@Christian-Ehrlicher Thanks for the flags , will check the results.

@Christian-Ehrlicher said in Need advice with QtConcurrency & lambda expression:

var is local inside calllambda() and gets destroyed when the function exits

Aha, sure, local variable will be destroyed before lambda function will try to get access from parallel thread.
To test I can add QThread::msleep(10); at the end of calllambda function (after QtConcurrency::run).

There is no problems in Qt or pure C++, the problem is in logic of example.

It's clear now, thank you!

Don't know what you really want to achieve by including the sqlite3 sources but Qt will not use it - it will use the QSqlite3 plugin provided by your Qt installation. And there json support is enabled since Qt 5.13

For a briefer discussion than a book, see https://softwareengineering.stackexchange.com/a/149576

Well, why are you creating a local pointer to a QtDemo object within a method that is part of QtDemo ?

https://en.cppreference.com/w/cpp/language/bit_field
Get familiar with this site. It contains just about everything with really good explanations of C and C++. It also has an offline copy you can download.

The other day I learned that gcc will convert a tail call optimized recursive routine into a while loop and thus no limit to recursion iterations (there are probably caveats to this, but it won't likely cause an SO). That is when I realized I should not spend my time on second guessing the compiler unless I am presented with a reason. Optimizing for the sake of optimizing is wasted effort. Find bottlenecks first. They are probably not where you think they are.

Hey,
Sorry for the delay, I didn't had/take enough time for Qt these days.

I think this is going a little bit far/too complex for me at the moment. I need to learn and pratice more on pointers.
Also my question was too 'general' so it could not be really answered (except with a all disseration and that was not the goal). But I've learn some really usefull informations and that's already something!

So thank you all for your replies.

Have a nice day.

it's a bit heavy weight but I always like a good RE engine for parsing most things.

[-+]?(\d+)%

I am glad to have started such a big discussion :)

For my own part, I shall:

Obviously, for any global/member variables which are genuinely meant to be const I will put that in. That was never the question.

When I choose to create a temporary pointer, I will (try to) make the effort to put in const (e.g. const QWidget *widg = something; widg->constMethod();) where I only need to use the variable to call something const. Just as I would it it were a formal parameter to a method.

But for non-pointer simple value types, whether as local variables or formal parameters, for right or for wrong I 'm not going to cover my code with const. It's just too much to type, and I'd rather save const for where it actually matters.

Thanks to all. I'll close this in a couple of days, give y'all some more time to disagree with each other ;-)

I too very much dislike c++ style casts... too fracking wordy to use them in places like

uint8_t c{ 0x43 }; cout << static_cast<int>(c); // would be shorter and just as obvious when done as cout << (int)c;

Since qtSt.toStdString() is returning a temporary why not just turn QStringList into a std::vector<string> and maintain for the lifetime or your pointers. That way the pointer returned by c_str remains valid. Probably better than managing the memory with new/delete.

@fcarney For this to work you have to use pointers:

QVector<drink*> drinks;

{ auto x1 = [](int i) -> int { qInfo() << "Invoked"; return i; }; typedef typename std::invoke_result<decltype(x1), int>::type x1ret_type; typedef decltype (x1(1)) rtype; Q_ASSERT(typeid(rtype) == typeid(x1ret_type)); qInfo() << typeid(rtype).name() << typeid(x1ret_type).name(); qInfo() << "before typeid"; typeid(x1(1)); qInfo() << "after typeid"; }

Pretty sure, I saw this behavior before (tested again):

i i before typeid after typeid

A quick look on bcrypt's webpage tells me this is not a library, but a program. If you want to have it as a library, there is not support for this out of the box. You need to figure it out by yourself.

The general instructions for building are to use make. (Usually, nmake is the equivalent on Windows.) I don't see any instructions for Windows on the webpage. You'll have to look through the README's, etc. to figure it out. cmake and qmake will not help as these will just generate makefiles and not compile anything. However, there is already a makefile, so you don't have to run cmake or qmake to create them.

No it's not, you can see that with its constructors that take a QSqlDatabase parameter.

No clear is not called, the destructor is.

There's no need to create QSqlQuery objects on the heap.

@IamSumit said in JSON/XML Conversion in C++ ?:

i want to convert JSON to an XML

I think Qt probably has enough in it to do this. Read up on how to read and parse through a JSON document using the Qt library. Do a recursive printout of the structure to text. Then once you are confident you are reading the entire document then use the Qt library to create your XML document. Also print this out to make sure it matches what you want. You will have to determine a convention of how to translate and this might just be a simple lookup table or a set of rules.