create QApplication or how to const cast in cppfront

Question

I am playing with cppfront (https://github.com/hsutter/cppfront), a possible C++ successor, in combination with Qt. I am trying to replicate the simplest possible widget application showing just an empty widget.

In C++ (aka cpp1) it would be like this:

#include <QApplication>
#include <QWidget>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget w;
    w.show();
    return a.exec();
}

In cppfront (aka cpp2) I am trying this:

#include <QApplication>
#include <QWidget>

main: (args) -> int =
{
    app := QApplication(args.argc, args.argv);
    w := QWidget();
    w.show();
    return app.exec();
}

I build it first with cppfront and then with MSVC. It results in errors like error: C2440: '<function-style-cast>': cannot convert from 'initializer list' to 'QApplication'. After more trial and error I found out that the problem is that args.argv is char const* const* while QApplication expects char** as the second argument. Just to prove this I tried this:

main: (args) -> int =
{
    app := QApplication(args.argc, nullptr);
    ...

Then the constructor types match well and the program builds and runs fine. Of course it is not correct because it does not pass the arguments. So I tried this:

main: (args) -> int =
{
    app := QApplication(args.argc, std::const_cast<char**>(args.argv));
    ...

But then cppfront complains: error: 'const_cast' is not supported in Cpp2 - the current C++ best practice is to never cast away const, and that is const_cast's only effective use. I fully understand this is for memory safety reasons to prevent possible programming errors.

So my question is: has anyone any simple solution to instantiate a QApplication with passing arguments correctly in vanilla cppfront?

PS: I am using qmake build with pro file:

QT = core gui widgets

CONFIG += c++20

SOURCES += \
        main.cpp

OTHER_FILES += \
        main.cpp2

INCLUDEPATH += C:\cppfront\include

of course you would need to change the cppfront include path to match your paths. I am compiling cpp2 to cpp manually with just cppfront main.cpp in the command line. So no fancy pre-build steps in qmake yet because I am still just experimenting... What is important in this file is c++20 flag and that include path. I am using Qt6.5.2.

HiFile.app - best file manager · Answer 1 · 2023-08-16T08:13:02.723

VERSION 1 (not working):

So after a few more trials and errors (you know, there is still no official documentation to cppfront available at the moment of writing this) I found this possibility:

#include <QApplication>
#include <QWidget>

main: (args) -> int =
{
    app := QApplication(args.argc, args.argv as **char); // note the position of **, that's a gotcha for for someone coming from C/C++
    w := QWidget();
    w.show();
    return app.exec();
}

It builds with cppfront with MSVC and runs well. But it does not compile with clang. It shows error error: static_assert failed due to requirement 'program_violates_type_safety_guarantee<char **, const char *const *>' "No safe 'as' cast available - please check your cast"

VERSION 2 (working as of 2023-08-16):

I posted query to project page https://github.com/hsutter/cppfront/issues/592 and the author reacted and changed args.argv member to char**. So the original code app := QApplication(args.argc, args.argv); now should work fine. Btw. args.argv remains for compatibility with C++. The new and safer way of working with arg is the fact that args inherits from std::vector<std::string_view> which provides more modern and safer way of working with args. This should be recommended to be used in cppfront code.

PS: Please note that cppfront project is still evolving and is just an experiment. Many things can change yet. Let's see how well it work together with Qt framework.

create QApplication or how to const cast in cppfront

1 Answers1