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« XPCOM API Reference


The nsACString abstract class represents a character string composed of single-byte storage units. This class is typically used to represent ASCII or UTF-8 character arrays.

#include "nsStringAPI.h"

  class nsACString { ... };


char_type [char]
Storage unit for this class.
self_type [nsACString]
An alias for the type of this class.
size_type [PRUint32]
Type used to represent the number of bytes stored in the string.
index_type [PRUint32]
Type used to represent an offset into the string.


The BeginReading function returns a const pointer to the first element of the string's internal buffer.
The EndReading function returns a const char_type pointer to the element just beyond the last element of the string's internal buffer.
The Length function returns the number of storage units in the string's internal buffer (not including the null-terminator if present).
The Assign family of functions sets the value of a string's internal buffer.
The Replace family of functions sets the value of a string's internal buffer.
The Append family of functions appends a value to the end of a string's internal buffer.
The Insert family of functions inserts a value into a string's internal buffer.
The Cut function removes a section of the string's internal buffer.


This operator is a shortcut for the Assign family of functions.
This operator is a shortcut for the Append family of functions.


The nsACString class is never instantiated directly. It should be instantiated using a subclass, such as nsEmbedCString.

nsACString corresponds to the ACString and AUTF8String XPIDL data types. nsACString by itself does not imply a particular character encoding. That is, it may be used to store UTF-8 characters, ASCII characters, or any random binary data. The character encoding in use for a particular nsACString instance depends on the context in which it is used. If used with XPIDL, then the character encodings of the corresponding XPIDL data types applies.

nsACString may even hold data with embedded null bytes. Thus, the length of the data contained in the nsACString should be determined by calling the nsACString::Length() method.

The methods defined on nsACString are implemented as inline wrappers around the XPCOM string functions, prefixed with NS_CString. See, for example, NS_CStringGetData.

See also