std vector, QVector and fields..

[J.Hilk]

@JonB
you said

I need to apply some initial changes to all of them

which means, you know (theoretically) during compile time what your values should be.

Now, if all your functions on QCheckBox were constexpr and find children would be as well, you could create a small constexpr function of your own, that finds all QCheckBox and sets the initial values and call that somewhere early on. The compiler would evaluate those calls during compile time and set the initial values accordingly without runtime overhead.

---

no kittens required to die, in fact you would have more time, due to longer compile times, to pet them

[JonB]

@J-Hilk said in std vector, QVector and fields..:

you could create a small constexpr function of your own, that finds all QCheckBox

Not that I can imagine doing this, but could you give an example of how I would do this, at compile-time? Or is that based on a hypothetical "if all your functions on QCheckBox were constexpr and find children would be as well"? I don't follow how you would get a list at compile-time of all QCheckBoxes I have created in Designer (without my having to type that into the source code manually)?

[J.Hilk]

@JonB it is hypothetical, because I'm pretty sure setupUi is not constexpr

but as example, take this compiletime calculation of fibonacci

constexpr int64_t fibonacci(const int64_t  n)
{
    return n < 1 ? -1 :
        (n == 1 || n == 2 ? 1 : fibonacci(n - 1) + fibonacci(n - 2));
}

int main (int argc, char *argv[])
{
    constexpr auto evaled = fibonacci(25);
        return evaled;
}

actually had to modify the example to force the compiletime evaluation , a little bit finicky the whole system (still)

[JonB]

@J-Hilk
OK, yes, I know about turning the C++ compiler into an interpreter with constexpr :) But that fibonacci function is self-contained. Compiler is not going to be able to execute anything like constexpr QObject::findChildren<QCheckBox *>, it's runtime-only.

Purely BTW. I'm not a great programmer (nor mathematician), so while I develop my code I have a go at:

constexpr int64_t fibonacci(const int64_t  n)
{
    int64_t result = 0;
    while (n > 0)
        result += n;
    return result;
}

Now I compile my code. Does the compilation simply hang??

[jsulm]

@JonB said in std vector, QVector and fields..:

Now I compile my code. Does the compilation simply hang??

No, because you're not calling it :-)
And it would not hang if you would call it with any number as parameter. It could probably slow down compilation if you would pass a huge number :-)

[JonB]

@jsulm
Oh yes I am! In @J-Hilk's code (at least originally) his main() calls fibonacci(10), so assume that, and then tell me what happens while I sit waiting for the compilation to finish?

And it would not hang if you would call it with any number as parameter. It could probably slow down compilation if you would pass a huge number :-)

I think you should look at the code again... :)

[jsulm]

@JonB OK, I see n > 0. You should try :-)

[J.Hilk]

@JonB Add at least a function call to your calculation, then you get an stack overflow(eventually)

[JonB]

@jsulm

You should try :-)

I'm terrified of seizing up my Linux VM :) It already does that if I accidentally debug from Creator when inside a QComboBox clicked slot, and I have to hard-switch-off the whole VM... :(

[J.Hilk]

@JonB

main.cpp:158:18: error: expression is not an integral constant expression
main.cpp:152:13: note: constexpr evaluation hit maximum step limit; possible infinite loop?
main.cpp:158:18: note: in call to 'fibonacci(25)'

smart things these compilers

[JonB]

@J-Hilk said in std vector, QVector and fields..:

@JonB Add at least a function call to your calculation, then you get an stack overflow(eventually)

But I don't want a stackoverflow to terminate! I want to know whether the compiler sits there forever! Which is why I wrote as I did.

OK, I'm off to try....

[JonB]

@J-Hilk said in std vector, QVector and fields..:

note: constexpr evaluation hit maximum step limit; possible infinite loop?

Ah ha! So..... this C++ constexpr has what "maximum step limit`? Where is that in the spec? :)

[J.Hilk]

@JonB up to you with the compiler option -fconstexpr-steps

[JonB]

@J-Hilk
Wow! They think of everything! I wonder if that would accept -fconstexpr-steps fibonacci(25) ;-)

OK, enough now, thanks!