having 2 header files:  header1.h #pragma once #include "header2.h" class A {   public:    B b; };  header2.h #pragma once #include "header1.h" class B {   public:    A a; };   running such code in visual studio gives several error like 'B' unknown override specifier. Why is it that including the two classes mutually in each other gives error?
I'm a C++ newbie, but I wasn't able to find the answer to this (most likely trivial) question online.  I am having some trouble compiling some code where two classes include each other.  To begin, should my #include statements go inside or outside of my macros?  In practice, this hasn't seemed to matter.  However, in this particular case, I am having trouble.  Putting the #include statements outside of the macros causes the compiler to recurse and gives me "#include nested too deeply" errors.  This seems to makes sense to me since neither class has been fully defined before #include has been invoked.  However, strangely, when I try to put them inside, I am unable to declare a type of one of the classes, for it is not recognized.  Here is, in essence, what I'm trying to compile:  A.h  #ifndef A_H_ #define A_H_  #include "B.h"  class A {     private:         B b;      public:         A() : b(*this) {} };  #endif /*A_H_*/   B.h  #ifndef B_H_ #define B_H_  #include "A.h"  class B {     private:             A&amp; a;      public:         B(A&amp; a) : a(a) {}  };  #endif /*B_H_*/   main.cpp  #include "A.h"  int main() {     A a; }   If it makes a difference, I am using g++ 4.3.2.  And just to be clear, in general, where should #include statements go?  I have always seen them go outside of the macros, but the scenario I described clearly seems to break this principle.  Thanks to any helpers in advance!  Please allow me to clarify my intent if I have made any silly mistakes!
Fix corrupt NTFS partition without Windows MY NTFS Partition has gotten corrupt somehow (it's a relic from the days when I had Windows installed).     I'm putting the debug output of fdisk and blkid .  At the same time, any OS is unable to mount my root partition, which is located next to my NTFS partition. I'm not sure if this has anything to do with it, though. I get the following error while trying to mount my root partition (sda5)  mount: wrong fs type, bad option, bad superblock on /dev/sda5,        missing codepage or helper program, or other error        In some cases useful info is found in syslog - try        dmesg | tail  or so  ubuntu@ubuntu:~$ dmesg | tail [ 1019.726530] Descriptor sense data with sense descriptors (in hex): [ 1019.726533]         72 03 11 04 00 00 00 0c 00 0a 80 00 00 00 00 00  [ 1019.726551]         1a 3e ed 92  [ 1019.726558] sd 0:0:0:0: [sda]  Add. Sense: Unrecovered read error - auto reallocate failed [ 1019.726568] sd 0:0:0:0: [sda] CDB: Read(10): 28 00 1a 3e ed 40 00 01 00 00 [ 1019.726584] end_request: I/O error, dev sda, sector 440331666 [ 1019.726602] JBD: Failed to read block at offset 462 [ 1019.726609] ata1: EH complete [ 1019.726612] JBD: recovery failed [ 1019.726617] EXT4-fs (sda5): error loading journal   When I open gparted (using live CD), I get an exclamation next to my NTFS drive which states   Is there a way to run chkdsk without using windows ?  My attempt to run fsck results in the following :  ubuntu@ubuntu:~$ sudo fsck /dev/sda fsck from util-linux-ng 2.17.2 e2fsck 1.41.14 (22-Dec-2010) fsck.ext2: Superblock invalid, trying backup blocks... fsck.ext2: Bad magic number in super-block while trying to open /dev/sda  The superblock could not be read or does not describe a correct ext2 filesystem.  If the device is valid and it really contains an ext2 filesystem (and not swap or ufs or something else), then the superblock is corrupt, and you might try running e2fsck with an alternate superblock:     e2fsck -b 8193 &lt;device&gt;   Update: I was able to fix the NTFS partition running chkdsk off Hiren's BootCD, but it seems that the superblock problem still remains.  Update 2: Fixed superblock issue using e2fsck -c /dev/sda5 Standard example where Jacobson radical not equal nilradical Is there a standard example in commutative algebra of a ring where the jacobson radical does not equal the nilradical? I came across this problem when I was at a book store inside of a book made to prepare Berkeley graduates to pass a mandatory exam. I wanted to buy the book, but, alas, I didn't have the money (forty bucks is a lot of money when you don't have a job). So I took my phone out and started taking as many pictures as I could. Unfortunately, I didn't take any pictures of the solutions!  Enough backstory. Time for math. The problem is as follows:  Prove that $\cos^p( \theta) \le \cos(p \theta)$ if $0\le\theta\le\frac\pi2$ and $0\le p\le 1$.  I tried using the series expansion for cosine, but that seemed to be a dead end. Then I tried using Euler's theorem, but I got stuck. Then I got distracted and started to think about other cosine identities. For example, $$\cos\left(\frac\theta2\right)=\pm\sqrt{\frac{1+\cos(\theta)}{2}}.$$ Then what is $\cos(\frac\theta3)$ equal to? I tried to figure it out then I realized that I needed to find the root of a cubic polynomial. Then I realized that I'm better at staying focused than finding the roots of cubic polynomials.  Anyways, a proof would be nice here. I really appreciate any hints or answers. I apologize for my digressions! How to get correct scale values? The scale of my map is wrong, no matter what coordinate system I use in my project. The map and all objects show the correct coordinates but the scale of the map shows nonsense and the measurement tool gives wrong dimension.   The map should be like 1:10000 but qgis shows 1:1. Anyone an idea? I am doing exercise 3.24 of Marcus which is the following.           Let $L,K$ be number fields with $L/K$ a finite extension (of degree $[L:K] = n$) with $R = \mathcal{O}_K$ and $S = \mathcal{O}_L$. A  prime $P$ of $R$ is said to be totally ramified in $L$ iff $PS = Q^n$ for some prime $Q$ of $S$.           (a) Show that if $P$ is totally ramified in $L$ then it is totally ramified in $M$ for any intermediate field $M$, $K \subseteq M \subseteq L$.          (b) If $P$ is totally ramified in $L$ and unramified in another extension $L'$ of $K$ then $L \cap L'= K$.           (c) Give a new proof that $\Bbb{Q}(w)$ where $w = e^{2\pi i/m}$ has degree $\varphi(m)$ over $\Bbb{Q}$ (First do it for $m = p^r$ by using the fact that $(p) = (1- w)^{\varphi(m)}$ and then build up to any $m$ by using (b) above.      Now I have done (a) and (b) but when I come to (c) I fail to see how the problem is essentially circular. Even in the case $m = p^r$, I would have to use the fact that the ring of integers of $\Bbb{Q}(w)$ is $\Bbb{Z}[w]$. The proof of this fact already uses knowledge of $[\Bbb{Q}(w) : \Bbb{Q}]$ being equal to $\varphi(m)$. Am I missing something here?  Thanks. How do you add a timed delay to a C++ program? I am trying to add a timed delay in a C++ program, and was wondering if anyone has any suggestions on what I can try or information I can look at?  I wish I had more details on how I am implementing this timed delay, but until I have more information on how to add a timed delay I am not sure on how I should even attempt to implement this. I'm applying for B1/B2 visa. In DS-160, it asks for      Monthly Salary in Local Currency (if employed)   However it doesn't mention anywhere if this should be gross (before tax deductions) or net (after tax deductions, whatever is credited into my account every month).  What should I fill here? Intuitive explanation of variance and moment in Probability While I understand the intuition behind expectation, I don't really understand the meaning of variance and moment.   What is a good way to think of those two terms? macOS: Is there a way to show an image constantly on screen (like a watermark)? I'm looking for a very simple way to add a watermark to my screen which should be shown whatever I'm doing, is it during a Zoom call, or is it when giving a presentation, or similar (I don't necessarily need it to show during fullscreen mode). The reason is that I want to record my screen using QuickTime Player, and the watermark should be on the video. I know that there are many post-production tools that offer adding watermarks, but I don't want to do any post-production, I want to have everything on screen while recording already. (I also know that there are screen recorders which offer placing a watermark to the end result. I'd rather also avoid using another screen recording software, but if I don't find any other way to do it, then I'm open to a simple, non-pricy alternative to QuickTime Player.) This post is an expansion of  For many graduated sites (eg Physics), moderators do not want to apply the unilateral close vote in all cases--it is better to let some community votes pile on first.  We only use the unilateral in obvious cases then. This renders the CV queue useless to us. If we &quot;skip&quot; something, we can't get back to it. If we say &quot;do not close&quot;, it's removed from the queue and our community can't see it. If we use &quot;close&quot;, we are using a unilateral mod vote--we don't want to do that. What I propose is a set of &quot;next/previous&quot; buttons, that let us navigate the queues, looking for closeable questions, and coming back to the others later. Thoughts? Update and bounty reason Due to the recent Great Flag Overhaul, mods no longer see close flags in the flag queue. Which means that the only way to get to them is via the review queue 1 Shog's solution is ideal in this case; let mods view the close queue as a list. (In fact, a list similar to the old flag queue -- which shows a list of mods who have seen the post and what mod actions have already been taken -- would be great) On Physics, the majority of the flags that we get are VTC flags. And the community isn't always able to close things on their own, so we have to step in. However, powering through the close queue isn't an option; because &quot;skip&quot;ing a post2 leads to it being dumped into the void (I can't get back to it unless I have a link). Extremely inconvenient. Could the workflow here be improved? 1. The /tools listing for posts with outstanding close votes doesn't work because it doesn't list posts which have been flagged but not VTCd 2. which I may want to take a closer look at later; gotta be careful with the binding votes. Switches needed in an n-location multiway switching system we can construct a two location multiway switching system with two 3-way switches:    They also teach us how to perform three location multiway switching with two 3-way switches and one 4-way switch:    Is it possible to make a similar system (three locations, all able to switch the light in any situation) with only three 3-way switches?   What can be said in general about the switches needed to make an n-location multiway switching system? Beamer Bibliography Icon I want to list my references in a beamer presentation using thebibliography environment, with the nice little    book icon (\beamertemplatebookbibitems) and  article icon (\beamertemplatearticlebibitems)   in front of each source.  Is there an icon for websites too? something like \beamertemplatewebsitebibitems? If not, is there a way to implement this easily ? No more updates for Google Chrome 32-bit I am using Google Chrome 48.0 32-bit. Today when I launched Google Chrome, I got this message:    This computer will soon stop receiving Google Chrome updates because this Linux system will no longer be supported   I thought Ubuntu 14.04 is supported for five years, what's exactly happening here?  And on 64-bit Ubuntu, I get an error from Update Manager and apt-get update:  Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/Release  Unable to find expected entry 'main/binary-i386/Packages' in Release file (Wrong sources.list entry or malformed file)   Some index files failed to download.   They have been ignored, or old ones used instead. "Deleted by {owner}" vs "Deleted by owner" in self deleted answers What is "my guy syndrome" and how do I handle it? I know some nice ways to prove that $\zeta(2) = \sum_{n=1}^{\infty} \frac{1}{n^2} = \pi^2/6$.  For example, see  or the answers to the question "?"  Are there any nice ways to prove that $$\zeta(4) = \sum_{n=1}^{\infty} \frac{1}{n^4} = \frac{\pi^4}{90}?$$  I already know some proofs that give all values of $\zeta(n)$ for positive even integers $n$ (like #7 on Robin Chapman's list or Qiaochu Yuan's answer in the linked question).  I'm not so much interested in those kinds of proofs as I am those that are specifically for $\zeta(4)$.  I would be particularly interested in a proof that isn't an adaption of one that $\zeta(2) = \pi^2/6$. I accepted an answer to this question:    but the bounty was awarded by Community&thinsp;♦ to another answer. Why did this happen? Let $(G,.)$ be a group and $m,n \in\mathbb Z$ such that $\gcd(m,n)=1$. Assume that $$ \forall a,b \in G, \,a^mb^m=b^ma^m,$$ $$\forall a,b \in G, \, a^nb^n=b^na^n.$$  Then how prove $G$ is an abelian group?  Some context: Some of these commutation relations often imply that $G$ is abelian, for example , or . This looks like another example of this phenomenon, but the same techniques do not apply. Springer llncs TOC not working I can't get the TOC to work. I remember it worked before, but there was something I did (unfortunately I can't remember what it was), and after that the TOC just stopped working.  Here is a part from my document:  \documentclass[deutsch]{llncs}  \usepackage{graphicx} \usepackage[labelformat=empty]{caption} \usepackage{subcaption} \captionsetup{compatibility=false}  \bibliographystyle{unsrt}  \pagestyle{headings} \begin{document}  \sffamily  \begin{titlepage}     %Some stuff here \end{titlepage}  \thispagestyle{headings} \tableofcontents \newpage %more stuff here blablabla \section{test} \subsection{test2} \end{document} $R/Ra$ is an injective module over itself Let $R$ be a PID, $a\in R$ be a nonzero nonunit in $R$. Prove that $R/Ra$ is an injective module over itself.   If $R$ is a PID, every $R$- divisible module is injective, but the question concerns with $R/Ra$-module, so I have no idea to solve this problem. Help me.