Classes: wxString, wxArrayString, wxStringTokenizer
Introduction
Comparison of wxString to other string classes
Some advice about using wxString
Other string related functions and classes
Reference counting and why you shouldn't care about it
Tuning wxString for your application
wxString is a class which represents a character string of arbitrary length (limited by 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.
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:
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:
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.
Probably the main trap with using this class is the implicit conversion operator to 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:
// 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;
}
There are two nasty bugs in these three lines. First of them is in the call to the
printf() function. Although the implicit conversion to C strings is applied
automatically by the compiler in the case of
puts(output);
because the argument of puts() is known to be of the type const char *,
this is not done for 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
printf("Hello, %s!\n", output.c_str());
The second bug is that returning 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 const wxString& (this makes assignment to the strings inside the function faster because of reference counting) and all functions returning strings should return wxString - this makes it safe to return local variables.
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 <wx/string.h> header also defines wxSnprintf and wxVsnprintf functions which should be used instead of the inherently dangerous standard sprintf() and which use 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 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.
wxString objects use a technique known as 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" operator[] (which is the same as GetChar()) cannot be chosen and the "read/write" 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 const wxString& (see the section Some advice) this situation will almost never arise because for constant references the correct operator is called automatically.
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:
// 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;
}
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 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.