+\subsection{Introduction}
+
+wxString is a class which represents a character string of arbitrary length (limited by
+{\it MAX\_INT} which is usually 2147483647 on 32 bit machines) and containing
+arbitrary characters. The ASCII NUL character is allowed, although care should be
+taken when passing strings containing it to other functions.
+
+wxString only works with ASCII (8 bit characters) strings as of this release,
+but support for UNICODE (16 but characters) is planned for the next one.
+
+This class has all the standard operations you can expect to find in a string class:
+dynamic memory management (string extends to accommodate new characters),
+construction from other strings, C strings and characters, assignment operators,
+access to individual characters, string concatenation and comparison, substring
+extraction, case conversion, trimming and padding (with spaces), searching and
+replacing and both C-like \helpref{Printf()}{wxstringprintf} and stream-like
+insertion functions as well as much more - see \helpref{wxString}{wxstring}
+for a list of all functions.
+
+\subsection{Comparison of wxString to other string classes}
+
+The advantages of using a special string class instead of working directly with
+C strings are so obvious that there is a huge number of such classes available.
+The most important advantage is the need to always
+remember to allocate/free memory for C strings; working with fixed size buffers almost
+inevitably leads to buffer overflows. At last, C++ has a standard string class
+(std::string). So why the need for wxString?
+
+There are several advantages:
+
+\begin{enumerate}\itemsep=0pt
+\item {\bf Efficiency} This class was made to be as efficient as possible: both
+in terms of size (each wxString objects takes exactly the same space as a {\it
+char *} pointer, sing \helpref{reference counting}{wxstringrefcount}) and speed.
+It also provides performance \helpref{statistics gathering code}{wxstringtuning}
+which may be enabled to fine tune the memory allocation strategy for your
+particular application - and the gain might be quite big.
+\item {\bf Compatibility} This class tries to combine almost full compatibility
+with the old wxWindows 1.xx wxString class, some reminiscence to MFC CString
+class and 90\% of the functionality of std::string class.
+\item {\bf Rich set of functions} Some of the functions present in wxString are
+very useful but don't exist in most of other string classes: for example,
+\helpref{AfterFirst}{wxstringafterfirst},
+\helpref{BeforeLast}{wxstringbeforelast}, \helpref{operator<<}{wxstringoperatorout}
+or \helpref{Printf}{wxstringprintf}. Of course, all the standard string
+operations are supported as well.
+\item {\bf UNICODE} In this release, wxString only supports {\it construction} from
+a UNICODE string, but in the next one it will be capable of also storing its
+internal data in either ASCII or UNICODE format.
+\item {\bf Used by wxWindows} And, of course, this class is used everywhere
+inside wxWindows so there is no performance loss which would result from
+conversions of objects of any other string class (including std::string) to
+wxString internally by wxWindows.
+\end{enumerate}
+
+However, there are several problems as well. The most important one is probably
+that there are often several functions to do exactly the same thing: for
+example, to get the length of the string either one of
+length(), \helpref{Len()}{wxstringlen} or
+\helpref{Length()}{wxstringlength} may be used. The first function, as almost
+all the other functions in lowercase, is std::string compatible. The second one
+is "native" wxString version and the last one is wxWindows 1.xx way. So the
+question is: which one is better to use? And the answer is that:
+
+{\bf The usage of std::string compatible functions is strongly advised!} It will
+both make your code more familiar to other C++ programmers (who are supposed to
+have knowledge of std::string but not of wxString), let you reuse the same code
+in both wxWindows and other programs (by just typedefing wxString as std::string
+when used outside wxWindows) and by staying compatible with future versions of
+wxWindows which will probably start using std::string sooner or later too.
+
+In the situations where there is no corresponding std::string function, please
+try to use the new wxString methods and not the old wxWindows 1.xx variants
+which are deprecated and may disappear in future versions.
+
+\subsection{Some advice about using wxString}\label{wxstringadvices}
+
+Probably the main trap with using this class is the implicit conversion operator to
+{\it const char *}. It is advised that you use \helpref{c\_str()}{wxstringcstr}
+instead to clearly indicate when the conversion is done. Specifically, the
+danger of this implicit conversion may be seen in the following code fragment:
+
+\begin{verbatim}
+// this function converts the input string to uppercase, output it to the screen
+// and returns the result
+const char *SayHELLO(const wxString& input)
+{
+ wxString output = input.Upper();
+
+ printf("Hello, %s!\n", output);
+
+ return output;
+}
+\end{verbatim}
+
+There are two nasty bugs in these three lines. First of them is in the call to the
+{\it printf()} function. Although the implicit conversion to C strings is applied
+automatically by the compiler in the case of
+
+\begin{verbatim}
+ puts(output);
+\end{verbatim}
+
+because the argument of {\it puts()} is known to be of the type {\it const char *},
+this is {\bf not} done for {\it printf()} which is a function with variable
+number of arguments (and whose arguments are of unknown types). So this call may
+do anything at all (including displaying the correct string on screen), although
+the most likely result is a program crash. The solution is to use
+\helpref{c\_str()}{wxstringcstr}: just replace this line with
+
+\begin{verbatim}
+ printf("Hello, %s!\n", output.c_str());
+\end{verbatim}
+
+The second bug is that returning {\it output} doesn't work. The implicit cast is
+used again, so the code compiles, but as it returns a pointer to a buffer
+belonging to a local variable which is deleted as soon as the function exits,
+its contents is totally arbitrary. The solution to this problem is also easy:
+just make the function return wxString instead of a C string.
+
+This leads us to the following general advice: all functions taking string
+arguments should take {\it const wxString\&} (this makes assignment to the
+strings inside the function faster because of
+\helpref{reference counting}{wxstringrefcount}) and all functions returning
+strings should return {\it wxString} - this makes it safe to return local
+variables.
+
+\subsection{Other string related functions and classes}
+
+As most programs use character strings, the standard C library provides quite a
+few functions to work with them. Unfortunately, some of them have rather
+counter-intuitive behaviour (like strncpy() which doesn't always terminate the resulting
+string with a NULL) and are in general not very safe (passing NULL to them will
+probably lead to program crash). Moreover, some very useful functions are not
+standard at all. This is why in addition to all wxString functions, there are
+also a few global string functions which try to correct these problems:
+\helpref{IsEmpty()}{IsEmpty} verifies whether the string is empty (returning
+TRUE for NULL pointers), \helpref{Strlen()}{Strlen} also handles NULLs correctly
+and returns 0 for them and \helpref{Stricmp()}{Stricmp} is just a
+platform-independent version of case-insensitive string comparison function
+known either as stricmp() or strcasecmp() on different platforms.
+
+The {\tt <wx/string.h>} header also defines \helpref{wxSnprintf}{wxsnprintf}
+and \helpref{wxVsnprintf}{wxvsnprintf} functions which should be used instead
+of the inherently dangerous standard {\tt sprintf()} and which use {\tt
+snprintf()} instead which does buffer size checks whenever possible. Of
+course, you may also use \helpref{wxString::Printf}{wxstringprintf} which is
+also safe.
+
+There is another class which might be useful when working with wxString:
+\helpref{wxStringTokenizer}{wxstringtokenizer}. It is helpful when a string must
+be broken into tokens and replaces the standard C library {\it
+strtok()} function.
+
+And the very last string-related class is \helpref{wxArrayString}{wxarraystring}: it
+is just a version of the "template" dynamic array class which is specialized to work
+with strings. Please note that this class is specially optimized (using its
+knowledge of the internal structure of wxString) for storing strings and so it is
+vastly better from a performance point of view than a wxObjectArray of wxStrings.
+
+\subsection{Reference counting and why you shouldn't care about it}\label{wxstringrefcount}
+
+wxString objects use a technique known as {\it copy on write} (COW). This means
+that when a string is assigned to another, no copying really takes place: only
+the reference count on the shared string data is incremented and both strings
+share the same data.
+
+But as soon as one of the two (or more) strings is modified, the data has to be
+copied because the changes to one of the strings shouldn't be seen in the
+others. As data copying only happens when the string is written to, this is
+known as COW.
+
+What is important to understand is that all this happens absolutely
+transparently to the class users and that whether a string is shared or not is
+not seen from the outside of the class - in any case, the result of any
+operation on it is the same.
+
+Probably the unique case when you might want to think about reference
+counting is when a string character is taken from a string which is not a
+constant (or a constant reference). In this case, due to C++ rules, the
+"read-only" {\it operator[]} (which is the same as
+\helpref{GetChar()}{wxstringgetchar}) cannot be chosen and the "read/write"
+{\it operator[]} (the same as
+\helpref{GetWritableChar()}{wxstringgetwritablechar}) is used instead. As the
+call to this operator may modify the string, its data is unshared (COW is done)
+and so if the string was really shared there is some performance loss (both in
+terms of speed and memory consumption). In the rare cases when this may be
+important, you might prefer using \helpref{GetChar()}{wxstringgetchar} instead
+of the array subscript operator for this reasons. Please note that
+\helpref{at()}{wxstringat} method has the same problem as the subscript operator in
+this situation and so using it is not really better. Also note that if all
+string arguments to your functions are passed as {\it const wxString\&} (see the
+section \helpref{Some advice}{wxstringadvices}) this situation will almost
+never arise because for constant references the correct operator is called automatically.
+
+\subsection{Tuning wxString for your application}\label{wxstringtuning}
+
+\normalbox{{\bf Note:} this section is strictly about performance issues and is
+absolutely not necessary to read for using wxString class. Please skip it unless
+you feel familiar with profilers and relative tools. If you do read it, please
+also read the preceding section about
+\helpref{reference counting}{wxstringrefcount}.}
+
+For the performance reasons wxString doesn't allocate exactly the amount of
+memory needed for each string. Instead, it adds a small amount of space to each
+allocated block which allows it to not reallocate memory (a relatively
+expensive operation) too often as when, for example, a string is constructed by
+subsequently adding one character at a time to it, as for example in:
+
+\begin{verbatim}
+// delete all vowels from the string
+wxString DeleteAllVowels(const wxString& original)
+{
+ wxString result;
+
+ size_t len = original.length();
+ for ( size_t n = 0; n < len; n++ )
+ {
+ if ( strchr("aeuio", tolower(original[n])) == NULL )
+ result += original[n];
+ }
+
+ return result;
+}
+\end{verbatim}
+
+This is quite a common situation and not allocating extra memory at all would
+lead to very bad performance in this case because there would be as many memory
+(re)allocations as there are consonants in the original string. Allocating too
+much extra memory would help to improve the speed in this situation, but due to
+a great number of wxString objects typically used in a program would also
+increase the memory consumption too much.
+
+The very best solution in precisely this case would be to use
+\helpref{Alloc()}{wxstringalloc} function to preallocate, for example, len bytes
+from the beginning - this will lead to exactly one memory allocation being
+performed (because the result is at most as long as the original string).
+
+However, using Alloc() is tedious and so wxString tries to do its best. The
+default algorithm assumes that memory allocation is done in granularity of at
+least 16 bytes (which is the case on almost all of wide-spread platforms) and so
+nothing is lost if the amount of memory to allocate is rounded up to the next
+multiple of 16. Like this, no memory is lost and 15 iterations from 16 in the
+example above won't allocate memory but use the already allocated pool.
+
+The default approach is quite conservative. Allocating more memory may bring
+important performance benefits for programs using (relatively) few very long
+strings. The amount of memory allocated is configured by the setting of {\it
+EXTRA\_ALLOC} in the file string.cpp during compilation (be sure to understand
+why its default value is what it is before modifying it!). You may try setting
+it to greater amount (say twice nLen) or to 0 (to see performance degradation
+which will follow) and analyse the impact of it on your program. If you do it,
+you will probably find it helpful to also define WXSTRING\_STATISTICS symbol
+which tells the wxString class to collect performance statistics and to show
+them on stderr on program termination. This will show you the average length of
+strings your program manipulates, their average initial length and also the
+percent of times when memory wasn't reallocated when string concatenation was
+done but the already preallocated memory was used (this value should be about
+98\% for the default allocation policy, if it is less than 90\% you should
+really consider fine tuning wxString for your application).
+
+It goes without saying that a profiler should be used to measure the precise
+difference the change to EXTRA\_ALLOC makes to your program.
projects / wxWidgets.git / blobdiff