IdentifiantMot de passe
Loading...
Mot de passe oublié ?Je m'inscris ! (gratuit)

QStringView Class

The QStringView class provides a unified view on UTF-16 strings with a read-only subset of the QString API.

This class was introduced in Qt 5.10.

All functions in this class are reentrant.

Article lu   fois.

L'auteur

Liens sociaux

Viadeo Twitter Facebook Share on Google+   

QStringView Class

  • Header: QStringView

  • Since: Qt 5.10

  • qmake: QT += core

  • Group: QStringView is part of tools, string-processing

Detailed Description

A QStringView references a contiguous portion of a UTF-16 string it does not own. It acts as an interface type to all kinds of UTF-16 string, without the need to construct a QString first.

The UTF-16 string may be represented as an array (or an array-compatible data-structure such as QString, std::basic_string, etc.) of QChar, ushort, char16_t (on compilers that support C++11 Unicode strings) or (on platforms, such as Windows, where it is a 16-bit type) wchar_t.

QStringView is designed as an interface type; its main use-case is as a function parameter type. When QStringViews are used as automatic variables or data members, care must be taken to ensure that the referenced string data (for example, owned by a QString) outlives the QStringView on all code paths, lest the string view ends up referencing deleted data.

When used as an interface type, QStringView allows a single function to accept a wide variety of UTF-16 string data sources. One function accepting QStringView thus replaces three function overloads (taking QString, QStringRef, and (const QChar*, int)), while at the same time enabling even more string data sources to be passed to the function, such as u"Hello World", a char16_t string literal.

QStringViews should be passed by value, not by reference-to-const:

 
Sélectionnez
    void myfun1(QStringView sv);        // preferred
    void myfun2(const QStringView &sv); // compiles and works, but slower

If you want to give your users maximum freedom in what strings they can pass to your function, accompany the QStringView overload with overloads for

  • QChar: this overload can delegate to the QStringView version:

     
    Sélectionnez
        void fun(QChar ch) { fun(QStringView(&ch, 1)); }

    even though, for technical reasons, QStringView cannot provide a QChar constructor by itself.

  • QString: if you store an unmodified copy of the string and thus would like to take advantage of QString's implicit sharing.

  • QLatin1String: if you can implement the function without converting the QLatin1String to UTF-16 first; users expect a function overloaded on QLatin1String to perform strictly less memory allocations than the semantically equivalent call of the QStringView version, involving construction of a QString from the QLatin1String.

QStringView can also be used as the return value of a function. If you call a function returning QStringView, take extra care to not keep the QStringView around longer than the function promises to keep the referenced string data alive. If in doubt, obtain a strong reference to the data by calling toString() to convert the QStringView into a QString.

QStringView is a Literal Type, but since it stores data as char16_t, iteration is not constexpr (casts from const char16_t* to const QChar*, which is not allowed in constexpr functions). You can use an indexed loop and/or utf16() in constexpr contexts instead.

We strongly discourage the use of QList<QStringView>, because QList is a very inefficient container for QStringViews (it would heap-allocate every element). Use QVector (or std::vector) to hold QStringViews instead.

See Also

See also QString, QStringRef

Member Type Documentation

 

QStringView::const_iterator

This typedef provides an STL-style const iterator for QStringView.

See Also

QStringView::const_pointer

Alias for value_type *. Provided for compatibility with the STL.

QStringView::const_reference

Alias for value_type &. Provided for compatibility with the STL.

QStringView::const_reverse_iterator

This typedef provides an STL-style const reverse iterator for QStringView.

See Also

QStringView::difference_type

Alias for std::ptrdiff_t. Provided for compatibility with the STL.

QStringView::iterator

This typedef provides an STL-style const iterator for QStringView.

QStringView does not support mutable iterators, so this is the same as const_iterator.

See Also

QStringView::pointer

Alias for value_type *. Provided for compatibility with the STL.

QStringView does not support mutable pointers, so this is the same as const_pointer.

QStringView::reference

Alias for value_type &. Provided for compatibility with the STL.

QStringView does not support mutable references, so this is the same as const_reference.

QStringView::reverse_iterator

This typedef provides an STL-style const reverse iterator for QStringView.

QStringView does not support mutable reverse iterators, so this is the same as const_reverse_iterator.

See Also

QStringView::size_type

Alias for qsizetype. Provided for compatibility with the STL.

Unlike other Qt classes, QStringView uses qsizetype as its size_type, to allow accepting data from std::basic_string without truncation. The Qt API functions, for example length(), return int, while the STL-compatible functions, for example size(), return size_type.

QStringView::storage_type

Alias for char16_t for non-Windows or if Q_COMPILER_UNICODE_STRINGS is defined. Otherwise, alias for wchar_t.

QStringView::value_type

Alias for const QChar. Provided for compatibility with the STL.

Member Function Documentation

 

[default] QStringView::QStringView()

Constructs a null string view.

See Also

See also isNull()

[default] QStringView::QStringView(std::nullptr_t)

Constructs a null string view.

See Also

See also isNull()

Vous avez aimé ce tutoriel ? Alors partagez-le en cliquant sur les boutons suivants : Viadeo Twitter Facebook Share on Google+