使用 () 或不使用 () 创建对象的区别

2021-12-09 00:00:00 qt c++ most-vexing-parse

我刚遇到这个问题

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

尝试编译一个简单的 qt 应用程序时:

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();
}

我通过替换

MainGUIWindow myWindow();

由

MainGUIWindow myWindow;

但我不明白其中的区别.我的问题:有什么区别?

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

问候，德克

推荐答案

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

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;

和

MainGUIWindow* p = new MainGUIWindow();

带括号的版本称为值初始化，而没有括号的版本称为默认初始化.对于非 POD 类，两者没有区别.然而，对于 POD 结构，值初始化涉及将所有成员设置为 0，

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

补充:一般来说，如果某个句法结构既可以解释为声明又可以解释为其他东西，编译器总是会以声明的方式解决歧义.

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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