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git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@1800 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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Julian Smart
1999-02-26 17:29:39 +00:00
parent 3a2629c0a2
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73
docs/latex/wx/tstring.tex
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@@ -4,52 +4,51 @@ Classes: \helpref{wxString}{wxstring}, \helpref{wxArrayString}{wxarray}, \helpre
\subsection{Introduction}
wxString is a class which represents a character string of arbitrary (limited by
{\it MAX\_INT} which is usually 2147483647 on 32 bit machines) length and containing
arbitrary characters (i.e. ASCII NUL character is allowed, although care should be
taken when passing strings containing it to other functions).
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,
however support for UNICODE (16 but characters) is planned for the next one.
but support for UNICODE (16 but characters) is planned for the next one.
This class has all standard operations you can expect to find in a string class:
This class has all the standard operations you can expect to find in a string class:
dynamic memory management (string extends to accomodate new characters),
construction from other strings, C strings and characters, assignment operators,
access to separate characters, string concatenation and comparison, substring
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 else - see \helpref{wxString}{wxstring}
for the list of all functions.
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 (the most imoprtant being, of course, the need to always
remember to allocate/free memory for C strings unless the programmer prefers
working with fixed size buffers which almost certainly leads to the dreaded
buffer overflows) that there is a huge number of such classes available and now,
finally, C++ even has one (std::string) in standard. Why use wxString then?
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 place as {\it
char *} pointer, \helpref{reference counting}{wxstringrefcount}) and speed.
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 functionality of std::string class.
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 construction from
an UNICODE string, but in the next one it will be capable of also storing its
\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
@@ -73,15 +72,15 @@ 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 when there is no correspondinw std::string function, please
In the situations where there is no correspondinw std::string function, please
try to use the new wxString methods and not the old wxWindows 1.xx variants
which are deprecated and risk to disappear in future versions.
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 of it to clearly indicate when the conversion is done. Specifically, the
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}
@@ -120,7 +119,7 @@ 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 C string.
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
@@ -131,13 +130,13 @@ variables.
\subsection{Other string related functions and classes}
As any program operates with character strings, the standard C library provides quite a
few of functions to work with them. Unfortunately, some of them have rather non
intuitive behaviour (like strncpy() which doesn't always terminate the resulting
string with a NUL) and are in general not very safe (passing NULL to them will
probably lead to program crash). Moreover, some of very useful functions are not
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 of global string functions which try to correct these problems:
also a few global string functions which try to correct these problems:
\helpref{IsEmpty()}{wxstringisempty} verifies whether the string is empty (returning
TRUE for NULL pointers), \helpref{Strlen()}{wxstringstrlen} also handles NULLs correctly
and returns 0 for them and \helpref{Stricmp()}{wxstringstricmp} is just a
@@ -149,17 +148,17 @@ There is another class which might be useful when working with wxString:
be broken into tokens and replaces the standard C library {\it
strtok()} function.
And the very last string related class is \helpref{wxArrayString}{wxarray}: it
And the very last string-related class is \helpref{wxArrayString}{wxarray}: 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 (it uses its
knowledge of internal structure of wxString) for storing strings and so it is
vastly better from performance point of view than wxObjectArray of wxString.
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 increased and both strings
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
@@ -183,8 +182,8 @@ 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 array subscript operator for this reasons. Please note that
\helpref{at()}{wxstringat} method has the same problem as subscript operator in
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
@@ -200,7 +199,7 @@ also read the preceding section about
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 (this is a relatively
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: