I get "LNK2019: unresolved external symbol" error when trying to create new slot in any window class with standard C++ library arguments.
private slots:
...
void setNewTableItemFromDialog(int number, int age, string rate, string name, string team);
...
private slots:
void replaceTableItem(int number, int age, string rate, string name, string team);
For some reason Qt can compile "setNewTableItemFromDialog()" method, but can't do this with "replaceTableItem()" method, whicn contains actually the same arguments. If I remove this particular method, all works just fine. I already tried to delete build derectory, run CMake, and clean project, nothing works.
Related
I'm using qt creator to create getter and setter automatically and i choose prefer getter name without "get". I find it's not working. I still got getter with "get";
like this
class A
{
int getTest() const;
void setTest(const int& test);
public:
int _test;
}
is "_test" wrong?
From the docs (https://doc.qt.io/qt-5/qtqml-cppintegration-data.html#value-types) and other posts I got the impression, that Q_GADGET can be used as a value type with copy semantics.
Let's take the Actor from the docs:
class Actor
{
Q_GADGET
Q_PROPERTY(QString name READ name WRITE setName)
public:
QString name() const { return m_name; }
void setName(const QString &name) { m_name = name; }
private:
QString m_name;
};
Q_DECLARE_METATYPE(Actor)
Now I expose an Actor with Q_PROPERTY on some C++ object like this:
Q_PROPERTY(Actor actor READ actor WRITE setActor NOTIFY actorChanged)
So far, everything is fine.
Now I have some javascript code in QML (context is the C++ object on which the Q_PROPERTY actor is exposed:
{
var actorCopy = context.actor
actorCopy.name = "Tom"
}
I always assumed that the Actor-Gadget will be copied, when assigning to the javascript variable actorCopy and I change the name on a copy.
Now I set a breakpoint in the setActor function of the defined Q_PROPERTY.
What happens is, when assigning the name of the actor in javascript, the setActor method will be called.
I would not expect that, since I work on a copy.
My question is, do I have a wrong understanding of Q_GADGET or is this a bug?
I mean, if I would have a O_OBJECT instead of Q_GADGET I would not expect the setActor function being called either, since I don't assign the property.
So Q_GADGET is neither a lightweight Q_OBJECT nor a value type.
Try something like this:
{
var actorCopy = context.actor
var otherCopy = actorCopy
actorCopy.name = "Tom"
otherCopy.name = "Gatis"
}
Then in your debugger you will see that setName() is called on two different instances. You can print "this" pointer to confirm.
I did not run this code, but I think that is how it should work.
I've got the following MQL code:
class Collection {
public: void *Get(void *_object) { return NULL; }
};
class Timer {
protected:
string name;
uint start, end;
public:
void Timer(string _name = "") : name(_name) { };
void TimerStart() { start = GetTickCount(); }
void TimerStop() { end = GetTickCount(); }
};
class Profiler {
public:
static Collection *timers;
static ulong min_time;
void Profiler() { };
void ~Profiler() { Deinit(); };
static void Deinit() { delete Profiler::timers; };
};
// Initialize static global variables.
Collection *Profiler::timers = new Collection();
ulong Profiler::min_time = 1;
void main() {
// Define local variable.
static Timer *_timer = new Timer(__FUNCTION__); // This line doesn't.
//Timer *_timer = new Timer(__FUNCTION__); // This line works.
// Start a timer.
((Timer *) Profiler::timers.Get(_timer)).TimerStart();
/* Some code here. */
// Stop a timer.
((Timer *) Profiler::timers.Get(_timer)).TimerStop();
}
which defines a Timer class which is used as a timer to profile the functions how long it took. The original version uses a list of timers to store time separately on each call, however, the code has been simplified to provide a minimum working example and focus on the actual compilation problem.
The problem is when I'm using the following line in order to initialize a static variable:
static Timer *_timer = new Timer(__FUNCTION__); // Line 30.
the compilation fails with:
'Timer' - local variables cannot be used TestProfiler.mqh 30 30
When I drop static word, the code compiles fine.
But it doesn't help me, as I want to define this variable as a static pointer to the class, as I don't want to destroy my object each time when the same function is called over and over again, so the timers can be added to the list which can be read later on. I don't really see why the MQL compiler would prevent from compiling the above code. I also believe this syntax worked fine in the previous builds.
I'm using MetaEditor 5.00 build 1601 (May 2017).
What is wrong with my static variable declaration and how can I correct it, so it can point to a Timer class?
Keyword static has two different meanings in MQL4/5: it indicates that a member of a class is static (which is obvious), and it also says that a variable is static... for instance, if you have a variable that is used only in one function, you probably do not need to declare it globally but as a static. You can find an example of isNewBar() function that has static datetime lastBar=0; in the articles about new bar at mql5.com. This keyword in such a function says that the variable is not deleted after function is finished, but remains in memory and is used with the next call. And if you need a variable in OnTick() function - it does not make sence to have it static, declare it globally.
I have a COM object that I am trying to wrap in a C# class in order to make it more readily available for other applications that wish to consume it.
I have the following code that creates an instance of the COM object, and then using reflection makes a call to a method to retrieve user data. This code works fine when it is located in an aspx page.
object jdObj = Server.CreateObject("jd_api.UserCookie");
string username = jdObj.GetType().InvokeMember("GetUserName", System.Reflection.BindingFlags.InvokeMethod, null, jdObj , null).ToString();
However, when I move the code to a class file (JD_API.cs) in order to abstract it from the actual website, I can no longer get it to work. For example, I have the following static method that is declared like such:
public static string GetUserName() {
object jdObj = Server.CreateObject("jd_api.UserCookie");
string username = jdObj.GetType().InvokeMember("GetUserName",
System.Reflection.BindingFlags.InvokeMethod, null, jdObj , null).ToString();
return username;
}
Unfortunately, the Server object is restricted to some ASP.NET libraries that are included by default in web applications, and so the above code was a no-go. So at this point I decided to try to create an instance of the COM object like such:
public static string GetUserName() {
Type type = Type.GetTypeFromProgID("jd_api.UserCookie");
object jdObj = Activator.CreateInstance(type);
string username = jdObj.GetType().InvokeMember("GetUserName", System.Reflection.BindingFlags.InvokeMethod, null, jdObj , null).ToString();
return username;
}
However at runtime I get an error that says, "Attempted to read or write protected memory. This is often an indication that other memory is corrupt.".
I'm not sure where to go from here. Any help on how to abstract creating an instance of this COM object to a layer that is not within the web application itself would greatly appreciated. Thanks!!
Declare DLL functions within a class. Then define a static method for each DLL function you want to call.
The following code sample creates a wrapper named Win32MessageBox that calls the MessageBox function in User32.dll each time a .NET app calls the object Show method.
It requeres the System.Runtime.InteropServices namespace.
using System;
using System.Runtime.InteropServices;
class Win32MessageBox
{
[DllImport("user32.dll")]
private static extern int MessageBox(IntPtr hWnd, String text,
String caption, uint type);
public static void Show(string message, string caption)
{
MessageBox(new IntPtr(0), message, caption, 0);
}
}
To call it, just type:
Win32MessageBox.Show("StackOverflow!", "my stack box");
The method where you call the above line doesn't need to be aware that it's a actually calling a function in an unmanaged DLL.
Resources: the MCTS Self-Paced Training Kit (Exam 70-536) by Tony Northrup.
Hove you tried usinsing interoperating
I've done the following in the past (working from memory so you might need to fiddle with this a bit):
Right Click "References" in your project
Select "Add Reference"
Selelct the "Com" Tab
Find and add your Com Instnace
In your class file
using yourComName;
public static string GetUserName()
{
yourComName.yourComClass jdObj = new yourComClass();
string username = jdObj.GetUserName(someParameters);
return username;
}
Hope this a) works and b) helps!
(C++/Qt) I have a smart pointer to a QObject. Let's say a QWeakPointer. For some external reason (something that might happen in another object or due to an event), it is possible that the pointed object gets destroyed. Since I have a smart pointer there will be no dangling reference, so there's no problem. But I always have to check if the pointer is null or not.
I'm thinking of using the null pattern in order to avoid checking this all the time but I'm not sure if this is possible or convenient with a QObject. The idea would be that the pointer points to the object and in case it gets destroyed, the smart pointer changes its pointed object to a null object. Is this a good idea or should I forget it and just check if the pointer is NULL all the time?
Let's show you an example. We have a worker who uses a tool to do its work:
class Worker : public QObject
{
Q_OBJECT
public:
Worker(QObject *parent = 0);
void work()
{
if(m_tool)
m_tool->use();
emit workCompleted();
};
signals:
workCompleted();
public slots:
void setTool(QWeakPointer<Tool> tool);
private:
QWeakPointer<Tool> m_tool;
};
class Tool : public QObject
{
Q_OBJECT
public:
Tool();
public slots:
void use() =0;
};
class Screwdriver : public Tool
{
Q_OBJECT
public:
Screwdriver() : Tool();
public slots:
void use()
{
// do something
};
};
class Hammer : public Tool;
class Saw : public Tool;
...
In this case, the Tool is a public domain object of a library, which is used by the Worker. I'm developing such library. So the worker is using a screwdriver but it gets broken and gets destroyed. No problem:
if(m_tool)
m_tool->use();
emit workCompleted();
m_tool is 0 so it simply does nothing. But we have to check that it's not null everytime.
Now let's say we had a NullTool object:
class NullTool : public Tool
{
Q_OBJECT
public:
NullTool() : Tool();
public slots:
void use()
{
// does nothing
};
};
When the tool was destroyed, our pointer would be smart and would know it should point to a NullTool instance. So Worker::work() could be implemented like this:
void Worker::work()
{
m_tool->use();
emit workCompleted();
};
m_tool->use() would then get called on the NullTool which does nothing, so there would be no need to check the pointer is not null.
Is this a good idea? Is it possible with the smart pointer classes Qt provides or should I subclass QWeakPointer?
I think the null object pattern makes most sense for value-like classes. Examples are QString or QVariant, were you don't want to have code like if ( str && !str->isEmpty() ) but just do if ( !str.isEmpty() ). For QObjects, which are not values but have "an identity", I never found this useful.
I don't understand clearly your use case, but your program can be signaled when the object has been destroy by connecting the following signal from QObject:
void destroyed ( QObject * obj = 0 );
I don't see any problem in your idea. You just have to compare the work that it takes to implement it compared to the work for checking the pointer every time. Let's your checking the pointer 10.000 times it's a good idea to use your approach. Side note: Your null object pattern rely on the fact that Tool::use() has no side effects whatsoever.
Take care that possible side affects in Tool::use() don't get in the way when you replace it polymorphically with NullTool::use(). In other words: Be sure you don't break the Liskov Substitution Principle.