[Development] A QStringList puzzle for Monday

[Igor Khanin]

Hi,

This is a really unfortunate (and all too common) case...

o4 is invoking the QList(InputIterator, InputIterator) constructor, with 
the `InputIterator` type being deduced as `const char *`. This is a 
valid iterator type with traits specialization and everything, so this 
constructor is a member of the overload set. As long as there is a 
implicit conversion path from `const char` to `QString`, it will 
compile... and there is such a path. Of course trying to do pointer 
arithmetic on pointers to string literals is UB. The only way I see to 
make it not compile is to remove the conversion path by defining 
QT_NO_CAST_FROM_ASCII.

I'm not sure that anything can be added on top of the meta-programming 
already in QList to stop this. This is also something you can run into 
with a `std::vector<std::string>`. Just one more data point to how the 
prevalent use of iterator pairs is another footgun in C++'s arsenal.

Igor

On 17/03/2025 20:32, Henry Skoglund wrote:
> Hello, there are a more than one way to initialize a QStringList but 
> sometimes they are not equal, consider this console app:
>
> main.cpp:
> #include <QCoreApplication>
> #include "qcommandlineparser.h"
> #include "qdebug.h"
>
> int main(int argc, char *argv[])
> {
>     QCoreApplication a(argc, argv);
>
>     QCommandLineParser lp;
>     QCommandLineOption o1(QStringList() << "o" << "ok");
>     QCommandLineOption o2(QStringList{"a","alsook"});
>     QCommandLineOption o3(QStringList("yesok"));
>     qDebug() << "geronimo";
>     QCommandLineOption o4(QStringList("g","goessouth"));
>     qDebug() << "landed";
>
>     lp.addOptions({o1,o2,o3,o4});
> }
>
> If I build it in debug mode on 6.8.2, either with MSVC 2022 or gcc 
> version 13.3.0 on Ubuntu and run it:
>
> 18:57:31: Starting 
> /home/henry/untitled/build/Desktop_Qt_6_8_2-Debug/untitled...
> geronimo
> ASSERT: "this->isMutable() || b == e" in file 
> /home/henry/Qt/6.8.2/gcc_64/include/QtCore/qarraydataops.h, line 884
> 18:57:32: The process crashed.
>
> In release mode:
> 19:12:47: Starting 
> /home/henry/untitled/build/Desktop_Qt_6_8_2-Release/untitled...
> geronimo
> landed
> QCommandLineParser: already having an option named "a"
> 19:12:47: /home/henry/untitled/build/Desktop_Qt_6_8_2-Release/untitled 
> exited with code 0
>
>
> Obviously I am doing something wrong when setting o4 but it would be 
> nice if I could get a compile error...
>
> Rgrds Henry
>
>