/*Graph.h*/ template&lt;typename Tv, typename Te&gt; class Graph { public:     virtual ~Graph() = 0;      /*顶点操作*/     ...      /*边操作*/     ...  protected:     int n;  //顶点数     int e;  //边数 };  #pragma once #include &quot;Graph.h&quot; #include &lt;vector&gt; #include &lt;iostream&gt;  template&lt;typename Tv&gt; struct Vertex {     ... };  template&lt;typename Te&gt; struct Edge {     ... };   template&lt;typename Tv, typename Te&gt; class GraphMatrix : public Graph&lt;Tv, Te&gt; { public: ...     ~GraphMatrix() {         for (int i = 0; i != n; ++i)         {             for (int j = 0; j != n; ++j)             {                 delete E[i][j];             }         }     } ... };  when i Comment out virtual ~Graph() = 0; in Graph.h, I can run the program again. The complier said 错误  LNK2019 无法解析的外部符号 &quot;public: virtual __thiscall Graph&lt;char,int&gt;::~Graph&lt;char,int&gt;(void)&quot; (??1?$Graph@DH@@UAE@XZ)，该符号在函数 &quot;public: virtual __thiscall
Is it wrong to write:  class A { public:     virtual ~A() = 0; };   for an abstract base class?  At least that compiles in MSVC... Will it crash at run time?
How to solve differential equations of the form $f'(x) = f(x + a)$ What could one do to find analytic solutions for $f'(x) = f(x + a)$ for various values of $a$?  I know that $c_1\sin(x + c_2)$ is solution when $a = \frac{1}{2}\pi$, and of course $c_1e^x$ when $a = 0$.  For instance, is there a function satisfying $f'(x) = f(x + 1)$?  What about negative or imaginary $a$?  Is there possibly a generalization? Build a basic Python iterator Images are escaping from the help center privilege page Images included in the fully expanded privilege pages are escaping from the body.    This is most noticeable on the  (which I know is going away soon anyway):    But is also apparent on:       In the edit page preview, the images sit correctly within the borders.  Chrome 27, Windows 7 Why the middle mouse button in Blender 2.80 changes the orthographic view to perspective? If I choose an orthographic view by using the numpad and then try to orbit the geometry by clicking the middle mouse button Blender goes from orthographic to perspective. Why is that the default now? Wouldn't it be more logical if instead of that Blender kept the orthographic mode? In other words: Why Auto Perspective is turned on by default? Why is energy in a wave proportional to amplitude squared? I'm a mathematics student trying to grasp some basics about wave propagation. A sentence I find very often in introductive physics textbooks is the following:      In a wave, energy is proportional to amplitude squared.   This is something I would like to understand better in the case of mechanical (linear) waves.  The simplest model is the vibrating string of mass density $\mu$ and tension $T$: here an element of string of rest length $dx$ and vertical displacement $y(x, t)$ possesses a kinetic energy $\frac{1}{2}\mu dx \left( \frac{\partial y}{\partial t}\right)^2$ and a potential elastic energy $\frac{1}{2}T dx \left( \frac{\partial y}{\partial x}\right)^2$. So we have the total energy   $$dE=\frac{1}{2}\mu dx \left( \frac{\partial y}{\partial t}\right)^2+\frac{1}{2}T dx \left( \frac{\partial y}{\partial x}\right)^2,$$  which completely explains the sentence. Can we obtain a similar formula for higher-dimensional waves? How should I modify the above formula for a (simplified model of an) elastic membrane, for example? I would expect something like   $$dE=\text{some constant}\left(\frac{\partial z}{\partial t}\right)^2+\text{some other constant}\left\lvert\nabla z\right\rvert^2,$$  where $z=z(x, y, t)$ is the vertical displacement. Am I right? My motherboard currently supports both SATA II and III (3 Gb/s and 6Gb/s respectively), but after reading some online, it seems as though connecting a HDD to the 6 Gb/s is pretty much pointless. If this is the case I'm going to leave my two SATA III ports open for something that can actually use it (which at this point I'm not really sure what can).  This is part of an article I read:     In terms of everything else, we saw basically no difference from the same single drive being plugged into a SATA 6Gb/s or SATA 3Gb/s port. All of this is obviously due to the hard drive not being able to actually take advantage of the 6Gb/s bus.   from . Partitions of $n$ into distinct odd and even parts proof Let $p_\text{odd}(n)$ denote the number of partitions of $n$ into an odd number of parts, and let $p_\text{even}(n)$ denote the number of partitions of $n$ into an even number of parts.  How do I prove that    $|p_\text{even}(n) - p_\text{odd}(n)|$ is equal to the partitions of $n$ into distinct odd parts. Show that the number of partitions of $n$ for which no part appears exactly once is equal to the number of partitions of n for which every part is divisible by 2 or 3. Show that the number of partitions of $n$ for which no part appears more than twice is equal to the number of partitions of $n$ for which no part is divisible by 3. Tips to extend battery life for laptops and notebooks What is the difference between "used space", "size" and "size on disk"? There is only one partition for both system files and personal data, which is C:. The other two partitions are for boot and for recovery.  Now I have two different reports on the size of C: on my Windows 7:   by clicking the property of C:, it shows 59.4 GB for "used space"; by selecting all contents (I have setup to view all hidden files and directories) under C: and viewing their property, it shows 46.9 GB for "size" and 47.6 GB for "size on disk".   Here are my questions:   I was wondering why the two reports are not the same, and where the difference comes? in the second way, why "size" and "size on disk" are different? What do they mean actually? What is the difference between them and "used space" in the first way?   Thanks in advance! What happened to the Yin half of Kurama? Did Naruto get both halves of Kurama after fighting Sasuke, or did the other half of Kurama die when Minato was killed? I am looking to display my iPad screen wirelessly to an external display to demo applications.  I found  showing exactly what I want to do with an iPhone but no explanation as to what the hardware dongle used was.  The main thing is that there isn't a wire tethering the user to the external display. How does Cypher exit/enter the Matrix without an operator? In The Matrix, Cypher enters the Matrix to speak with the Agents, then leaves again. We never see if he has an operator, and as he kills Tank and Dozer you might assume he doesn't have one.  If he didn't have an operator how does he jack in/out of the Matrix? Determine the Set of a Sum of Numbers I want to determine the set of natural numbers that can be expressed as the sum of some non-negative number of 3s and 5s.  $$S=\{3k+5j∣k,j∈\mathbb{N}∪\{0\}\}$$  I want to check whether that would be: 0,3, 5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and so on.  Meaning that it would include 0, 3, 5, 8. Then from 9 and on, every Natural Number. But how would I explain it as a set? or prove that these are the numbers in the set? Is Einstein's Special Relativity completely inclusive of Newton's 3 laws of motion? Relativity has always been explained to me (in books I've read, etc) as a superset of newton's laws - that is; it encapsulates all of Newton's mechanics in addition to other effects (observer effect, time dilation, space-time geometry, etc).   I can kind of imagine 2 of Newton's 3 laws of motion to be incapsulated within Einstein's Special Relativity, but the one about "for every action there is an equal and opposite reaction", I'm struggling to find a place for that within Special Relativity.   Does this sit outside Special Relativity's explanatory power or is it inferred within somehow? When to use ' (or quote) in Lisp? After making it through the major parts of an introductory Lisp book, I still couldn't understand what the special operator (quote) (or equivalent ') function does, yet this has been all over Lisp code that I've seen.    What does it do? Knowing the interface of ubuntu and editing them Where are the files for the desktop or the interface that appears on the screen stored? And how can we edit them. How are they programmed?  What programming language is used to built the interfaces? Is there a Google Drive client available? No option to remove Bluetooth device in iOS Settings app? I would like to remove my Raspberry Pi from my iPhone 8 (running iOS 11.2.1) as a connected bluetooth device.  Normally, I would go into my Settings -> Bluetooth and select the Info button (a small blue circled "i") to show the "Disconnect" and "Forget Device" dialogs:    Unfortunately, iOS does not give me this option for my device. The bluetooth device list does not contain an "i" for the item, and there is seemingly no other way to disconnect.    How do I unpair this device if there is no button to trigger to the unpair prompt?  Due to work restrictions I cannot completely reset the device. Let $f(z)$ be entire function. Show that if $f(z)$ is real when $|z| = 1$, then $f(z)$ must be a constant function using Maximum Modulus theorem Let $f(z)$ be entire function. Consider the functions $e^{if(z)}$ and $e^{−if(z)}$ and applying the Maximum Modulus Theorem, show that if $f(z)$ is real when $|z| = 1$, then $f(z)$ must be a constant function.  (We take $f(z)=u(z)+iv(z)$)  I am confused as so far I have $|g(z)|=|e^{if(z)}|=|e^{-v(z)}|$ and then since $f(z)$ is real, $f(z)=u(z)$ and $v(z)=0$ so I assumed it would follow that $|g(z)|=|e^{v(z)}|=1$.  Similarly, $|g(z)|=|e^{-if(z)}|=|e^{v(z)}|=1$.  Using Liouville I assumed one could say that both $g(z)$ and $h(z)$ are bounded entire functions, they are constant and so it follows that $v(z)$ is constant, meaning that both its partial derivatives are equal to 0 and, due to Cauchy Riemann, both of the partial derivatives of $u(z)$ are equal to zero.  It would then follow that $f(z)$ is constant.  I don't know how to go about the question using the Maximum Modulus Theorem, also I feel I am overlooking the importance of $|z|=1$ perhaps?  Any help would be much appreciated!! How do you calculate the probability density function of the maximum of a sample of IID uniform random variables? Given the random variable  $$Y = \max(X_1, X_2, \ldots, X_n)$$  where $X_i$ are IID uniform variables, how do I calculate the PDF of $Y$?