[Development] QProperty and library coding guide

Thiago Macieira thiago.macieira at intel.com
Fri Jul 17 17:10:30 CEST 2020

On Friday, 17 July 2020 04:54:08 PDT Giuseppe D'Angelo via Development wrote:
> Il 16/07/20 17:40, Volker Hilsheimer ha scritto:
> > The struct has no data itself, so ideally would be of size zero.
> I'm missing some piece of the puzzle: if you take action->text, and text
> is a zero-size struct, how does the operator() applied to it figure out
> which action needs to read the text property from? E.g. like so:

The same way this works:

  auto l1 = [] { return 1; };
  auto l2 = [] { return 2; };

Both l1 and l2 are "zero" sized closure types with an operator(). When you 
call them, you get different results, because the C++ compiler will place a 
call to different functions.

Note: that's different from:
  auto f1 = +[] { return 1; };
  auto f2 = +[] { return 2; };

The unary + forces the stateless lambda to cast to void(void), so f1 and f2 
are the same type (a function pointer) and have different values.

> > QAction *action = ~~~;
> > auto prop = action->text;
> > QString text = prop();

Note on the above: the type should not have a cast operator. Otherwise:

  auto prop = action->text;
  QString value = action->text;

would be different things. Not to mention QString builder failures. People 
should always write action->text().

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel System Software Products

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