Alex Rivera | Logout

Difference between creating object with () or without

Asked 2011-02-25T11:15:25.963
24

i just run into the problem 

error: request for member ‘show’ in ‘myWindow’, which is of non-class type ‘MainGUIWindow()’

when trying to compile a simple qt-application:

#include <QApplication>
#include "gui/MainGUIWindow.h"


int main( int argc, char** argv )
{
  QApplication app( argc, argv );


  MainGUIWindow myWindow();
  myWindow.show();


  return app.exec();
}

I solved this by replacing 

MainGUIWindow myWindow();

by

MainGUIWindow myWindow;

but I don't understand the difference. My question: What is the difference?

Regards, Dirk

c++ qt most-vexing-parse
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1 Answer

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In C++ every expression that looks like a function declaration is a declaration of a function. Consider more complex sample that in your question:

#include <iostream>

struct X
{
  X( int value ) : x(value) {}
  int x;
};

struct Y
{
  Y( const X& x ) : y(x.x) {}
  int y;
};

int main()
{
  int test = 10;
  Y var( X(test) );                 // 1
  std::cout << var.y << std::endl;  // 2

  return 0;
}

At first glance (1) is a declaration of the local variable var which should be initialized with a temporary of a type X. But this looks like a function declaration for a compiler and you will get an error in (2):

 error: request for member ‘y’ in ‘var’, which is of non-class type ‘Y(X)’

The compiler considers that (1) is the function with name var:

Y                var(             X                     test            );
^- return value  ^-function name  ^-type of an argument ^-argument name

Now, how to say to the compiler that you do not want to declare a function? You could use additional parentheses as follows:

Y var( (X(test)) );

In your case MainGUIWindow myWindow() for the compiler looks like function declaration:

MainGUIWindow    myWindow(        void                  )
^- return value  ^-function name  ^-type of an argument
answered 2011-02-28T20:53:29.597

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