[Development] QPair vs std::pair difference with narrowing conversion

[Elvis Stansvik]

Den lör 3 juni 2023 kl 15:08 skrev Giuseppe D'Angelo <dangelog at gmail.com>:
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>
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> Il sab 3 giu 2023, 14:37 Elvis Stansvik <elvstone at 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 at 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 at 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 at 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
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> https://codebrowser.dev/qt5/qtbase/src/corelib/tools/qpair.h.html
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> That constructor hides the narrowing into std::pair's code.

Ah yes, thanks!

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>> Note the warning only appears when using aggregate initialization with { }.
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> It's list initialization, but it's not aggregate initialization, neither class is an aggregate as they have user defined constructors.

Right, my bad, had the terminology wrong.

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>> Using GCC 11.3.0 and Qt 5.15.3.
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> Note: I made QPair an alias to std::pair in Qt 6.

Nice, thanks.

Elvis

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> Hth,
>
>>
>> Cheers,
>> Elvis
>> --
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