Difference between creating object with () or without

Question

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

Answer 1

The other answers correctly state that the parentheses version is actually a function declaration. To understand it intuitively, suppose you wrote MainGUIWindow f(); Looks more like a function, doesn't it? :) The more interesting question is what is the difference between

MainGUIWindow* p = new MainGUIWindow; 

and

MainGUIWindow* p = new MainGUIWindow(); 

The version with parentheses is called value-initialization, whereas the version without is called default-initialization. For non-POD classes there is no difference between the two. For POD-structs, however, value-initialization involves setting all members to 0,

my2c

Addition: In general, if some syntactic construct can be interpreted both as a declaration and something else, the compiler always resolves the ambiguity in favor of the declaration.

Answer 2

The following:

MainGUIWindow myWindow(); 

declares a function that takes no arguments and returns MainGUIWindow. I.e. myWindow is a function name.

MainGUIWindow myWindow; 

on the other hand creates an object myWindow of type MainGUIWindow.