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).


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 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:











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 static, 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.


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>.


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:

// 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.


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

 * 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:

* 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:

 * 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;


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 returned 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 NSString strings

Use mozilla::CopyNSStringToXPCOMString() in mozilla/MacStringHelpers.h to convert NSString strings to XPCOM strings.

Searching strings - looking for substrings, characters, etc.

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

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:

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

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 - 16-bit encoding for a subset of Unicode, 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 - 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. (Gtk and Rust natively use UTF-8.)

  • 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. Conversions from these encodings to UTF-8 and UTF-16 are provided by mozilla::Encoding. Additionally, 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

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:

/* signature: void HandleUnicodeString(const nsAString& str); */

/* signature: void HandleUnicodeBuffer(const char16_t* str); */
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.

/* signature: void HandleUTF8String(const nsACString& str); */

/* signature: void HandleUTF8Buffer(const char* str); */
CopyUTF8toUTF16(const nsACString&, nsAString&)

converts and copies:

// return a UTF-16 value
void Foo::GetUnicodeValue(nsAString& result) {
  CopyUTF8toUTF16(mLocalUTF8Value, result);
AppendUTF8toUTF16(const nsACString&, nsAString&)

converts and appends:

// return a UTF-16 value
void Foo::GetUnicodeValue(nsAString& result) {
  AppendUTF8toUTF16(mLocalUTF8Value, result);
CopyUTF16toUTF8(const nsAString&, nsACString&)

converts and copies:

// return a UTF-8 value
void Foo::GetUTF8Value(nsACString& result) {
  CopyUTF16toUTF8(mLocalUTF16Value, result);
AppendUTF16toUTF8(const nsAString&, nsACString&)

converts and appends:

// return a UTF-8 value
void Foo::GetUnicodeValue(nsACString& result) {
  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.

NS_LossyConvertUTF16toASCII(const nsAString&)

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

void LossyCopyUTF16toASCII(Span<const char16_t>, nsACString&)

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

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.)

NS_ConvertASCIItoUTF16(const nsACString&)

A nsAutoString which holds a temporary buffer containing the value of the Latin1 to UTF-16 conversion.

void CopyASCIItoUTF16(Span<const char>, nsAString&)

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

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.

bool nsAString::EqualsASCII(const char*)

Compares with an ASCII C string.

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):

// 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&:

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:

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&);

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).

// 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.

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.

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.

// 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

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

// 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


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:

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.


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.


C++ Type




Raw character pointer to ASCII (7-bit) string, no string classes used.

High bit is not guaranteed across XPConnect boundaries.



Raw character pointer to UTF-16 string, no string classes used.



UTF-16 string.



8-bit string. All bits are preserved across XPConnect boundaries.



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.

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

class nsTSubstring<T>


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

size_type Length() const
bool IsEmpty() const
bool IsVoid() const
const char_type *BeginReading() const
const char_type *EndReading() const
bool Equals(const self_type&, comparator_type = ...) const
char_type First() const
char_type Last() const
size_type CountChar(char_type) const
int32_t FindChar(char_type, index_type aOffset = 0) const
void Assign(const self_type&)
void Append(const self_type&)
void Insert(const self_type&, index_type aPos)
void Cut(index_type aCutStart, size_type aCutLength)
void Replace(index_type aCutStart, size_type aCutLength, const self_type &aStr)
void Truncate(size_type aLength)
void SetIsVoid(bool)

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

char_type *BeginWriting()
char_type *EndWriting()
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.

void SetLength(size_type)
Result<BulkWriteHandle<char_type>, nsresult> BulkWrite(size_type aCapacity, size_type aPrefixToPreserve, bool aAllowShrinking)