In short, Qt uses private implementation to manage data. For classes that do this, there usually is an X class and an XPrivate class. The Q_D macro defines the &quot;d&quot; pointer so if you write d-&gt;whatever, you have access to that private data part.

This article should pretty much cover most of your questions:

https://wiki.qt.io/D-Pointer

Q_D is part of Qt&#39;s implementation of d-pointers, or the pimpl pattern (private implementation). A bit more information on all of this can be found on the Qt DevNet wiki: https://wiki.qt.io/D-Pointer

I have the following constraint being applied to one of my tables, but I don&#39;t know what PAD_INDEX means.

Can someone enlighten me?

    CONSTRAINT [PK_Employees] PRIMARY KEY CLUSTERED 
    (
    	[EmployeeId] ASC
    ) WITH (PAD_INDEX  = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]
            ^--------------^
             this part here

What is the purpose of PAD_INDEX in this SQL Server constraint?

Many dynamic languages implement (or want to implement) a JIT Compiler in order to speed up their execution times. Inevitably, someone from the peanut gallery asks why they don&#39;t use LLVM. The answer is often, &quot;LLVM is unsuitable for building a JIT.&quot; (For Example, Armin Rigo&#39;s comment [here.][1])

**Why is LLVM Unsuitable for building a JIT?**

Note: I know LLVM has its own JIT. If LLVM used to be unsuitable, but now is suitable, please say what changed. I&#39;m not talking about running LLVM Bytecode on the LLVM JIT, I&#39;m talking about using the LLVM libraries to implement a JIT for a dynamic language.


  [1]: http://morepypy.blogspot.com/2008/04/float-operations-for-jit.html?showComment=1208773800000#c6322995253226647458

Why is llvm considered unsuitable for implementing a JIT?

I am exploring Qt source code and came across this `Q_D` macro. Could some one please explain what it does? 

Almost all the time it should return a widget of the type given as a parameter which is the `d` variable. Need more clarification on this.


What does Q_D macro do in Qt

<p>I am exploring Qt source code and came across this <code>Q_D</code> macro. Could some one please explain what it does?</p>
<p>Almost all the time it should return a widget of the type given as a parameter which is the <code>d</code> variable. Need more clarification on this.</p>


Is there a way to change a .pro , .ui and .h names if the project is done? How to do it?

How to change a project&#39;s name in QtCreator?

I have a simple question - what is Shadow Building? I&#39;ve seen it in Qt in Target Setup dialog, but don&#39;t know what it does or if I should use it.

It&#39;s also present in Projects tab, under General group.

Qt - &#39;Shadow Building&#39;?

I&#39;m writing a program in C++ using the Qt library. There is a symbolic link in my home bin directory to the executable. I would like the current working directory of my program to be the directory in which I am with my terminal (ie. the result of the `pwd` command). I saw the `QDir::currentPath()` function, but it gives back the directory where the binary is.

How can I find my current working directory?

Get current working directory in a Qt application

I want to get `QString` from another `QString`, when I know necessary indexes.
For example:
Main string: *&quot;This is a string&quot;*.
I want to create new `QString` from first 5 symbols and get *&quot;This &quot;*.   
**input :** first and last char number.  
**output :** new `QString`.

How to create it ?  

**P.S.** Not only first several letters, also from the middle of the line, for example from 5 till 8.

Qt. get part of QString

I have a qmake file generated by Qt creator.
I am modifying it but I do not manage to understand how to create a variable.

For example, I want to declare the library MYPATH as I did here:

    MYPATH = /lib/aaa/bbb
    unix:!macx:!symbian: LIBS += -L$(MYPATH)

When I run qmake I find in the generated makefile

    LIBS = ....... -L$(MYPATH) .....

But the MYPATH variable is not declared anywhere.

Does anyone know how to declare such a variable properly?

QMake - How to add and use a variable into the .pro file

The standard predefined macro `__FILE__` available in C shows the full path to the file. Is there any way to short the path? I mean instead of

    /full/path/to/file.c

I see

    to/file.c

or 

    file.c

__FILE__ macro shows full path

I usually pass macro definitions from &quot;make command line&quot; to a &quot;makefile&quot; using the option :
-Dname=value. The definition is accessible inside the makefile.

I also pass macro definitions from the &quot;makefile&quot; to the &quot;source code&quot; using the similar compiler option :
-Dname=value (supported in many compilers). This definition is accessible in the source code.

What I need now, is to allow the user of my makefile to be able to pass arbitrary macro definitions from the &quot;make.exe commandline&quot; to &quot;source code&quot; right away, without having to change anything in the makefile.

so the user can type :
make -f mymakefile.mk -SOMEOPTION var=5

then directly the code main.c can see var :

    int main()
    {
      int i = var;
    }

How to pass macro definition from &quot;make&quot; command line arguments (-D) to C source code?

I bumped into this strange macro code in [/usr/include/linux/kernel.h][1]:

    /* Force a compilation error if condition is true, but also produce a
       result (of value 0 and type size_t), so the expression can be used
       e.g. in a structure initializer (or where-ever else comma expressions
       aren&#39;t permitted). */
    #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
    #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))

What does `:-!!` do?

  [1]: https://github.com/torvalds/linux/blob/ff2d8b19a3a62559afba1c53360c8577a7697714/include/linux/kernel.h#L677-L682


What is &quot;:-!!&quot; in C code?

Sorry for asking very basic question. I would like to set OR condition in #ifdef directive.?
How to do that ?
I tried

    #ifdef LINUX | ANDROID
    ...
    ..
    #endif 
It did not work? What is the proper way?

how to use #ifdef with an OR condition?

I have two macros `FOO2` and `FOO3`:

    #define FOO2(x,y) ...
    #define FOO3(x,y,z) ...

I want to define a new macro `FOO` as follows:

    #define FOO(x,y) FOO2(x,y)
    #define FOO(x,y,z) FOO3(x,y,z)

But this doesn&#39;t work because macros do not overload on number of arguments.

Without modifying `FOO2` and `FOO3`, is there some way to define a macro `FOO` (using `__VA_ARGS__` or otherwise) to get the same effect of dispatching `FOO(x,y)` to `FOO2`, and `FOO(x,y,z)` to `FOO3`?


Content Type	Original Author	Original Content on Stackoverflow
Question	Tharanga	View Question on Stackoverflow
Solution 1 - Qt	Tim Meyer	View Answer on Stackoverflow
Solution 2 - Qt	leinir	View Answer on Stackoverflow

What does Q_D macro do in Qt

Qt Problem Overview

Qt Solutions

Solution 1 - Qt

Solution 2 - Qt

Why is llvm considered unsuitable for implementing a JIT?

What is the purpose of PAD_INDEX in this SQL Server constraint?

Attributions