If I am NOT using a reference to a pointer when sending the base-pointer to another class constructor - and in this class binding the basepointer to a derived object of the base - the basepointer returns NULL.   Simply - what is the reason for this?   #include &lt;iostream&gt; using namespace std;  class Base {    public:       virtual void print() = 0; };  class Sub1 : public Base { public:    void print() {       cout &lt;&lt; "I am allocated!\n";   } };  class App { public:     App(Base *b) {       b = new Sub1;    } };  int main() {    Base *b;    b = NULL;     App app(b);     if (b != NULL)       b-&gt;print();    else       cout &lt;&lt; "Basepointer is NULL\n";     return 0; }   The crucial part here is the signature of the App-class constructor  when not using a reference to the basepointer i.e     App(Base *b)   the output is:     Basepointer is NULL   And when using a reference to the base-class i.e.    App(Base *&amp;b)   the output is:   I am allocated!
I would like to clarify the differences between by value and by reference.  I drew a picture     So, for passing by value,  a copy of an identical object is created with a different reference, and the local variable is assigned the new reference, so to point to the new copy  How to understand the words: " If the function modifies that value, the modifications appear also within the scope of the calling function for both passing by value and by reference "  Thanks!
Prove that $f'(0)$ exists and $f'(0) = b/(a - 1)$ Problem:   If $f(x)$ is continous at $x=0$, and $\lim\limits_{x\to 0} \dfrac{f(ax)-f(x)}{x}=b$, $a, b$ are constants and $|a|&gt;1$, prove that $f'(0)$ exists and $f'(0)=\dfrac{b}{a-1}$.   This approach is definitely wrong:    \begin{align} b&amp;=\lim_{x\to 0} \frac{f(ax)-f(x)}{x}\\ &amp;=\lim_{x\to 0} \frac{f(ax)-f(0)-(f(x)-f(0))}{x}\\ &amp;=af'(0)-f'(0)\\ &amp;=(a-1)f'(0) \end{align}  I will show you a case why this approach is wrong:     \[f(x)= \begin{cases} 1,&amp;x\neq0\\ 0,&amp;x=0 \end{cases}\]   $\lim_{x\to0}\dfrac{f(3x)-f(x)}{x}=\lim_{x\to0} \dfrac{1-1}{x}=0$   but $\lim_{x\to0}\dfrac{f(3x)}{x}=\infty$,$\lim_{x\to0}\dfrac{f(x)}{x}=\infty$   Does anyone know how to prove it? Thanks in advance! Derive a formula to find the number of trailing zeroes in $n!$ Possible Duplicate:          I know that I have to find the number of factors of $5$'s, $25$'s, $125$'s etc. in order to do this. But how can you derive such a formula for any number $n$? Can I make any injective function bijective by patching the codomain? If I know that a certain key has been pressed (eg Key.D3), and that the Shift key is also down (Keyboard.IsKeyDown(Key.LeftShift) || Keyboard.IsKeyDown(Key.RightShift)), how can I find out what character that refers to (eg, # on US keyboard, UK pound sign on UK keyboard, etc)?  Put another way, how can I find out, programatically, that Shift + 3 produces # (it wouldn't on a non-US keyboard). Prove that $ AB$ is a subgroup of order $mn$ if $m$ and $n$ are relatively prime. Is it correct to say "I slump/slumped on/onto the stool/barstool?" I wrote this sentence: "I buy a bottle of beer and slump on/onto the last stool."  A native English speaker told me that "slump" doesn't work here. That you can't slump on a bar/barstool.  Is this correct? If so, why? Which types of particles are affected by the wave-particle duality? According to the special theory of relativity, distant simultaneity depends on the observer's reference frame.  And, according to the quantum theory, in the case of two entangled particles, a measure on one of the particles simultaneously affects the second one. Under which reference frame is this simultaneous? I want to block access to my web server by default as a precaution but I keep getting the following errors showing up in my error log.      [Wed Jun 27 23:30:54 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/Edu.jar      [Wed Jun 27 23:32:40 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/REST.jar      [Wed Jun 27 23:35:39 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/Set.jar      [Thu Jun 28 01:01:17 2012] [error] [client 58.218.199.227] client denied by server configuration: /home/www/default/proxyheader.php      [Thu Jun 28 02:34:57 2012] [error] [client 58.218.199.227] client denied by server configuration: /home/www/default/proxy.php      [Thu Jun 28 05:41:33 2012] [error] [client 58.218.199.227] client denied by server configuration: /home/www/default/proxyheader.php      [Thu Jun 28 06:55:10 2012] [error] [client 180.76.6.20] client denied by server configuration: /home/www/default/      [Thu Jun 28 07:31:26 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/Edu.jar      [Thu Jun 28 07:32:25 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/REST.jar      [Thu Jun 28 07:36:10 2012] [error] [client 86.77.20.107] client denied by server configuration: /home/www/default/Set.jar   I don't really want these errors to show up but whatever I do, I can't get rid of them. Does anyone know how I can achieve this?  Here is a copy of my configuration.  &lt;VirtualHost *:80&gt;     DocumentRoot /home/www/default      &lt;Directory /&gt;            AllowOverride None            Order Deny,Allow            Deny from all     &lt;/Directory&gt;      #ErrorLog /var/log/apache2/error.log     #LogLevel warn     CustomLog /var/log/apache2/access.log combined &lt;/VirtualHost&gt; Is "89.9%" singular or plural? I’m writing a dissertation on network security. I’m unsure which of the following to use:   ...89.9%, which is 57 million users.  ...89.9%, which are 57 million users.   Technically it should be are because it refers to a plural amount of people, but it doesn’t sound right to me considering that 57 million is in a sense one group. How to connect to Salesforce REST API without OAuth? How does Jitterbit do it? Derivation of $E=pc$ for a massless particle? In classical mechanics, massless particles don't exist because for $m=0$, $p=0$.  The relativistic relation between energy, mass and spatial momentum is: $E^2= (pc)^2 + (mc^2)^2$ . So it is said that setting $m=0$ in the first equation you get $E=pc$.  How could setting $m=0$ in that equation give you $E=pc$ whilst $p$ appears in the equation and we know $p=γmu$? If you set $m=0$ you will have indeterminacy due to "$γm$". It seems to me like we are doing a "trick" in order to get the $E=pc$. Perhaps there is another proof for this relation? What is the monomorphism restriction? Windows 8 Setting an image on the login screen In Windows 8.1 when you log in there's the prelogin screen that has an image you can change, but then there's also the login (password entry) screen that as far as I can see is just a solid color.  How could I replace that with an actual picture? (ie, if I wanted a company logo to be displayed in the large space underneath the password entry box. On determinants and common divisors In JavaScript:  encodeURIComponent("©√") == "%C2%A9%E2%88%9A"   Is there an equivalent for C# applications? For escaping HTML characters I used:  txtOut.Text = Regex.Replace(txtIn.Text, @"[\u0080-\uFFFF]",     m =&gt; @"&amp;#" + ((int)m.Value[0]).ToString() + ";");   But I'm not sure how to convert the match to the correct hexadecimal format that JS uses. For example this code:  txtOut.Text = Regex.Replace(txtIn.Text, @"[\u0080-\uFFFF]",     m =&gt; @"%" + String.Format("{0:x}", ((int)m.Value[0])));   Returns "%a9%221a" for "©√" instead of "%C2%A9%E2%88%9A". It looks like I need to split the string up into bytes or something.  Edit: This is for a windows app, the only items available in System.Web are: AspNetHostingPermission, AspNetHostingPermissionAttribute, and AspNetHostingPermissionLevel. Physical Explanation for "Kolmogorov -5/3 spectrum" in Fluid Mechanics According to Kolmogorov, the energy spectrum function of a turbulent fluid is given as,  $E(k)=C\epsilon^{\frac{2}{3}}k^{\frac{-5}{3}}$  where $\epsilon$ is the energy flux and $k=\frac{2\pi}{r}$ where $r$ is the length scale.   The normal explanation I see in most physics texts and articles is that the -5/3 exponent is found purely through dimensional analysis. Obviously dimensional analysis is very useful, but often times there is a more physical explanation as well. Does anyone have any insights? Can I have a hint on how to construct a ring $A$ such that there are $a, b \in A$ for which $ab = 1$ but $ba \neq 1$, please? It seems that square matrices over a field are out of question because of the determinants, and that implies that no faithful finite-dimensional representation must exist, and my imagination seems to have given up on me :) adding an item to filtered table My project has a table with '+add' button and filtering. The add button opens a dialog to add item, and then to 'save' which closes it too Today I noticed the following flow:  Filter table on subject with value 'dummySubject'  Add new item with subject 'somethingelse'  save and close the new item ==&gt; the new item is shown in the table   So the outcome is that an item that does not qualify the filtering appears in the table. When I asked around I got the following answer  We don't want the new item to 'disappear' on creation just because it does not qualify the filtering. So we display it If you filter again, it will disappear as it should  I can almost understand that logic, but still it sounds wrong. I would rather have like a toaster saying 'new item create' with a 'goto' link. My question is  is the current user experience considered a reasonable one? what is the best practice way to handle such scenario?      &ndash; Wireframes created with Is there any way, shape, or form that ultraviolet light is recorded by a digital sensor?  Ideally, this question is related strictly to the sensor, without even a glass lens in between which could cut or block UV light.  But of course I don't take pictures with a bare sensor, I also have a lens, so I'd also like to know how much UV gets to the sensor to begin with.