使用（）或不使用创建对象的区别 [英] Difference between creating object with () or without

问题描述

我刚刚遇到问题

 错误：请求成员'show'在'myWindow'当尝试编译一个简单的qt应用程序时，类型类型'MainGUIWindow（）'
  

< >

  #include< QApplication> 
 #includegui / MainGUIWindow.h
 
 
 int main（int argc，char ** argv）
 {
 QApplication app argv）; 
 
 
 MainGUIWindow myWindow（）; 
 myWindow.show（）; 
 
 
 return app.exec（）; 
} 
  

我解决了这个问题，替换

  MainGUIWindow myWindow（）; 
  

由

  MainGUIWindow myWindow; 
  

但我不明白差别。

解决方案

其他答案正确地说明括号版本实际上是一个函数声明。为了直观地理解它，假设你写了 MainGUIWindow f（）; 看起来更像一个函数，不是吗？
更有趣的问题是

之间的区别是什么。

  MainGIUWindow * p = new MainGIUWindow; 
  

和

  MainGIUWindow * p = new MainGIUWindow（）; 
 

带括号的版本称为值初始化，而不带版本的版本称为默认初始化。对于非POD类，两者之间没有区别。对于POD结构，值初始化包括将所有成员设置为0，

my2c

一般来说，如果一些句法结构可以解释为声明和别的东西，编译器 总是解决有利于声明 的歧义。

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

解决方案

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

MainGIUWindow* p = new MainGIUWindow;

and

MainGIUWindow* p = new MainGIUWindow();

The version with parentheses is called value-initialization, whereas the version without is called default-initialization. For non-POD classes there is no differnce between the two. For POD-structs, however, value-initialization involes 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.

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