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Last doxygen topic overview cleanup assigned to myself.

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///////////////////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////////////////
// Name: string // Name: string.h
// Purpose: topic overview // Purpose: topic overview
// Author: wxWidgets team // Author: wxWidgets team
// RCS-ID: $Id$ // RCS-ID: $Id$
@@ -8,255 +8,271 @@
/*! /*!
@page string_overview wxString overview @page overview_string wxString Overview
Classes: #wxString, #wxArrayString, #wxStringTokenizer Classes: wxString, wxArrayString, wxStringTokenizer
#Introduction
@ref otherstringclasses_overview @li @ref overview_string_intro
@ref stringadvices_overview @li @ref overview_string_comparison
@ref relatedtostring_overview @li @ref overview_string_advice
@ref stringrefcount_overview @li @ref overview_string_related
@ref stringtuning_overview @li @ref overview_string_refcount
@li @ref overview_string_tuning
@section introductiontowxstring Introduction <hr>
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 @section overview_string_intro Introduction
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 wxString is a class which represents a character string of arbitrary length
which may be enabled to fine tune the memory allocation strategy for your (limited by @c MAX_INT which is usually 2147483647 on 32 bit machines) and
particular application - and the gain might be quite big. containing arbitrary characters. The ASCII NUL character is allowed, but be
@b Compatibility This class tries to combine almost full compatibility aware that in the current string implementation some methods might not work
with the old wxWidgets 1.xx wxString class, some reminiscence to MFC CString correctly in this case.
class and 90% of the functionality of std::string class.
@b Rich set of functions Some of the functions present in wxString are wxString works with both ASCII (traditional, 7 or 8 bit, characters) as well as
very useful but don't exist in most of other string classes: for example, Unicode (wide characters) strings.
#AfterFirst,
#BeforeLast, #operator This class has all the standard operations you can expect to find in a string
or #Printf. Of course, all the standard string class: dynamic memory management (string extends to accommodate new
operations are supported as well. characters), construction from other strings, C strings and characters,
@b Unicode wxString is Unicode friendly: it allows to easily convert assignment operators, access to individual characters, string concatenation and
to and from ANSI and Unicode strings in any build mode (see the comparison, substring extraction, case conversion, trimming and padding (with
@ref unicode_overview for more details) and maps to either spaces), searching and replacing and both C-like @c printf (wxString::Printf)
@c string or @c wstring transparently depending on the current mode. and stream-like insertion functions as well as much more - see wxString for a
@b Used by wxWidgets And, of course, this class is used everywhere list of all functions.
@section overview_string_comparison Comparison 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:
@li <b>Efficiency:</b> 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
<tt>char*</tt> pointer, see @ref overview_string_refcount
"reference counting") and speed. It also provides performance
@ref overview_string_tuning "statistics gathering code" which may be
enabled to fine tune the memory allocation strategy for your particular
application - and the gain might be quite big.
@li <b>Compatibility:</b> 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.
@li <b>Rich set of functions:</b> Some of the functions present in wxString are very
useful but don't exist in most of other string classes: for example,
wxString::AfterFirst, wxString::BeforeLast, wxString::operators or
wxString::Printf. Of course, all the standard string operations are
supported as well.
@li <b>Unicode wxString is Unicode friendly:</b> it allows to easily convert to
and from ANSI and Unicode strings in any build mode (see the
@ref overview_unicode "unicode overview" for more details) and maps to
either @c string or @c wstring transparently depending on the current mode.
@li <b>Used by wxWidgets:</b> And, of course, this class is used everywhere
inside wxWidgets so there is no performance loss which would result from inside wxWidgets so there is no performance loss which would result from
conversions of objects of any other string class (including std::string) to conversions of objects of any other string class (including std::string) to
wxString internally by wxWidgets. 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 @c length(),
wxString::Len() or wxString::Length() may be used. The first function, as
almost all the other functions in lowercase, is std::string compatible. The
second one is the "native" wxString version and the last one is the wxWidgets
1.xx way.
However, there are several problems as well. The most important one is probably So which is better to use? The usage of the std::string compatible functions is
that there are often several functions to do exactly the same thing: for strongly advised! It will both make your code more familiar to other C++
example, to get the length of the string either one of programmers (who are supposed to have knowledge of std::string but not of
length(), #Len() or wxString), let you reuse the same code in both wxWidgets and other programs (by
#Length() may be used. The first function, as almost just typedefing wxString as std::string when used outside wxWidgets) and by
all the other functions in lowercase, is std::string compatible. The second one staying compatible with future versions of wxWidgets which will probably start
is "native" wxString version and the last one is wxWidgets 1.xx way. So the using std::string sooner or later too.
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 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.
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 @section overview_string_advice Advice About Using wxString
// this function converts the input string to uppercase, output it to the screen
// and returns the result Probably the main trap with using this class is the implicit conversion
const char *SayHELLO(const wxString& input) operator to <tt>const char*</tt>. It is advised that you use wxString::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(); wxString output = input.Upper();
printf("Hello, %s!\n", output); printf("Hello, %s!\n", output);
return output; return output;
} }
@endcode @endcode
There are two nasty bugs in these three lines. First of them is in the call to the There are two nasty bugs in these three lines. The first is in the call to the
@e printf() function. Although the implicit conversion to C strings is applied @c printf() function. Although the implicit conversion to C strings is applied
automatically by the compiler in the case of automatically by the compiler in the case of
@code @code
puts(output); puts(output);
@endcode @endcode
because the argument of @e puts() is known to be of the type @e const char *, because the argument of @c puts() is known to be of the type
this is @b not done for @e printf() which is a function with variable <tt>const char*</tt>, this is @b not done for @c printf() which is a function
number of arguments (and whose arguments are of unknown types). So this call may with variable number of arguments (and whose arguments are of unknown types).
do anything at all (including displaying the correct string on screen), although So this call may do any number of things (including displaying the correct
the most likely result is a program crash. The solution is to use string on screen), although the most likely result is a program crash. The
#c_str(): just replace this line with solution is to use wxString::c_str(). Just replace this line with this:
@code @code
printf("Hello, %s!\n", output.c_str()); printf("Hello, %s!\n", output.c_str());
@endcode @endcode
The second bug is that returning @e output doesn't work. The implicit cast is The second bug is that returning @c output doesn't work. The implicit cast is
used again, so the code compiles, but as it returns a pointer to a buffer 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, 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: its contents are completely arbitrary. The solution to this problem is also
just make the function return wxString instead of a C string. 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 This leads us to the following general advice: all functions taking string
arguments should take <tt>const wxString</tt> (this makes assignment to the
As most programs use character strings, the standard C library provides quite strings inside the function faster because of
a few functions to work with them. Unfortunately, some of them have rather @ref overview_string_refcount "reference counting") and all functions returning
counter-intuitive behaviour (like strncpy() which doesn't always terminate the strings should return wxString - this makes it safe to return local variables.
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 @section overview_string_related String Related Functions and Classes
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 As most programs use character strings, the standard C library provides quite
memory needed for each string. Instead, it adds a small amount of space to each a few functions to work with them. Unfortunately, some of them have rather
allocated block which allows it to not reallocate memory (a relatively counter-intuitive behaviour (like @c strncpy() which doesn't always terminate
expensive operation) too often as when, for example, a string is constructed by the resulting string with a @NULL) and are in general not very safe (passing
subsequently adding one character at a time to it, as for example in: @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 @c stricmp() or
@c strcasecmp() on different platforms.
@code The <tt>@<wx/string.h@></tt> header also defines wxSnprintf and wxVsnprintf
// delete all vowels from the string functions which should be used instead of the inherently dangerous standard
wxString DeleteAllVowels(const wxString& original) @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 @c 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 overview_string_refcount Reference Counting and Why You Shouldn't Care
All considerations for wxObject-derived
@ref overview_refcount "reference counted" 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"
@c operator[] (which is the same as wxString::GetChar()) cannot be chosen and
the "read/write" @c operator[] (the same as wxString::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 wxString::GetChar()
instead of the array subscript operator for this reasons. Please note that
wxString::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 <tt>const wxString</tt> (see the
@ref overview_string_advice section) this situation will almost never arise
because for constant references the correct operator is called automatically.
@section overview_string_tuning Tuning wxString for Your Application
@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 overview_string_refcount "reference counting".
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; wxString result;
size_t len = original.length(); size_t len = original.length();
for ( size_t n = 0; n len; n++ ) for ( size_t n = 0; n < len; n++ )
{ {
if ( strchr("aeuio", tolower(original[n])) == @NULL ) if ( strchr("aeuio", tolower(original[n])) == NULL )
result += original[n]; result += original[n];
} }
return result; return result;
} }
@endcode @endcode
This is quite a common situation and not allocating extra memory at all would 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 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 (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 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 a great number of wxString objects typically used in a program would also
increase the memory consumption too much. 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.
*/ The very best solution in precisely this case would be to use wxString::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 wxString::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
@c 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 @c 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 @c EXTRA_ALLOC makes to your program.
*/