Qt 6 officially requires C++17, however we are constantly working on supporting new C++ language features to make Qt faster and safer, and provide a better experience for our users.
This page gives a brief overview of C++20 features available in Qt.
std::chrono
Various classes related to date and time have support for
std::chrono
features from older versions of C++. With the arrival of C++20,
std::chrono
has added new
calendar types
，譬如
year_month_day
, plus
date and time representations
, such as the
system_clock
及其
time_point
types. When Qt is built with C++20, it can now make use of these additions.
QDate and QDateTime now support conversion to and from the various calendar, date and date-time types, along with addition of duration types . QTimeZone now supports construction from a time_zone .
std::span
The std::span class template is an addition to the standard library that provides a uniform way to access a contiguous portion of any contiguous container.
使用
std::span
in the public APIs of Qt could provide significant advantages. However, Qt 6 only requires C++17, meaning it cannot have C++20 types in the API and ABI.
Qt 6.7 introduced
QSpan
- a Qt version of
std::span
. The API of the class is compatible with the std version.
QSpan
and
std::span
can be implicitly converted into each other. However, there are some differences between the classes. See the corresponding section in the
QSpan class documentation
了解更多细节。
C++20 introduced
operator<=>()
, also known as the
three-way comparison operator
, and three ordering types to represent the results of the comparison:
In Qt 6.8, many of the
QtCore
classes got support for
operator<=>()
. To use the new operator, the user project must be compiled in C++20 mode.
However, starting from Qt 6.7, C++17 users can use our own equivalents of the std ordering types.
As a C++20 language feature, backporting
operator<=>()
to C++17 is not possible, but you can use the Qt::compareThreeWay() function, which acts like a C++17 version of
operator<=>()
for built-in C++ types (such as integers, floating-point, and enumeration types).
Qt also defines helper functions
compareThreeWay()
for various classes in
QtCore
. These are all implemented as
hidden friends
. Users can implement their own
compareThreeWay()
functions for custom types.
Finally, Qt provides a
qCompareThreeWay
() function template, which serves as a generic three-way comparison implementation. It relies on Qt::compareThreeWay() and the above-mentioned free
compareThreeWay()
函数在其实现中。
Qt has also adopted a few more features from C++20, such as:
constinit
keyword, or to compiler-specific attributes if they are available
[[nodiscard]]
attribute if the compiler supports
[[nodiscard]]
on constructors
[[nodiscard("reason")]]