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[wxWidgets.git] / docs / doxygen / overviews / string.h

/////////////////////////////////////////////////////////////////////////////
// Name:        string
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/*!

 @page string_overview wxString overview

 Classes: #wxString, #wxArrayString, #wxStringTokenizer
 #Introduction
 @ref otherstringclasses_overview
 @ref stringadvices_overview
 @ref relatedtostring_overview
 @ref stringrefcount_overview
 @ref stringtuning_overview


 @section introductiontowxstring Introduction

 wxString is a class which represents a character string of arbitrary length (limited by
 @e MAX_INT which is usually 2147483647 on 32 bit machines) and containing
 arbitrary characters. The ASCII NUL character is allowed, but be aware that
 in the current string implementation some methods might not work correctly
 in this case.
 wxString works with both ASCII (traditional, 7 or 8 bit, characters) as well as
 Unicode (wide characters) strings.
 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 #Printf() and stream-like
 insertion functions as well as much more - see #wxString
 for a list of all functions.

 @section otherstringclasses 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:


  @b 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 @e char * pointer, sing @ref stringrefcount_overview) and speed.
 It also provides performance @ref stringtuning_overview
 which may be enabled to fine tune the memory allocation strategy for your
 particular application - and the gain might be quite big.
  @b Compatibility This class tries to combine almost full compatibility
 with the old wxWidgets 1.xx wxString class, some reminiscence to MFC CString
 class and 90% of the functionality of std::string class.
  @b 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,
 #AfterFirst,
 #BeforeLast, #operator
 or #Printf. Of course, all the standard string
 operations are supported as well.
  @b Unicode wxString is Unicode friendly: it allows to easily convert
 to and from ANSI and Unicode strings in any build mode (see the
 @ref unicode_overview for more details) and maps to either
 @c string or @c wstring transparently depending on the current mode.
  @b Used by wxWidgets And, of course, this class is used everywhere
 inside wxWidgets 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 wxWidgets.


 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(), #Len() or
 #Length() 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 wxWidgets 1.xx way. So the
 question is: which one is better to use? And the answer is that:
 @b 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 wxWidgets and other programs (by just typedefing wxString as std::string
 when used outside wxWidgets) and by staying compatible with future versions of
 wxWidgets 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 wxWidgets 1.xx variants
 which are deprecated and may disappear in future versions.

 @section wxstringadvices Some advice about using wxString

 Probably the main trap with using this class is the implicit conversion operator to
 @e const char *. It is advised that you use #c_str()
 instead to clearly indicate when the conversion is done. Specifically, the
 danger of this implicit conversion may be seen in the following code fragment:

 @code
 // 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;
 }
 @endcode

 There are two nasty bugs in these three lines. First of them is in the call to the
 @e printf() function. Although the implicit conversion to C strings is applied
 automatically by the compiler in the case of

 @code
 puts(output);
 @endcode

 because the argument of @e puts() is known to be of the type @e const char *,
 this is @b not done for @e 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
 #c_str(): just replace this line with

 @code
 printf("Hello, %s!\n", output.c_str());
 @endcode

 The second bug is that returning @e 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 @e const wxString (this makes assignment to the
 strings inside the function faster because of
 @ref stringrefcount_overview) and all functions returning
 strings should return @e wxString - this makes it safe to return local
 variables.

 @section relatedtostring 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: #wxIsEmpty() verifies whether the string
 is empty (returning @true for @NULL pointers),
 #wxStrlen() also handles @NULLs correctly and returns 0 for
 them and #wxStricmp() is just a platform-independent
 version of case-insensitive string comparison function known either as
 stricmp() or strcasecmp() on different platforms.
 The @c wx/string.h header also defines #wxSnprintf
 and #wxVsnprintf functions which should be used instead
 of the inherently dangerous standard @c sprintf() and which use @c snprintf() instead which does buffer size checks whenever possible. Of
 course, you may also use wxString::Printf which is
 also safe.
 There is another class which might be useful when working with wxString:
 #wxStringTokenizer. It is helpful when a string must
 be broken into tokens and replaces the standard C library @e strtok() function.
 And the very last string-related class is #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.

 @section wxstringrefcount Reference counting and why you shouldn't care about it

 All considerations for wxObject-derived @ref trefcount_overview objects
 are valid also for wxString, even if it does not derive from wxObject.
 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" @e operator[] (which is the same as
 #GetChar()) cannot be chosen and the "read/write"
 @e operator[] (the same as
 #GetWritableChar()) 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 #GetChar() instead
 of the array subscript operator for this reasons. Please note that
 #at() 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 @e const wxString (see the
 section @ref stringadvices_overview) this situation will almost
 never arise because for constant references the correct operator is called automatically.

 @section wxstringtuning Tuning wxString for your application


 @b 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
 @ref stringrefcount_overview.

 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:

 @code
 // 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;
 }
 @endcode

 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
 #Alloc() 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 @e 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.

 */