in UCS2 is stored in exactly one 16-bit
  ``char16_t`` in a string class.

* `UTF-8 <http://www.faqs.org/rfcs/rfc3629.html>`_ - 8-bit encoding for
  Unicode characters. Each Unicode characters is stored in up to 4 bytes in a
  string class. UTF-8 is capable of representing the entire Unicode character
  repertoire, and it efficiently maps to `UTF-32
  <http://www.unicode.org/glossary/#UTF_32>`_. (Gtk and Rust natively use
  UTF-8.)

* `UTF-16 <http://www.unicode.org/glossary/#UTF_16>`_ - 16-bit encoding for
  Unicode storage, backwards compatible with UCS2. The Unicode value of a
  character stored in UTF-16 may require one or two 16-bit ``char16_t`` in a
  string class. The contents of ``nsAString`` always has to be regarded as in
  this encoding instead of UCS2. UTF-16 is capable of representing the entire
  Unicode character repertoire, and it efficiently maps to UTF-32. (Win32 W
  APIs and Mac OS X natively use UTF-16.)

* Latin1 - 8-bit encoding for the first 256 Unicode code points. Used for
  HTTP headers and for size-optimized storage in text node and SpiderMonkey
  strings. Latin1 converts to UTF-16 by zero-extending each byte to a 16-bit
  code unit. Note that this kind of "Latin1" is not available for encoding
  HTML, CSS, JS, etc. Specifying ``charset=latin1`` means the same as
  ``charset=windows-1252``. Windows-1252 is a similar but different encoding
  used for interchange.

In addition, there exist multiple other (legacy) encodings. The Web-relevant
ones are defined in the `Encoding Standard <https://encoding.spec.whatwg.org/>`_.
Conversions from these encodings to
UTF-8 and UTF-16 are provided by `mozilla::Encoding
<https://searchfox.org/mozilla-central/source/intl/Encoding.h#109>`_.
Additonally, on Windows the are some rare cases (e.g. drag&drop) where it's
necessary to call a system API with data encoded in the Windows
locale-dependent legacy encoding instead of UTF-16. In those rare cases, use
``MultiByteToWideChar``/``WideCharToMultiByte`` from kernel32.dll. Do not use
``iconv`` on *nix. We only support UTF-8-encoded file paths on *nix, non-path
Gtk strings are always UTF-8 and Cocoa and Java strings are always UTF-16.

When working with existing code, it is important to examine the current usage
of the strings that you are manipulating, to determine the correct conversion
mechanism.

When writing new code, it can be confusing to know which storage class and
encoding is the most appropriate. There is no single answer to this question,
but the important points are:


* **Surprisingly many strings are very often just ASCII.** ASCII is a subset of
  UTF-8 and is, therefore, efficient to represent as UTF-8. Representing ASCII
  as UTF-16 bad both for memory usage and cache locality.

* **Rust strongly prefers UTF-8.** If your C++ code is interacting with Rust
  code, using UTF-8 in ``nsACString`` and merely validating it when converting
  to Rust strings is more efficient than using ``nsAString`` on the C++ side.

* **Networking code prefers 8-bit strings.** Networking code tends to use 8-bit
  strings: either with UTF-8 or Latin1 (byte value is the Unicode scalar value)
  semantics.

* **JS and DOM prefer UTF-16.** Most Gecko code uses UTF-16 for compatibility
  with JS strings and DOM string which are potentially-invalid UTF-16. However,
  both DOM text nodes and JS strings store strings that only contain code points
  below U+0100 as Latin1 (byte value is the Unicode scalar value).

* **Windows and Cocoa use UTF-16.** Windows system APIs take UTF-16. Cocoa
  ``NSString`` is UTF-16.

* **Gtk uses UTF-8.** Gtk APIs take UTF-8 for non-file paths. In the Gecko
  case, we support only UTF-8 file paths outside Windows, so all Gtk strings
  are UTF-8 for our purposes though file paths received from Gtk may not be
  valid UTF-8.

To assist with ASCII, Latin1, UTF-8, and UTF-16 conversions, there are some
helper methods and classes. Some of these classes look like functions,
because they are most often used as temporary objects on the stack.

Short zero-terminated ASCII strings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you have a short zero-terminated string that you are certain is always
ASCII, use these special-case methods instead of the conversions described in
the later sections.

* If you are assigning an ASCII literal to an ``nsACString``, use
  ``AssignLiteral()``.
* If you are assigning a literal to an ``nsAString``, use ``AssignLiteral()``
  and make the literal a ``u""`` literal. If the literal has to be a ``""``
  literal (as opposed to ``u""``) and is ASCII, still use ``AppendLiteral()``,
  but be aware that this involves a run-time inflation.
* If you are assigning a zero-terminated ASCII string that's not a literal from
  the compiler's point of view at the call site and you don't know the length
  of the string either (e.g. because it was looked up from an array of literals
  of varying lengths), use ``AssignASCII()``.

UTF-8 / UTF-16 conversion
~~~~~~~~~~~~~~~~~~~~~~~~~

.. cpp:function:: NS_ConvertUTF8toUTF16(const nsACString&)

    a ``nsAutoString`` subclass that converts a UTF-8 encoded ``nsACString``
    or ``const char*`` to a 16-bit UTF-16 string. If you need a ``const
    char16_t*`` buffer, you can use the ``.get()`` method. For example:

    .. code-block:: c++

        /* signature: void HandleUnicodeString(const nsAString& str); */
        object->HandleUnicodeString(NS_ConvertUTF8toUTF16(utf8String));

        /* signature: void HandleUnicodeBuffer(const char16_t* str); */
        object->HandleUnicodeBuffer(NS_ConvertUTF8toUTF16(utf8String).get());

.. cpp:function:: NS_ConvertUTF16toUTF8(const nsAString&)

    a ``nsAutoCString`` which converts a 16-bit UTF-16 string (``nsAString``)
    to a UTF-8 encoded string. As above, you can use ``.get()`` to access a
    ``const char*`` buffer.

    .. code-block:: c++

        /* signature: void HandleUTF8String(const nsACString& str); */
        object->HandleUTF8String(NS_ConvertUTF16toUTF8(utf16String));

        /* signature: void HandleUTF8Buffer(const char* str); */
        object->HandleUTF8Buffer(NS_ConvertUTF16toUTF8(utf16String).get());

.. cpp:function:: CopyUTF8toUTF16(const nsACString&, nsAString&)

    converts and copies:

    .. code-block:: c++

        // return a UTF-16 value
        void Foo::GetUnicodeValue(nsAString& result) {
          CopyUTF8toUTF16(mLocalUTF8Value, result);
        }

.. cpp:function:: AppendUTF8toUTF16(const nsACString&, nsAString&)

    converts and appends:

    .. code-block:: c++

        // return a UTF-16 value
        void Foo::GetUnicodeValue(nsAString& result) {
          result.AssignLiteral("prefix:");
          AppendUTF8toUTF16(mLocalUTF8Value, result);
        }

.. cpp:function:: CopyUTF16toUTF8(const nsAString&, nsACString&)

    converts and copies:

    .. code-block:: c++

        // return a UTF-8 value
        void Foo::GetUTF8Value(nsACString& result) {
          CopyUTF16toUTF8(mLocalUTF16Value, result);
        }

.. cpp:function:: AppendUTF16toUTF8(const nsAString&, nsACString&)

    converts and appends:

    .. code-block:: c++

        // return a UTF-8 value
        void Foo::GetUnicodeValue(nsACString& result) {
          result.AssignLiteral("prefix:");
          AppendUTF16toUTF8(mLocalUTF16Value, result);
        }


Latin1 / UTF-16 Conversion
~~~~~~~~~~~~~~~~~~~~~~~~~~

The following should only be used when you can guarantee that the original
string is ASCII or Latin1 (in the sense that the byte value is the Unicode
scalar value; not in the windows-1252 sense). These helpers are very similar
to the UTF-8 / UTF-16 conversion helpers above.


UTF-16 to Latin1 converters
```````````````````````````

These converters are **very dangerous** because they **lose information**
during the conversion process. You should **avoid UTF-16 to Latin1
conversions** unless your strings are guaranteed to be Latin1 or ASCII. (In
the future, these conversions may start asserting in debug builds that their
input is in the permissible range.) If the input is actually in the Latin1
range, each 16-bit code unit in narrowed to an 8-bit byte by removing the
high half. Unicode code points above U+00FF result in garbage whose nature
must not be relied upon. (In the future the nature of the garbage will be CPU
architecture-dependent.) If you want to ``printf()`` something and don't care
what happens to non-ASCII, please convert to UTF-8 instead.


.. cpp:function:: NS_LossyConvertUTF16toASCII(const nsAString&)

    A ``nsAutoCString`` which holds a temporary buffer containing the Latin1
    value of the string.

.. cpp:function:: void LossyCopyUTF16toASCII(Span<const char16_t>, nsACString&)

    Does an in-place conversion from UTF-16 into an Latin1 string object.

.. cpp:function:: void LossyAppendUTF16toASCII(Span<const char16_t>, nsACString&)

    Appends a UTF-16 string to a Latin1 string.

Latin1 to UTF-16 converters
```````````````````````````

These converters are very dangerous because they will **produce wrong results
for non-ASCII UTF-8 or windows-1252 input** into a meaningless UTF-16 string.
You should **avoid ASCII to UTF-16 conversions** unless your strings are
guaranteed to be ASCII or Latin1 in the sense of the byte value being the
Unicode scalar value. Every byte is zero-extended into a 16-bit code unit.

It is correct to use these on most HTTP header values, but **it's always
wrong to use these on HTTP response bodies!** (Use ``mozilla::Encoding`` to
deal with response bodies.)

.. cpp:function:: NS_ConvertASCIItoUTF16(const nsACString&)

    A ``nsAutoString`` which holds a temproary buffer contianing the value of
    the Latin1 to UTF-16 conversion.

.. cpp:function:: void CopyASCIItoUTF16(Span<const char>, nsAString&)

    does an in-place conversion from Latin1 to UTF-16.

.. cpp:function:: void AppendASCIItoUTF16(Span<const char>, nsAString&)

    appends a Latin1 string to a UTF-16 string.

Comparing ns*Strings with C strings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can compare ``ns*Strings`` with C strings by converting the ``ns*String``
to a C string, or by comparing directly against a C String.

.. cpp:function:: bool nsAString::EqualsASCII(const char*)

    Compares with an ASCII C string.

.. cpp:function:: bool nsAString::EqualsLiteral(...)

    Compares with a string literal.

Common Patterns
---------------

Literal Strings
~~~~~~~~~~~~~~~

A literal string is a raw string value that is written in some C++ code. For
example, in the statement ``printf("Hello World\n");`` the value ``"Hello
World\n"`` is a literal string. It is often necessary to insert literal
string values when an ``nsAString`` or ``nsACString`` is required. Two
user-defined literals are provided that implicitly convert to ``const
nsString&`` resp. ``const nsCString&``:

* ``""_ns`` for 8-bit literals, converting implicitly to ``const nsCString&``
* ``u""_ns`` for 16-bit literals, converting implicitly to ``const nsString&``

The benefits of the user-defined literals may seem unclear, given that
``nsDependentCString`` will also wrap a string value in an ``nsCString``. The
advantage of the user-defined literals is twofold.

* The length of these strings is calculated at compile time, so the string does
  not need to be scanned at runtime to determine its length.

* Literal strings live for the lifetime of the binary, and can be moved between
  the ``ns[C]String`` classes without being copied or freed.

Here are some examples of proper usage of the literals (both standard and
user-defined):

.. code-block:: c++

    // call Init(const nsLiteralString&) - enforces that it's only called with literals
    Init(u"start value"_ns);

    // call Init(const nsAString&)
    Init(u"start value"_ns);

    // call Init(const nsACString&)
    Init("start value"_ns);

In case a literal is defined via a macro, you can just convert it to
``nsLiteralString`` or ``nsLiteralCString`` using their constructor. You
could consider not using a macro at all but a named ``constexpr`` constant
instead.

In some cases, an 8-bit literal is defined via a macro, either within code or
from the environment, but it can't be changed or is used both as an 8-bit and
a 16-bit string. In these cases, you can use the
``NS_LITERAL_STRING_FROM_CSTRING`` macro to construct a ``nsLiteralString``
and do the conversion at compile-time.

String Concatenation
~~~~~~~~~~~~~~~~~~~~

Strings can be concatenated together using the + operator. The resulting
string is a ``const nsSubstringTuple`` object. The resulting object can be
treated and referenced similarly to a ``nsAString`` object. Concatenation *does
not copy the substrings*. The strings are only copied when the concatenation
is assigned into another string object. The ``nsSubstringTuple`` object holds
pointers to the original strings. Therefore, the ``nsSubstringTuple`` object is
dependent on all of its substrings, meaning that their lifetime must be at
least as long as the ``nsSubstringTuple`` object.

For example, you can use the value of two strings and pass their
concatenation on to another function which takes an ``const nsAString&``:

.. code-block:: c++

    void HandleTwoStrings(const nsAString& one, const nsAString& two) {
      // call HandleString(const nsAString&)
      HandleString(one + two);
    }

NOTE: The two strings are implicitly combined into a temporary ``nsString``
in this case, and the temporary string is passed into ``HandleString``. If
``HandleString`` assigns its input into another ``nsString``, then the string
buffer will be shared in this case negating the cost of the intermediate
temporary. You can concatenate N strings and store the result in a temporary
variable:

.. code-block:: c++

    constexpr auto start = u"start "_ns;
    constexpr auto middle = u"middle "_ns;
    constexpr auto end = u"end"_ns;
    // create a string with 3 dependent fragments - no copying involved!
    nsString combinedString = start + middle + end;

    // call void HandleString(const nsAString&);
    HandleString(combinedString);

It is safe to concatenate user-defined literals because the temporary
``nsLiteral[C]String`` objects will live as long as the temporary
concatenation object (of type ``nsSubstringTuple``).

.. code-block:: c++

    // call HandlePage(const nsAString&);
    // safe because the concatenated-string will live as long as its substrings
    HandlePage(u"start "_ns + u"end"_ns);

Local Variables
~~~~~~~~~~~~~~~

Local variables within a function are usually stored on the stack. The
``nsAutoString``/``nsAutoCString`` classes are subclasses of the
``nsString``/``nsCString`` classes. They own a 64-character buffer allocated
in the same storage space as the string itself. If the ``nsAutoString`` is
allocated on the stack, then it has at its disposal a 64-character stack
buffer. This allows the implementation to avoid allocating extra memory when
dealing with small strings. ``nsAutoStringN``/``nsAutoCStringN`` are more
general alternatives that let you choose the number of characters in the
inline buffer.

.. code-block:: c++

    ...
    nsAutoString value;
    GetValue(value); // if the result is less than 64 code units,
                    // then this just saved us an allocation
    ...

Member Variables
~~~~~~~~~~~~~~~~

In general, you should use the concrete classes ``nsString`` and
``nsCString`` for member variables.

.. code-block:: c++

    class Foo {
      ...
      // these store UTF-8 and UTF-16 values respectively
      nsCString mLocalName;
      nsString mTitle;
    };

A common incorrect pattern is to use ``nsAutoString``/``nsAutoCString``
for member variables. As described in `Local Variables`_, these classes have
a built in buffer that make them very large. This means that if you include
them in a class, they bloat the class by 64 bytes (``nsAutoCString``) or 128
bytes (``nsAutoString``).


Raw Character Pointers
~~~~~~~~~~~~~~~~~~~~~~

``PromiseFlatString()`` and ``PromiseFlatCString()`` can be used to create a
temporary buffer which holds a null-terminated buffer containing the same
value as the source string. ``PromiseFlatString()`` will create a temporary
buffer if necessary. This is most often used in order to pass an
``nsAString`` to an API which requires a null-terminated string.

In the following example, an ``nsAString`` is combined with a literal string,
and the result is passed to an API which requires a simple character buffer.

.. code-block:: c++

    // Modify the URL and pass to AddPage(const char16_t* url)
    void AddModifiedPage(const nsAString& url) {
      constexpr auto httpPrefix = u"http://"_ns;
      const nsAString& modifiedURL = httpPrefix + url;

      // creates a temporary buffer
      AddPage(PromiseFlatString(modifiedURL).get());
    }

``PromiseFlatString()`` is smart when handed a string that is already
null-terminated. It avoids creating the temporary buffer in such cases.

.. code-block:: c++

    // Modify the URL and pass to AddPage(const char16_t* url)
    void AddModifiedPage(const nsAString& url, PRBool addPrefix) {
        if (addPrefix) {
            // MUST create a temporary buffer - string is multi-fragmented
            constexpr auto httpPrefix = u"http://"_ns;
            AddPage(PromiseFlatString(httpPrefix + modifiedURL));
        } else {
            // MIGHT create a temporary buffer, does a runtime check
            AddPage(PromiseFlatString(url).get());
        }
    }

.. note::

    It is **not** possible to efficiently transfer ownership of a string
    class' internal buffer into an owned ``char*`` which can be safely
    freed by other components due to the COW optimization.

    If working with a legacy API which requires malloced ``char*`` buffers,
    prefer using ``ToNewUnicode``, ``ToNewCString`` or ``ToNewUTF8String``
    over ``strdup`` to create owned ``char*`` pointers.

``printf`` and a UTF-16 string
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For debugging, it's useful to ``printf`` a UTF-16 string (``nsString``,
``nsAutoString``, etc). To do this usually requires converting it to an 8-bit
string, because that's what ``printf`` expects. Use:

.. code-block:: c++

    printf("%s\n", NS_ConvertUTF16toUTF8(yourString).get());

Sequence of appends without reallocating
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``SetCapacity()`` allows you to give the string a hint of the future string
length caused by a sequence of appends (excluding appends that convert
between UTF-16 and UTF-8 in either direction) in order to avoid multiple
allocations during the sequence of appends. However, the other
allocation-avoidance features of XPCOM strings interact badly with
``SetCapacity()`` making it something of a footgun.

``SetCapacity()`` is appropriate to use before a sequence of multiple
operations from the following list (without operations that are not on the
list between the ``SetCapacity()`` call and operations from the list):

* ``Append()``
* ``AppendASCII()``
* ``AppendLiteral()``
* ``AppendPrintf()``
* ``AppendInt()``
* ``AppendFloat()``
* ``LossyAppendUTF16toASCII()``
* ``AppendASCIItoUTF16()``

**DO NOT** call ``SetCapacity()`` if the subsequent operations on the string
do not meet the criteria above. Operations that undo the benefits of
``SetCapacity()`` include but are not limited to:

* ``SetLength()``
* ``Truncate()``
* ``Assign()``
* ``AssignLiteral()``
* ``Adopt()``
* ``CopyASCIItoUTF16()``
* ``LossyCopyUTF16toASCII()``
* ``AppendUTF16toUTF8()``
* ``AppendUTF8toUTF16()``
* ``CopyUTF16toUTF8()``
* ``CopyUTF8toUTF16()``

If your string is an ``nsAuto[C]String`` and you are calling
``SetCapacity()`` with a constant ``N``, please instead declare the string as
``nsAuto[C]StringN<N+1>`` without calling ``SetCapacity()`` (while being
mindful of not using such a large ``N`` as to overflow the run-time stack).

There is no need to include room for the null terminator: it is the job of
the string class.

Note: Calling ``SetCapacity()`` does not give you permission to use the
pointer obtained from ``BeginWriting()`` to write past the current length (as
returned by ``Length()``) of the string. Please use either ``BulkWrite()`` or
``SetLength()`` instead.

.. _stringguide.xpidl:

XPIDL
-----

The string library is also available through IDL. By declaring attributes and
methods using the specially defined IDL types, string classes are used as
parameters to the corresponding methods.

XPIDL String types
~~~~~~~~~~~~~~~~~~

The C++ signatures follow the abstract-type convention described above, such
that all method parameters are based on the abstract classes. The following
table describes the purpose of each string type in IDL.

+-----------------+----------------+----------------------------------------------------------------------------------+
| XPIDL Type      | C++ Type       | Purpose                                                                          |
+=================+================+==================================================================================+
| ``string``      | ``char*``      | Raw character pointer to ASCII (7-bit) string, no string classes used.           |
|                 |                |                                                                                  |
|                 |                | High bit is not guaranteed across XPConnect boundaries.                          |
+-----------------+----------------+----------------------------------------------------------------------------------+
| ``wstring``     | ``char16_t*``  | Raw character pointer to UTF-16 string, no string classes used.                  |
+-----------------+----------------+----------------------------------------------------------------------------------+
| ``AString``     | ``nsAString``  | UTF-16 string.                                                                   |
+-----------------+----------------+----------------------------------------------------------------------------------+
| ``ACString``    | ``nsACString`` | 8-bit string. All bits are preserved across XPConnect boundaries.                |
+-----------------+----------------+----------------------------------------------------------------------------------+
| ``AUTF8String`` | ``nsACString`` | UTF-8 string.                                                                    |
|                 |                |                                                                                  |
|                 |                | Converted to UTF-16 as necessary when value is used across XPConnect boundaries. |
+-----------------+----------------+----------------------------------------------------------------------------------+

Callers should prefer using the string classes ``AString``, ``ACString`` and
``AUTF8String`` over the raw pointer types ``string`` and ``wstring`` in
almost all situations.

C++ Signatures
~~~~~~~~~~~~~~

In XPIDL, ``in`` parameters are read-only, and the C++ signatures for
``*String`` parameters follows the above guidelines by using ``const
nsAString&`` for these parameters. ``out`` and ``inout`` parameters are
defined simply as ``nsAString&`` so that the callee can write to them.

.. code-block::

    interface nsIFoo : nsISupports {
        attribute AString utf16String;
        AUTF8String getValue(in ACString key);
    };

.. code-block:: c++

    class nsIFoo : public nsISupports {
      NS_IMETHOD GetUtf16String(nsAString& aResult) = 0;
      NS_IMETHOD SetUtf16String(const nsAString& aValue) = 0;
      NS_IMETHOD GetValue(const nsACString& aKey, nsACString& aResult) = 0;
    };

In the above example, ``utf16String`` is treated as a UTF-16 string. The
implementation of ``GetUtf16String()`` will use ``aResult.Assign`` to
"return" the value. In ``SetUtf16String()`` the value of the string can be
used through a variety of methods including `Iterators`_,
``PromiseFlatString``, and assignment to other strings.

In ``GetValue()``, the first parameter, ``aKey``, is treated as a raw
sequence of 8-bit values. Any non-ASCII characters in ``aKey`` will be
preserved when crossing XPConnect boundaries. The implementation of
``GetValue()`` will assign a UTF-8 encoded 8-bit string into ``aResult``. If
the this method is called across XPConnect boundaries, such as from a script,
then the result will be decoded from UTF-8 into UTF-16 and used as a Unicode
value.

String Guidelines
-----------------

Follow these simple rules in your code to keep your fellow developers,
reviewers, and users happy.

* Use the most abstract string class that you can. Usually this is:
  * ``nsAString`` for function parameters
  * ``nsString`` for member variables
  * ``nsAutoString`` for local (stack-based) variables
* Use the ``""_ns`` and ``u""_ns`` user-defined literals to represent literal strings (e.g. ``"foo"_ns``) as nsAString-compatible objects.
* Use string concatenation (i.e. the "+" operator) when combining strings.
* Use ``nsDependentString`` when you have a raw character pointer that you need to convert to an nsAString-compatible string.
* Use ``Substring()`` to extract fragments of existing strings.
* Use `iterators`_ to parse and extract string fragments.

Class Reference
---------------

.. cpp:class:: template<T> nsTSubstring<T>

    .. note::

        The ``nsTSubstring<char_type>`` class is usually written as
        ``nsAString`` or ``nsACString``.

    .. cpp:function:: size_type Length() const

    .. cpp:function:: bool IsEmpty() const

    .. cpp:function:: bool IsVoid() const

    .. cpp:function:: const char_type* BeginReading() const

    .. cpp:function:: const char_type* EndReading() const

    .. cpp:function:: bool Equals(const self_type&, comparator_type = ...) const

    .. cpp:function:: char_type First() const

    .. cpp:function:: char_type Last() const

    .. cpp:function:: size_type CountChar(char_type) const

    .. cpp:function:: int32_t FindChar(char_type, index_type aOffset = 0) const

    .. cpp:function:: void Assign(const self_type&)

    .. cpp:function:: void Append(const self_type&)

    .. cpp:function:: void Insert(const self_type&, index_type aPos)

    .. cpp:function:: void Cut(index_type aCutStart, size_type aCutLength)

    .. cpp:function:: void Replace(index_type aCutStart, size_type aCutLength, const self_type& aStr)

    .. cpp:function:: void Truncate(size_type aLength)

    .. cpp:function:: void SetIsVoid(bool)

        Make it null. XPConnect and WebIDL will convert void nsAStrings to
        JavaScript ``null``.

    .. cpp:function:: char_type* BeginWriting()

    .. cpp:function:: char_type* EndWriting()

    .. cpp:function:: void SetCapacity(size_type)

        Inform the string about buffer size need before a sequence of calls
        to ``Append()`` or converting appends that convert between UTF-16 and
        Latin1 in either direction. (Don't use if you use appends that
        convert between UTF-16 and UTF-8 in either direction.) Calling this
        method does not give you permission to use ``BeginWriting()`` to
        write past the logical length of the string. Use ``SetLength()`` or
        ``BulkWrite()`` as appropriate.

    .. cpp:function:: void SetLength(size_type)

    .. cpp:function:: Result<BulkWriteHandle<char_type>, nsresult> BulkWrite(size_type aCapacity, size_type aPrefixToPreserve, bool aAllowShrinking)


Original Document Information
-----------------------------

This document was originally hosted on MDN as part of the XPCOM guide.

* Author: `Alec Flett <mailto:alecf@flett.org>`_
* Copyright Information: Portions of this content are © 1998–2007 by individual mozilla.org contributors; content available under a Creative Commons license.
* Thanks to David Baron for `actual docs <http://dbaron.org/mozilla/coding-practices>`_,
* Peter Annema for lots of direction
* Myk Melez for some more docs
* David Bradley for a diagram
* Revised by Darin Fisher for Mozilla 1.7
* Revised by Jungshik Shin to clarify character encoding issues
* Migrated to in-tree documentation by Nika Layzell