docs/doxygen/overviews/string.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        string.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /**
  10
  11 @page overview_string wxString Overview
  12
  13 Classes: wxString, wxArrayString, wxStringTokenizer
  14
  15 @li @ref overview_string_intro
  16 @li @ref overview_string_comparison
  17 @li @ref overview_string_advice
  18 @li @ref overview_string_related
  19 @li @ref overview_string_tuning
  20
  21
  22 <hr>
  23
  24
  25 @section overview_string_intro Introduction
  26
  27 wxString is a class which represents a character string of arbitrary length and
  28 containing arbitrary characters. The ASCII NUL character is allowed, but be
  29 aware that in the current string implementation some methods might not work
  30 correctly in this case.
  31
  32 Since wxWidgets 3.0 wxString internally uses UCS-2 (basically 2-byte per
  33 character wchar_t) under Windows and UTF-8 under Unix, Linux and
  34 OS X to store its content. Much work has been done to make
  35 existing code using ANSI string literals work as before.
  36
  37 This class has all the standard operations you can expect to find in a string
  38 class: dynamic memory management (string extends to accommodate new
  39 characters), construction from other strings, C strings, wide character C strings
  40 and characters, assignment operators, access to individual characters, string
  41 concatenation and comparison, substring extraction, case conversion, trimming and padding (with
  42 spaces), searching and replacing and both C-like @c printf (wxString::Printf)
  43 and stream-like insertion functions as well as much more - see wxString for a
  44 list of all functions.
  45
  46
  47 @section overview_string_comparison Comparison to Other String Classes
  48
  49 The advantages of using a special string class instead of working directly with
  50 C strings are so obvious that there is a huge number of such classes available.
  51 The most important advantage is the need to always remember to allocate/free
  52 memory for C strings; working with fixed size buffers almost inevitably leads
  53 to buffer overflows. At last, C++ has a standard string class (std::string). So
  54 why the need for wxString? There are several advantages:
  55
  56 @li <b>Efficiency:</b> Since wxWidgets 3.0 wxString uses std::string (UTF8
  57     mode under Linux, Unix and OS X) or std::wstring (MSW) internally by
  58     default to store its constent. wxString will therefore inherit the
  59     performance characteristics from std::string.
  60 @li <b>Compatibility:</b> This class tries to combine almost full compatibility
  61     with the old wxWidgets 1.xx wxString class, some reminiscence to MFC
  62     CString class and 90% of the functionality of std::string class.
  63 @li <b>Rich set of functions:</b> Some of the functions present in wxString are very
  64     useful but don't exist in most of other string classes: for example,
  65     wxString::AfterFirst, wxString::BeforeLast, wxString::operators or
  66     wxString::Printf. Of course, all the standard string operations are
  67     supported as well.
  68 @li <b>Unicode wxString is Unicode friendly:</b> it allows to easily convert to
  69     and from ANSI and Unicode strings (see the @ref overview_unicode "unicode overview"
  70     for more details) and maps to @c wstring transparently.
  71 @li <b>Used by wxWidgets:</b> And, of course, this class is used everywhere
  72     inside wxWidgets so there is no performance loss which would result from
  73     conversions of objects of any other string class (including std::string) to
  74     wxString internally by wxWidgets.
  75
  76 However, there are several problems as well. The most important one is probably
  77 that there are often several functions to do exactly the same thing: for
  78 example, to get the length of the string either one of wxString::length(),
  79 wxString::Len() or wxString::Length() may be used. The first function, as
  80 almost all the other functions in lowercase, is std::string compatible. The
  81 second one is the "native" wxString version and the last one is the wxWidgets
  82 1.xx way.
  83
  84 So which is better to use? The usage of the std::string compatible functions is
  85 strongly advised! It will both make your code more familiar to other C++
  86 programmers (who are supposed to have knowledge of std::string but not of
  87 wxString), let you reuse the same code in both wxWidgets and other programs (by
  88 just typedefing wxString as std::string when used outside wxWidgets) and by
  89 staying compatible with future versions of wxWidgets which will probably start
  90 using std::string sooner or later too.
  91
  92 In the situations where there is no corresponding std::string function, please
  93 try to use the new wxString methods and not the old wxWidgets 1.xx variants
  94 which are deprecated and may disappear in future versions.
  95
  96
  97 @section overview_string_advice Advice About Using wxString
  98
  99 Probably the main trap with using this class is the implicit conversion
 100 operator to <tt>const char*</tt>. It is advised that you use wxString::c_str()
 101 instead to clearly indicate when the conversion is done. Specifically, the
 102 danger of this implicit conversion may be seen in the following code fragment:
 103
 104 @code
 105 // this function converts the input string to uppercase,
 106 // output it to the screen and returns the result
 107 const char *SayHELLO(const wxString& input)
 108 {
 109     wxString output = input.Upper();
 110     printf("Hello, %s!\n", output);
 111     return output;
 112 }
 113 @endcode
 114
 115 There are two nasty bugs in these three lines. The first is in the call to the
 116 @c printf() function. Although the implicit conversion to C strings is applied
 117 automatically by the compiler in the case of
 118
 119 @code
 120 puts(output);
 121 @endcode
 122
 123 because the argument of @c puts() is known to be of the type
 124 <tt>const char*</tt>, this is @b not done for @c printf() which is a function
 125 with variable number of arguments (and whose arguments are of unknown types).
 126 So this call may do any number of things (including displaying the correct
 127 string on screen), although the most likely result is a program crash. The
 128 solution is to use wxString::c_str(). Just replace this line with this:
 129
 130 @code
 131 printf("Hello, %s!\n", output.c_str());
 132 @endcode
 133
 134 The second bug is that returning @c output doesn't work. The implicit cast is
 135 used again, so the code compiles, but as it returns a pointer to a buffer
 136 belonging to a local variable which is deleted as soon as the function exits,
 137 its contents are completely arbitrary. The solution to this problem is also
 138 easy, just make the function return wxString instead of a C string.
 139
 140 This leads us to the following general advice: all functions taking string
 141 arguments should take <tt>const wxString</tt> (this makes assignment to the
 142 strings inside the function faster) and all functions returning strings
 143 should return wxString - this makes it safe to return local variables.
 144
 145
 146 @section overview_string_related String Related Functions and Classes
 147
 148 As most programs use character strings, the standard C library provides quite
 149 a few functions to work with them. Unfortunately, some of them have rather
 150 counter-intuitive behaviour (like @c strncpy() which doesn't always terminate
 151 the resulting string with a @NULL) and are in general not very safe (passing
 152 @NULL to them will probably lead to program crash). Moreover, some very useful
 153 functions are not standard at all. This is why in addition to all wxString
 154 functions, there are also a few global string functions which try to correct
 155 these problems: wxIsEmpty() verifies whether the string is empty (returning
 156 @true for @NULL pointers), wxStrlen() also handles @NULL correctly and returns
 157 0 for them and wxStricmp() is just a platform-independent version of
 158 case-insensitive string comparison function known either as @c stricmp() or
 159 @c strcasecmp() on different platforms.
 160
 161 The <tt>@<wx/string.h@></tt> header also defines wxSnprintf and wxVsnprintf
 162 functions which should be used instead of the inherently dangerous standard
 163 @c sprintf() and which use @c snprintf() instead which does buffer size checks
 164 whenever possible. Of course, you may also use wxString::Printf which is also
 165 safe.
 166
 167 There is another class which might be useful when working with wxString:
 168 wxStringTokenizer. It is helpful when a string must be broken into tokens and
 169 replaces the standard C library @c strtok() function.
 170
 171 And the very last string-related class is wxArrayString: it is just a version
 172 of the "template" dynamic array class which is specialized to work with
 173 strings. Please note that this class is specially optimized (using its
 174 knowledge of the internal structure of wxString) for storing strings and so it
 175 is vastly better from a performance point of view than a wxObjectArray of
 176 wxStrings.
 177
 178
 179 @section overview_string_tuning Tuning wxString for Your Application
 180
 181 @note This section is strictly about performance issues and is absolutely not
 182 necessary to read for using wxString class. Please skip it unless you feel
 183 familiar with profilers and relative tools.
 184
 185 For the performance reasons wxString doesn't allocate exactly the amount of
 186 memory needed for each string. Instead, it adds a small amount of space to each
 187 allocated block which allows it to not reallocate memory (a relatively
 188 expensive operation) too often as when, for example, a string is constructed by
 189 subsequently adding one character at a time to it, as for example in:
 190
 191 @code
 192 // delete all vowels from the string
 193 wxString DeleteAllVowels(const wxString& original)
 194 {
 195     wxString vowels( "aeuioAEIOU" );
 196     wxString result;
 197     wxString::const_iterator i;
 198     for ( i = original.begin(); i != original.end(); ++i )
 199     {
 200         if (vowels.Find( *i ) == wxNOT_FOUND)
 201             result += *i;
 202     }
 203
 204     return result;
 205 }
 206 @endcode
 207
 208 This is quite a common situation and not allocating extra memory at all would
 209 lead to very bad performance in this case because there would be as many memory
 210 (re)allocations as there are consonants in the original string. Allocating too
 211 much extra memory would help to improve the speed in this situation, but due to
 212 a great number of wxString objects typically used in a program would also
 213 increase the memory consumption too much.
 214
 215 The very best solution in precisely this case would be to use wxString::Alloc()
 216 function to preallocate, for example, len bytes from the beginning - this will
 217 lead to exactly one memory allocation being performed (because the result is at
 218 most as long as the original string).
 219
 220 However, using wxString::Alloc() is tedious and so wxString tries to do its
 221 best. The default algorithm assumes that memory allocation is done in
 222 granularity of at least 16 bytes (which is the case on almost all of
 223 wide-spread platforms) and so nothing is lost if the amount of memory to
 224 allocate is rounded up to the next multiple of 16. Like this, no memory is lost
 225 and 15 iterations from 16 in the example above won't allocate memory but use
 226 the already allocated pool.
 227
 228 The default approach is quite conservative. Allocating more memory may bring
 229 important performance benefits for programs using (relatively) few very long
 230 strings. The amount of memory allocated is configured by the setting of
 231 @c EXTRA_ALLOC in the file string.cpp during compilation (be sure to understand
 232 why its default value is what it is before modifying it!). You may try setting
 233 it to greater amount (say twice nLen) or to 0 (to see performance degradation
 234 which will follow) and analyse the impact of it on your program. If you do it,
 235 you will probably find it helpful to also define @c WXSTRING_STATISTICS symbol
 236 which tells the wxString class to collect performance statistics and to show
 237 them on stderr on program termination. This will show you the average length of
 238 strings your program manipulates, their average initial length and also the
 239 percent of times when memory wasn't reallocated when string concatenation was
 240 done but the already preallocated memory was used (this value should be about
 241 98% for the default allocation policy, if it is less than 90% you should
 242 really consider fine tuning wxString for your application).
 243
 244 It goes without saying that a profiler should be used to measure the precise
 245 difference the change to @c EXTRA_ALLOC makes to your program.
 246
 247 */
 248