Some instructions on whether to call destructor when QT window closes and application stops

I have been imitating these days QQ Make an instant messaging software , The process is not very smooth , When the window closes , Applications still exist , The application ended abnormally , Close subwindow , The main window closes , So I summarized some contents , Convenient for future access .

If you are interested in this project, you can go to GitHub：https://github.com/FdogMain/FdogInstantMessaging

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main Function , Windows created on the stack , Call destructor automatically when closing , This situation cannot be used this->setAttribute(Qt::WA_DeleteOnClose);, An exception occurs when closing , If it is created on the heap , You can use this->setAttribute(Qt::WA_DeleteOnClose), When calling destructors , There will be no exceptions .

When the window is closed close function , This function sends a close event QCloseEvent, Then the window will be hidden , Ask if you want to close , Can intercept QCloseEvent event , Which is rewriting QCloseEvent, Users can choose to close , Or cancel .

In does not add Qt::WA_DeleteOnClose when , Choose to turn off , The window will disappear , Actually called hide, If you add it , In addition to the call hide, It also calls deleteLater Method to release the window , Without this attribute ,close and hide, also servisibel The function of is the same , It just hides the window object , It doesn't destroy objects .

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stay main, Create a window on the stack A, close window A when , Will call the destructor .
If in this window A Create another window in the constructor of B, And in A In the destructor of B To release .

The first form ：

MainWindow * b = new MainWindow();

When closing the window A, Then close the window B when , establish B The destructor of is called , window A The destructor of is called （ This closing method has obvious jamming , When off A, According to the rules ,B Should be turned off , Release , however B The window is also displayed on the desktop , Multiple runs , Find that there will still exist A The problem of non execution of deconstruction （ The printed statements in the destructor are not printed on the console ）, So there are problems in this way ）

In turn, , Close the window first B, Then close the window A,B The destructor of is called , window A The destructor of is called （ This closing method has no jamming , It's actually B The window is hidden , Failure to actively perform deconstruction , And in the A Passive execution in the destructor of , That's why it's closed B when , The display does not call B destructor , Shut down A when , To display the call B Cause of destructions ）

We give the window B add to Qt::WA_DeleteOnClose Have a try （ Add... To the constructor setAttribute(Qt::WA_DeleteOnClose)）, Because of the window B It's the window on the pile , have access to setAttribute(Qt::WA_DeleteOnClose)）,

Or close the window first A, Then close the window B, Show that the window was called B Destructor of , Then there's an exception , This exception should be repeated destructions B Sent exception .

Put the window A About windows B Remove the released code , Show that the window was called B Destructor of , Call window A Destructor of , But there were no exceptions （ There's stuck , Multiple runs , Find that there will still exist A The problem of non execution of deconstruction （ The printed statements in the destructor are not printed on the console ））.

Now the reverse is true , Close the window first B, Because of the window B Set up setAttribute(Qt::WA_DeleteOnClose)） attribute , The destructor is executed immediately , Then close the window A, If nothing happens , There should be an exception , Because of the window B Has been released , In the window A Release again B Abnormal transactions , hold A Release in destructor in B The code is commented again , function , The display calls the windows in turn B Destructor of , window A Destructor of （ No caton ）.

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The second form , Specify the parent window

MainWindow * b = new MainWindow(this);

A The window destructor has no write release B Window code case ：

close A window （ Be released ）,B The window closes （ Be released ）（ No caton ）.

close B window （ Just hide ）, close A window （ Be released ）（ No caton ）.

Try again b add to setAttribute(Qt::WA_DeleteOnClose)） attribute , close B window , perform B destructor , To shut down A, perform A The analysis of （ No caton ）.

close A window , perform A The analysis of , perform B The analysis of , also B The window is closed （ No caton ）.

This is because QT The parent-child object mechanism of is at work , The reason lies in that this.

When we use the parent object to create an object , The parent object will add this object to its child object list . When the parent object is deleted , It will traverse its sub object class table and delete each sub object , Then the sub objects themselves delete their own sub objects , This calls recursively until all objects are deleted , So if new Out of the control , If a parent object is specified , We don't need to delete it manually .

Another discovery , If other controls are specified A Window as parent window , Will be embedded in A Window , however MainWindow Windows of this class will not be embedded
The opposite is not true .

But the above is only in the basic case , When I set the window property to borderless , After no taskbar and other different attributes , Close the window again , Destructors are not called automatically , In other words, the window is just closed , But the application itself has not been closed , The most obvious feature is when you close the window ,qt The output window of the application still shows red squares instead of green triangles .

At this time, you can add the following code where you want to close , The application will be closed .

    QApplication* app;
    app->quit();

There is also a situation where MainWindow Created in widget window , But one closes the last widget,MainWindow Will be shut down , Is it inconceivable , Before finding a solution , I can only judge whether it is the last widget, If it was me, I would hide , Instead of closing , for instance .

for example qq The main interface is MainWindow Double click friend generation widget window .

When closing these widget When the window comes to the last , The main window will close , One of the reasons for this symptom , Is to set the properties of the window

That is to use this function setWindowFlags(), In for a penny, in for a pound , Put this widget Add another attribute to the window setWindowFlag(Qt::CoverWindow);

This problem may not exist ~

Another problem is to close the window , The program may end abnormally , This mistake is also related to main The location of the window created in .

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