virtual | 易学教程

Where does virtual method table store in C++?

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问题 I want to know how does class object (not instances, but exactly classes) store in memory? class A { public: int a; virtual void f(); virtual ~A(); }; class B : public A { public: int b; void f() final override; }; I know, that usually (not strongly described by standard) in case of this inheritance (B derived from A) we have: memory: ....AB... where AB is a class object of B (if I understand it correctly). If we go deeper (tried with clang and gcc ), we can see something like (again, not

关于C++虚函数，纯虚函数以及模板等重要概念的深入讨论（二）

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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 2.析构与虚析构函数为了说明基类的析构函数必须为虚析构函数，我们来实践一下：在A的析构函数中加入输出cout << "This is A class." << endl;同理在B的析构函数中也加入输出cout << "This is B class." << endl; 最后在main函数objectC->print();下方加入语句：delete objectC; objectC = NULL;从而销毁对象，调用析构函数。如果基类A的析构函数非虚析构的话，运行结果如下：如果将基类A的析构函数改写为虚析构函数：virtual ~A();那么运行的结果如下：可以清晰的看到，在基类A中如果是非虚析构函数时，销毁objectC 对象的时候则没有调用B析构函数，而若为虚析构函数的话，就会调用B的析构函数。因此为了防止释放指向子类的基类指针时，子类发生内存泄露现象，我们必须将基类的析构函数设置为虚析构！ 3.虚函数与纯虚函数虚函数即是在声明函数的时候在前面加入virtual 关键字如：virtual void print(); 而纯虚函数则是这样定义的：virtual void print() = 0; 即纯虚函数没有实体，实体是需要它的继承重写来实现的。我们来看一段实例代码： // Test1.cpp :

how to redirect the output of serial console (e.g. /dev/ttyS0) to a buffer or file

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问题 Is it possible to pipe serial console output to a file or a buffer or some virtual or pseudo device (in /dev)? The Kernel command line has in startup at this point "console=null,115200". (Normally it has "console=ttyS0,115200" - my requirement is: if "console=null,115200", should the output go to some other place than ttyS0, e.g. a virtual or pseudo device or to a file/buffer) Maybe somebody know if there is good solution available? Thanks a lot in advance! 回答1: There are two ways that I am

Abstract classes and Pointers

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问题 I have a class // i want an abstract class. class Foo { public: virtual void bar()=0; }; // i want this abstract calss to be used all over the program :) to enjoy polymorphism. class EatFoo { public: vector<Foo> fooV; // not working vector<Foo *> fooPV; }; I get a compile time error that abstract class cannot be instantiated. Yes its true but i really want or i want to learn : how to make other programmers "have to - have to" implement some function and i donot want to use pointers in my

How to release the VM / private bytes of a Desktop application in c#

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问题 I have desktop application developed in C#. The VM Size used by application is very high. I want to add watermark to a pdf file, which has more that 10,000 pages, 10776 pages to be exact, the VM size inscreases and some times the application freezes or it throws out of memory exception. Is there a solution to release / decrease the VM size programatically in C# 回答1: Environment.FailFast :) In all seriousness though, a large VM size is not necessarily an indication of a memory problem. I

Genymotion Virtual Device OpenGl Error

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问题 I have installed Genymotion Virtual Device for Android Studio and also created virtual device in it ,but the problem is that virtual device(in Genymotion) is not getting started and showing the following error : Unable to start virtual device To start virtual device,make sure that your video card supports OpenGl 2.0 and update the drivers Details: Failed to initialize backend EGL display (error: 4) If possible, update your video card drivers. I tried to update the video card through device

Add arguments in a virtual getter

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问题 What I'm trying to do is something like that : Schema .virtual('getSomething') .get(function(what) { if (!what) { return this.somethingElse } else { return this.something[what] } }) The problem is that we can't pass arguments in a virtual getter, how can I achieve something like that without having to duplicate my code ? 回答1: Add it as an instance method instead of a virtual getter. schema.methods.getSomething = function(what) { if (!what) { return this.somethingElse } else { return this

Missing 'virtual' qualifier in function declarations

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问题 Whilst trawling through some old code I came across something similar to the following: class Base { public: virtual int Func(); ... }; class Derived : public Base { public: int Func(); // Missing 'virtual' qualifier ... }; The code compiles fine (MS VS2008) with no warnings (level 4) and it works as expected - Func is virtual even though the virtual qualifier is missing in the derived class. Now, other than causing some confusion, are there any dangers with this code or should I change it

How do you change the return key text in Mobile Safari keyboard without the form tag?

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问题 I would like to change the the "return" button text on the Mobile Safari keyboard when my input element is focused. I know you can do this: <form action="somewebsite.com"> <input id='SearchTextBox' type="search"/> <input id='SearchButton' type="button" value="Search" /> </form> But I don't want the form tag because the search is ajaxian (I don't want to reload the page). Does anyone know if this is possible? 回答1: Heh, should have thought of this (coworker solved). Just cancel the form

Operator== in derived class never gets called

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问题 Can someone please put me out of my misery with this? I'm trying to figure out why a derived operator== never gets called in a loop. To simplify the example, here's my Base and Derived class: class Base { // ... snipped bool operator==( const Base& other ) const { return name_ == other.name_; } }; class Derived : public Base { // ... snipped bool operator==( const Derived& other ) const { return ( static_cast<const Base&>( *this ) == static_cast<const Base&>( other ) ? age_ == other.age_ :