Il sab 3 giu 2023, 14:37 Elvis Stansvik <elvst...@gmail.com> ha scritto:
> Hi all,
>
> I was going through some legacy code and came across a behavioral
> difference between QPair and std::pair:
>
> estan@edison:~$ cat test.cpp
> #include <QPair>
> #include <utility>
>
> int main(void) {
> int i = 1;
> QPair<unsigned, unsigned>{i, i};
> std::pair<unsigned, unsigned>{i, i};
> return 0;
> }
> estan@edison:~$ g++ -isystem /usr/include/x86_64-linux-gnu/qt5
> -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -Wall -fPIC -c
> test.cpp
> test.cpp: In function ‘int main()’:
> test.cpp:6:31: warning: narrowing conversion of ‘i’ from ‘int’ to
> ‘unsigned int’ [-Wnarrowing]
> 6 | QPair<unsigned, unsigned>{i, i};
> | ^
> test.cpp:6:34: warning: narrowing conversion of ‘i’ from ‘int’ to
> ‘unsigned int’ [-Wnarrowing]
> 6 | QPair<unsigned, unsigned>{i, i};
> | ^
> estan@edison:~$
>
> Just curious if anyone know why what difference between QPair and
> std::pair makes the narrowing conversion warning pop up in the QPair
> case and not the std::pair case?
>
Because QPair is lacking constructor 3
https://en.cppreference.com/w/cpp/utility/pair/pair
https://codebrowser.dev/qt5/qtbase/src/corelib/tools/qpair.h.html
That constructor hides the narrowing into std::pair's code.
> Note the warning only appears when using aggregate initialization with { }.
>
It's list initialization, but it's not aggregate initialization, neither
class is an aggregate as they have user defined constructors.
> Using GCC 11.3.0 and Qt 5.15.3.
>
Note: I made QPair an alias to std::pair in Qt 6.
Hth,
> Cheers,
> Elvis
> --
> Development mailing list
> Development@qt-project.org
> https://lists.qt-project.org/listinfo/development
>
--
Development mailing list
Development@qt-project.org
https://lists.qt-project.org/listinfo/development
