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Docs: Review and improve QString documentation

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Changed section titles to sentence case.
Added or removed commas as required.
Simplified language were appropriate.
Varied terms to improve engagement.
Corrected resultant text to within the 80 character
width, so there will be whitespace change warnings.

Fixes: QTBUG-119553
Pick-to: 6.6
Change-Id: I5f40605fde4639a6dfcdb3816f32ad7599572fae
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mats Honkamaa <mats.honkamaa@qt.io>
(cherry picked from commit ef01f32388ad2eb69aa58879b56a7891a492619b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Nicholas Bennett 2023-12-01 15:29:31 +02:00 committed by Qt Cherry-pick Bot
parent fe2a4baa49
commit ebccd49dbb
1 changed files with 121 additions and 115 deletions
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src/corelib/text/qstring.cpp
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@ -1718,7 +1718,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
QString stores a string of 16-bit \l{QChar}s, where each QChar QString stores a string of 16-bit \l{QChar}s, where each QChar
corresponds to one UTF-16 code unit. (Unicode characters corresponds to one UTF-16 code unit. (Unicode characters
with code values above 65535 are stored using surrogate pairs, with code values above 65535 are stored using surrogate pairs,
i.e., two consecutive \l{QChar}s.) that is, two consecutive \l{QChar}s.)
\l{Unicode} is an international standard that supports most of the \l{Unicode} is an international standard that supports most of the
writing systems in use today. It is a superset of US-ASCII (ANSI writing systems in use today. It is a superset of US-ASCII (ANSI
@ -1734,17 +1734,15 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
store raw bytes and traditional 8-bit '\\0'-terminated strings. store raw bytes and traditional 8-bit '\\0'-terminated strings.
For most purposes, QString is the class you want to use. It is For most purposes, QString is the class you want to use. It is
used throughout the Qt API, and the Unicode support ensures that used throughout the Qt API, and the Unicode support ensures that
your applications will be easy to translate if you want to expand your applications are easy to translate if you want to expand
your application's market at some point. The two main cases where your application's market at some point. Two prominent cases
QByteArray is appropriate are when you need to store raw binary where QByteArray is appropriate are when you need to store raw
data, and when memory conservation is critical (like in embedded binary data, and when memory conservation is critical (like in
systems). embedded systems).
\tableofcontents \section1 Initializing a string
\section1 Initializing a String One way to initialize a QString is to pass a \c{const char
One way to initialize a QString is simply to pass a \c{const char
*} to its constructor. For example, the following code creates a *} to its constructor. For example, the following code creates a
QString of size 5 containing the data "Hello": QString of size 5 containing the data "Hello":
@ -1755,17 +1753,18 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
In all of the QString functions that take \c{const char *} In all of the QString functions that take \c{const char *}
parameters, the \c{const char *} is interpreted as a classic parameters, the \c{const char *} is interpreted as a classic
C-style '\\0'-terminated string encoded in UTF-8. It is legal for C-style \c{'\\0'}-terminated string. Except where the function's
the \c{const char *} parameter to be \nullptr. name overtly indicates some other encoding, such \c{const char *}
parameters are assumed to be encoded in UTF-8.
You can also provide string data as an array of \l{QChar}s: You can also provide string data as an array of \l{QChar}s:
\snippet qstring/main.cpp 1 \snippet qstring/main.cpp 1
QString makes a deep copy of the QChar data, so you can modify it QString makes a deep copy of the QChar data, so you can modify it
later without experiencing side effects. (If for performance later without experiencing side effects. You can avoid taking a
reasons you don't want to take a deep copy of the character data, deep copy of the character data by using QStringView or
use QString::fromRawData() instead.) QString::fromRawData() instead.
Another approach is to set the size of the string using resize() Another approach is to set the size of the string using resize()
and to initialize the data character per character. QString uses and to initialize the data character per character. QString uses
@ -1782,7 +1781,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
\snippet qstring/main.cpp 3 \snippet qstring/main.cpp 3
The at() function can be faster than \l operator[](), because it The at() function can be faster than \l operator[]() because it
never causes a \l{deep copy} to occur. Alternatively, use the never causes a \l{deep copy} to occur. Alternatively, use the
first(), last(), or sliced() functions to extract several characters first(), last(), or sliced() functions to extract several characters
at a time. at a time.
@ -1804,11 +1803,11 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
You can also pass string literals to functions that take QStrings You can also pass string literals to functions that take QStrings
as arguments, invoking the QString(const char *) as arguments, invoking the QString(const char *)
constructor. Similarly, you can pass a QString to a function that constructor. Similarly, you can pass a QString to a function that
takes a \c{const char *} argument using the \l qPrintable() macro takes a \c{const char *} argument using the \l qPrintable() macro,
which returns the given QString as a \c{const char *}. This is which returns the given QString as a \c{const char *}. This is
equivalent to calling <QString>.toLocal8Bit().constData(). equivalent to calling <QString>.toLocal8Bit().constData().
\section1 Manipulating String Data \section1 Manipulating string data
QString provides the following basic functions for modifying the QString provides the following basic functions for modifying the
character data: append(), prepend(), insert(), replace(), and character data: append(), prepend(), insert(), replace(), and
@ -1816,19 +1815,19 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
\snippet qstring/main.cpp 5 \snippet qstring/main.cpp 5
In the above example the replace() function's first two arguments are the In the above example, the replace() function's first two arguments are the
position from which to start replacing and the number of characters that position from which to start replacing and the number of characters that
should be replaced. should be replaced.
When data-modifying functions increase the size of the string, When data-modifying functions increase the size of the string,
they may lead to reallocation of memory for the QString object. When QString may reallocate the memory in which it holds its data. When
this happens, QString expands by more than it immediately needs so as this happens, QString expands by more than it immediately needs so as
to have space for further expansion without reallocation until the size to have space for further expansion without reallocation until the size
of the string has greatly increased. of the string has significantly increased.
The insert(), remove() and, when replacing a sub-string with one of The insert(), remove(), and, when replacing a sub-string with one of
different size, replace() functions can be slow (\l{linear time}) for different size, replace() functions can be slow (\l{linear time}) for
large strings, because they require moving many characters in the string large strings because they require moving many characters in the string
by at least one position in memory. by at least one position in memory.
If you are building a QString gradually and know in advance If you are building a QString gradually and know in advance
@ -1846,32 +1845,32 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
method of the QString is called. Accessing such an iterator or reference method of the QString is called. Accessing such an iterator or reference
after the call to a non-\c{const} method leads to undefined behavior. When after the call to a non-\c{const} method leads to undefined behavior. When
stability for iterator-like functionality is required, you should use stability for iterator-like functionality is required, you should use
indexes instead of iterators as they are not tied to QString's internal indexes instead of iterators, as they are not tied to QString's internal
state and thus do not get invalidated. state and thus do not get invalidated.
\note Due to \l{implicit sharing}, the first non-\c{const} operator or \note Due to \l{implicit sharing}, the first non-\c{const} operator or
function used on a given QString may cause it to, internally, perform a deep function used on a given QString may cause it to internally perform a deep
copy of its data. This invalidates all iterators over the string and copy of its data. This invalidates all iterators over the string and
references to individual characters within it. After the first non-\c{const} references to individual characters within it. Do not call non-const
operator, operations that modify QString may completely (in case of functions while keeping iterators. Accessing an iterator or reference
reallocation) or partially invalidate iterators and references, but other after it has been invalidated leads to undefined behavior. See the
methods (such as begin() or end()) will not. Accessing an iterator or \l{Implicit sharing iterator problem} section for more information.
reference after it has been invalidated leads to undefined behavior.
A frequent requirement is to remove whitespace characters from a A frequent requirement is to remove or simplify the spacing between
string ('\\n', '\\t', ' ', etc.). If you want to remove whitespace visible characters in a string. The characters that make up that spacing
from both ends of a QString, use the trimmed() function. If you are those for which \l {QChar::}{isSpace()} returns \c true, such as
want to remove whitespace from both ends and replace multiple the simple space \c{' '}, the horizontal tab \c{'\\t'} and the newline \c{'\\n'}.
consecutive whitespaces with a single space character within the To obtain a copy of a string leaving out any spacing from its start and end,
string, use simplified(). use \l trimmed(). To also replace each sequence of spacing characters within
the string with a simple space, \c{' '}, use \l simplified().
If you want to find all occurrences of a particular character or If you want to find all occurrences of a particular character or
substring in a QString, use the indexOf() or lastIndexOf() substring in a QString, use the indexOf() or lastIndexOf()
functions. The former searches forward starting from a given index functions.The former searches forward, the latter searches backward.
position, the latter searches backward. Both return the index Either can be told an index position from which to start their search.
position of the character or substring if they find it; otherwise, Each returns the index position of the character or substring if they
they return -1. For example, here is a typical loop that finds all find it; otherwise, they return -1. For example, here is a typical loop
occurrences of a particular substring: that finds all occurrences of a particular substring:
\snippet qstring/main.cpp 6 \snippet qstring/main.cpp 6
@ -1880,52 +1879,57 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
setNum() functions, the number() static functions, and the setNum() functions, the number() static functions, and the
toInt(), toDouble(), and similar functions. toInt(), toDouble(), and similar functions.
To get an upper- or lowercase version of a string use toUpper() or To get an uppercase or lowercase version of a string, use toUpper() or
toLower(). toLower().
Lists of strings are handled by the QStringList class. You can Lists of strings are handled by the QStringList class. You can
split a string into a list of strings using the split() function, split a string into a list of strings using the split() function,
and join a list of strings into a single string with an optional and join a list of strings into a single string with an optional
separator using QStringList::join(). You can obtain a list of separator using QStringList::join(). You can obtain a filtered list
strings from a string list that contain a particular substring or from a string list by selecting the entries in it that contain a
that match a particular QRegularExpression using the QStringList::filter() particular substring or match a particular QRegularExpression.
function. See QStringList::filter() for details.
\section1 Querying String Data \section1 Querying string data
If you want to see if a QString starts or ends with a particular To see if a QString starts or ends with a particular substring, use
substring use startsWith() or endsWith(). If you simply want to startsWith() or endsWith(). To check whether a QString contains a
check whether a QString contains a particular character or specific character or substring, use the contains() function. To
substring, use the contains() function. If you want to find out find out how many times a particular character or substring occurs
how many times a particular character or substring occurs in the in a string, use count().
string, use count().
To obtain a pointer to the actual character data, call data() or To obtain a pointer to the actual character data, call data() or
constData(). These functions return a pointer to the beginning of constData(). These functions return a pointer to the beginning of
the QChar data. The pointer is guaranteed to remain valid until a the QChar data. The pointer is guaranteed to remain valid until a
non-\c{const} function is called on the QString. non-\c{const} function is called on the QString.
\section2 Comparing Strings \section2 Comparing strings
QStrings can be compared using overloaded operators such as \l QStrings can be compared using overloaded operators such as \l
operator<(), \l operator<=(), \l operator==(), \l operator>=(), operator<(), \l operator<=(), \l operator==(), \l operator>=(),
and so on. Note that the comparison is based exclusively on the and so on. The comparison is based exclusively on the lexicographical
numeric Unicode values of the characters. It is very fast, but is order of the two strings, seen as sequences of UTF-16 code units.
not what a human would expect; the QString::localeAwareCompare() It is very fast but is not what a human would expect; the
function is usually a better choice for sorting user-interface QString::localeAwareCompare() function is usually a better choice for
strings, when such a comparison is available. sorting user-interface strings, when such a comparison is available.
On Unix-like platforms (including Linux, \macos and iOS), when Qt When Qt is linked with the ICU library (which it usually is), its
is linked with the ICU library (which it usually is), its locale-aware sorting is used. Otherwise, platform-specific solutions
locale-aware sorting is used. Otherwise, on \macos and iOS, \l are used:
localeAwareCompare() compares according the "Order for sorted \list
lists" setting in the International preferences panel. On other \li On Windows, localeAwareCompare() uses the current user locale,
Unix-like systems without ICU, the comparison falls back to the as set in the \uicontrol{regional} and \uicontrol{language}
system library's \c strcoll(), options portion of \uicontrol{Control Panel}.
\li On \macos and iOS, \l localeAwareCompare() compares according
to the \uicontrol{Order for sorted lists} setting in the
\uicontrol{International preferences} panel.
\li On other Unix-like systems, the comparison falls back to the
system library's \c strcoll().
\endlist
\section1 Converting Between Encoded Strings Data and QString \section1 Converting between encoded string data and QString
QString provides the following three functions that return a QString provides the following functions that return a
\c{const char *} version of the string as QByteArray: toUtf8(), \c{const char *} version of the string as QByteArray: toUtf8(),
toLatin1(), and toLocal8Bit(). toLatin1(), and toLocal8Bit().
@ -1956,7 +1960,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
\li \l QT_NO_CAST_FROM_ASCII disables automatic conversions from \li \l QT_NO_CAST_FROM_ASCII disables automatic conversions from
C string literals and pointers to Unicode. C string literals and pointers to Unicode.
\li \l QT_RESTRICTED_CAST_FROM_ASCII allows automatic conversions \li \l QT_RESTRICTED_CAST_FROM_ASCII allows automatic conversions
from C characters and character arrays, but disables automatic from C characters and character arrays but disables automatic
conversions from character pointers to Unicode. conversions from character pointers to Unicode.
\li \l QT_NO_CAST_TO_ASCII disables automatic conversion from QString \li \l QT_NO_CAST_TO_ASCII disables automatic conversion from QString
to C strings. to C strings.
@ -1964,7 +1968,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
You then need to explicitly call fromUtf8(), fromLatin1(), You then need to explicitly call fromUtf8(), fromLatin1(),
or fromLocal8Bit() to construct a QString from an or fromLocal8Bit() to construct a QString from an
8-bit string, or use the lightweight QLatin1StringView class, for 8-bit string, or use the lightweight QLatin1StringView class. For
example: example:
\snippet code/src_corelib_text_qstring.cpp 1 \snippet code/src_corelib_text_qstring.cpp 1
@ -1985,7 +1989,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
\snippet qstring/main.cpp 7 \snippet qstring/main.cpp 7
The \c result variable, is a normal variable allocated on the The \c result variable is a normal variable allocated on the
stack. When \c return is called, and because we're returning by stack. When \c return is called, and because we're returning by
value, the copy constructor is called and a copy of the string is value, the copy constructor is called and a copy of the string is
returned. No actual copying takes place thanks to the implicit returned. No actual copying takes place thanks to the implicit
@ -1993,12 +1997,12 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
\endtable \endtable
\section1 Distinction Between Null and Empty Strings \section1 Distinction between null and empty strings
For historical reasons, QString distinguishes between a null For historical reasons, QString distinguishes between null
string and an empty string. A \e null string is a string that is and empty strings. A \e null string is a string that is
initialized using QString's default constructor or by passing initialized using QString's default constructor or by passing
(\c{const char *})0 to the constructor. An \e empty string is any \nullptr to the constructor. An \e empty string is any
string with size 0. A null string is always empty, but an empty string with size 0. A null string is always empty, but an empty
string isn't necessarily null: string isn't necessarily null:
@ -2006,10 +2010,10 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
All functions except isNull() treat null strings the same as empty All functions except isNull() treat null strings the same as empty
strings. For example, toUtf8().constData() returns a valid pointer strings. For example, toUtf8().constData() returns a valid pointer
(\e not nullptr) to a '\\0' character for a null string. We (not \nullptr) to a '\\0' character for a null string. We
recommend that you always use the isEmpty() function and avoid isNull(). recommend that you always use the isEmpty() function and avoid isNull().
\section1 Number Formats \section1 Number formats
When a QString::arg() \c{'%'} format specifier includes the \c{'L'} locale When a QString::arg() \c{'%'} format specifier includes the \c{'L'} locale
qualifier, and the base is ten (its default), the default locale is qualifier, and the base is ten (its default), the default locale is
@ -2019,16 +2023,16 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
C locale's representation of numbers. C locale's representation of numbers.
When QString::arg() applies left-padding to numbers, the fill character When QString::arg() applies left-padding to numbers, the fill character
\c{'0'} is treated specially. If the number is negative, its minus sign will \c{'0'} is treated specially. If the number is negative, its minus sign
appear before the zero-padding. If the field is localized, the appears before the zero-padding. If the field is localized, the
locale-appropriate zero character is used in place of \c{'0'}. For locale-appropriate zero character is used in place of \c{'0'}. For
floating-point numbers, this special treatment only applies if the number is floating-point numbers, this special treatment only applies if the number is
finite. finite.
\section2 Floating-point Formats \section2 Floating-point formats
In member functions (e.g., arg(), number()) that represent floating-point In member functions (for example, arg() and number()) that format floating-point
numbers (\c float or \c double) as strings, the form of display can be numbers (\c float or \c double) as strings, the representation used can be
controlled by a choice of \e format and \e precision, whose meanings are as controlled by a choice of \e format and \e precision, whose meanings are as
for \l {QLocale::toString(double, char, int)}. for \l {QLocale::toString(double, char, int)}.
@ -2037,14 +2041,14 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
the exponent shows its sign and includes at least two digits, left-padding the exponent shows its sign and includes at least two digits, left-padding
with zero if needed. with zero if needed.
\section1 More Efficient String Construction \section1 More efficient string construction
Many strings are known at compile time. The QString constructor from Many strings are known at compile time. The QString constructor from
C++ string literals will copy the contents of the string, C++ string literals will copy the contents of the string,
treating the contents as UTF-8. This requires a memory allocation and the treating the contents as UTF-8. This requires memory allocation and
re-encoding of the string data, operations that will happen at runtime. re-encoding string data, operations that will happen at runtime.
If the string data is known at compile time, you can use the QStringLiteral macro If the string data is known at compile time, you can use the QStringLiteral
or similarly \c{operator""_s} to create QString's payload at compile macro or similarly \c{operator""_s} to create QString's payload at compile
time instead. time instead.
Using the QString \c{'+'} operator, it is easy to construct a Using the QString \c{'+'} operator, it is easy to construct a
@ -2056,7 +2060,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
There is nothing wrong with either of these string constructions, There is nothing wrong with either of these string constructions,
but there are a few hidden inefficiencies: but there are a few hidden inefficiencies:
First, multiple uses of the \c{'+'} operator usually means First, repeated use of the \c{'+'} operator may lead to
multiple memory allocations. When concatenating \e{n} substrings, multiple memory allocations. When concatenating \e{n} substrings,
where \e{n > 2}, there can be as many as \e{n - 1} calls to the where \e{n > 2}, there can be as many as \e{n - 1} calls to the
memory allocator. memory allocator.
@ -2078,55 +2082,57 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
then called \e{once} to get the required space, and the substrings then called \e{once} to get the required space, and the substrings
are copied into it one by one. are copied into it one by one.
Additional efficiency is gained by inlining and reduced reference Additional efficiency is gained by inlining and reducing reference
counting (the QString created from a \c{QStringBuilder} counting (the QString created from a \c{QStringBuilder}
has a ref count of 1, whereas QString::append() needs an extra has a ref count of 1, whereas QString::append() needs an extra
test). test).
There are two ways you can access this improved method of string There are two ways you can access this improved method of string
construction. The straightforward way is to include construction. The straightforward way is to include
\c{QStringBuilder} wherever you want to use it, and use the \c{QStringBuilder} wherever you want to use it and use the
\c{'%'} operator instead of \c{'+'} when concatenating strings: \c{'%'} operator instead of \c{'+'} when concatenating strings:
\snippet qstring/stringbuilder.cpp 5 \snippet qstring/stringbuilder.cpp 5
A more global approach, which is more convenient but not entirely source A more global approach, which is more convenient but not entirely
compatible, is to define \c QT_USE_QSTRINGBUILDER (by adding it to the compiler source-compatible, is to define \c QT_USE_QSTRINGBUILDER (by adding
flags) at build time. This will make concatenating strings with \c{'+'} work the it to the compiler flags) at build time. This will make concatenating
same way as \c{QStringBuilder} \c{'%'}. strings with \c{'+'} work the same way as \c{QStringBuilder's} \c{'%'}.
\note Using automatic type deduction (e.g. by using the \c auto keyword) \note Using automatic type deduction (for example, by using the \c
with the result of string concatenation when QStringBuilder is enabled will auto keyword) with the result of string concatenation when QStringBuilder
show that the concatenation is indeed an object of a QStringBuilder specialization: is enabled will show that the concatenation is indeed an object of a
QStringBuilder specialization:
\snippet qstring/stringbuilder.cpp 6 \snippet qstring/stringbuilder.cpp 6
This does not cause any harm, as QStringBuilder will implictly convert to This does not cause any harm, as QStringBuilder will implicitly convert to
QString when required. If this is undesirable, then one should specify QString when required. If this is undesirable, then one should specify
the required types instead of having the compiler deduce them: the necessary types instead of having the compiler deduce them:
\snippet qstring/stringbuilder.cpp 7 \snippet qstring/stringbuilder.cpp 7
\section1 Maximum Size and Out-of-memory Conditions \section1 Maximum size and out-of-memory conditions
The maximum size of QString depends on the architecture. Most 64-bit The maximum size of QString depends on the architecture. Most 64-bit
systems can allocate more than 2 GB of memory, with a typical limit systems can allocate more than 2 GB of memory, with a typical limit
of 2^63 bytes. The actual value also depends on the overhead required for of 2^63 bytes. The actual value also depends on the overhead required for
managing the data block. As a result, you can expect the maximum size managing the data block. As a result, you can expect a maximum size
of 2 GB minus overhead on 32-bit platforms, and 2^63 bytes minus overhead of 2 GB minus overhead on 32-bit platforms and 2^63 bytes minus overhead
on 64-bit platforms. The number of elements that can be stored in a on 64-bit platforms. The number of elements that can be stored in a
QString is this maximum size divided by the size of QChar. QString is this maximum size divided by the size of QChar.
When memory allocation fails, QString throws a \c std::bad_alloc When memory allocation fails, QString throws a \c std::bad_alloc
exception if the application was compiled with exception support. exception if the application was compiled with exception support.
Out of memory conditions in Qt containers are the only case where Qt Out-of-memory conditions in Qt containers are the only cases where Qt
will throw exceptions. If exceptions are disabled, then running out of will throw exceptions. If exceptions are disabled, then running out of
memory is undefined behavior. memory is undefined behavior.
Note that the operating system may impose further limits on applications \note Target operating systems may impose limits on how much memory an
holding a lot of allocated memory, especially large, contiguous blocks. application can allocate, in total, or on the size of individual allocations.
Such considerations, the configuration of such behavior or any mitigation This may further restrict the size of string a QString can hold.
are outside the scope of the Qt API. Mitigating or controlling the behavior these limits cause is beyond the
scope of the Qt API.
\sa fromRawData(), QChar, QStringView, QLatin1StringView, QByteArray \sa fromRawData(), QChar, QStringView, QLatin1StringView, QByteArray
*/ */
@ -2412,8 +2418,8 @@ encoded in \1, and is converted to QString using the \2 function.
/*! \fn std::wstring QString::toStdWString() const /*! \fn std::wstring QString::toStdWString() const
Returns a std::wstring object with the data contained in this Returns a std::wstring object with the data contained in this
QString. The std::wstring is encoded in utf16 on platforms where QString. The std::wstring is encoded in UTF-16 on platforms where
wchar_t is 2 bytes wide (e.g. windows) and in ucs4 on platforms wchar_t is 2 bytes wide (for example, Windows) and in UTF-32 on platforms
where wchar_t is 4 bytes wide (most Unix systems). where wchar_t is 4 bytes wide (most Unix systems).
This method is mostly useful to pass a QString to a function This method is mostly useful to pass a QString to a function
@ -2565,7 +2571,7 @@ QString::QString(QChar ch)
can be useful if you want to ensure that all user-visible strings can be useful if you want to ensure that all user-visible strings
go through QObject::tr(), for example. go through QObject::tr(), for example.
\note: any null ('\\0') bytes in the byte array will be included in this \note Any null ('\\0') bytes in the byte array will be included in this
string, converted to Unicode null characters (U+0000). This behavior is string, converted to Unicode null characters (U+0000). This behavior is
different from Qt 5.x. different from Qt 5.x.
@ -2712,20 +2718,20 @@ void QString::resize(qsizetype newSize, QChar fillChar)
Ensures the string has space for at least \a size characters. Ensures the string has space for at least \a size characters.
If you know in advance how large the string will be, you can call this If you know in advance how large a string will be, you can call this
function to save repeated reallocation in the course of building it. function to save repeated reallocation while building it.
This can improve performance when building a string incrementally. This can improve performance when building a string incrementally.
A long sequence of operations that add to a string may trigger several A long sequence of operations that add to a string may trigger several
reallocations, the last of which may leave you with significantly more reallocations, the last of which may leave you with significantly more
space than you really need, which is less efficient than doing a single space than you need. This is less efficient than doing a single
allocation of the right size at the start. allocation of the right size at the start.
If in doubt about how much space shall be needed, it is usually better to If in doubt about how much space shall be needed, it is usually better to
use an upper bound as \a size, or a high estimate of the most likely size, use an upper bound as \a size, or a high estimate of the most likely size,
if a strict upper bound would be much bigger than this. If \a size is an if a strict upper bound would be much bigger than this. If \a size is an
underestimate, the string will grow as needed once the reserved size is underestimate, the string will grow as needed once the reserved size is
exceeded, which may lead to a larger allocation than your best overestimate exceeded, which may lead to a larger allocation than your best
would have and will slow the operation that triggers it. overestimate would have and will slow the operation that triggers it.
\warning reserve() reserves memory but does not change the size of the \warning reserve() reserves memory but does not change the size of the
string. Accessing data beyond the end of the string is undefined behavior. string. Accessing data beyond the end of the string is undefined behavior.