I'm new to C++ OOP, and I am trying to implement a Hashmap and these are the files I have: unoreder_map.h: template &lt;typename K, typename V&gt; class unordered_map { .... public:     unordered_map();     ~unordered_map();     int size();     V getValue(K key);     void insert(K key, V value);     V remove(K key);     double getLoadFactor(); };   unordered_map.cpp: #include &quot;unordered_map.h&quot; template&lt;typename K, typename V&gt; unordered_map&lt;K, V&gt;::unordered_map() : count(0), numBuckets(5) { ... } template&lt;typename K, typename V&gt; unordered_map&lt;K, V&gt;::~unordered_map() {  ... }  main.cpp: #include &quot;unordered_map.h&quot;  int main(void) {     unordered_map&lt;int, int&gt; map;      map.insert(1, 1337);      std::cout &lt;&lt; map.getValue(1) &lt;&lt; '\n';      return 0; }  And I'm compiling as follow: g++ unordered_map.cpp main.cpp -o main And I get: /usr/bin/ld: /tmp/ccXdGpTS.o: in function `main':                                                                       main.cpp:(.text+0x24): undefined reference to `unordered_map&lt;int, int&gt;::unordered_map()'                                /usr/bin/ld: main.cpp:(.text+0x3a): undefined reference to `unordered_map&lt;int, int&gt;::insert(int, int)'                  /usr/bin/ld: main.cpp:(.text+0x4b): undefined reference to `unordered_map&lt;int, int&gt;::getValue(int)'                     /usr/bin/ld: main.cpp:(.text+0x77): undefined reference to `unordered_map&lt;int, int&gt;::~unordered_map()'                  /usr/bin/ld: main.cpp:(.text+0x9d): undefined reference to `unordered_map&lt;int, int&gt;::~unordered_map()'  What's this refering to?
Quote from :     The only portable way of using templates at the moment is to implement them in header files by using inline functions.   Why is this?  (Clarification: header files are not the only portable solution. But they are the most convenient portable solution.)
Calculate the product of distances for a line perpendicular to hyperbola Let us have a hyperbola $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ and assume the normal at P meets the transverse (horizontal) axis $AA'$ in $G$ and the conjugate (vertical) axis $BB'$ in $g$. Assume CF be perpendicular to the normal from the center.   Find $\frac{PF \cdot PG}{CB^2}$ Find $PF \cdot Pg$   How can I solve the question in least amount of steps?  My attempt:  I first formed the equation of the normal and then I found out the points where it cuts the conjugate and transverse axis. After finding these points I used distance formula to find the distance required and convert it in the form that the question asked. But its a long method and took lot of steps I am foreign graduate student in USA. I recently completed my undergrad (from USA) and have a bit of money saved up. I know that F1 students cannot work off campus and other business activities such stock trading etc are prohibited as well. I wanted to know if opening an account in an investment management firm like Charles Schwabb or Betterment is allowed. I would not be working but I will still be earning a small interest on my deposits. If that is not allowed, are there any good and legal ways for foreign students to earn a little interest on savings? I have come across numerous questions where I am given the picture such as the above one been asked "how many rectangles are there?". I have even come across some slightly different images that instead of rectangles you are supposed to find the number of triangles. Well, I was thinking whether there is any formula or strategy that is used to solve these problems without having to manually count every shape.  Help would be appreciated.  Thank you :) Why do we say "is it" when asking a question rather than "it is"? What it says on the tin, a foreign friend of mine has asked and I can't tell him; apart from it sounding horrible.  For example:  "Why is it raining today?"   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Wouldn't it have been better to not have an aerial bombardment and simply have the troops surround the cantina, keeping as many people as possible bottled up inside it while they searched both the interior and surrounding area for BB-8?  Instead of a surgical extraction, the strategy that they chose seemed to create chaos and even risked destroying BB-8.  Perhaps this was a sign of the inexperience and emotionally-charged state of Kylo Ren, but General Hux seemed to have more direct authority over the deployment of troops and fighters.  So what was the strategy behind these tactics? How to produce a longer \sim with a small letter on top Footnotes in tables? I would like to have footnotes in tables, but the \footnote command does not work inside a table environment.  Googling one finds that this is , at least in the UK.  That link contains a list of solutions to the problem, but since the author of the FAQ does not believe that this is good style, no comparison is made between the different methods.  I am sure that there are good typographical reasons why in most situations it is undesirable to have footnotes in tables (although I have never come across them), however I have recently felt the need to do just this in a paper I am currently writing.  Therefore I'd like to ask two questions:   What are the arguments against footnotes in tables? If one really must go against the advice in 1., which method (whether considered in the  or not) would you recommend?   Thanks in advance.  Added  In response to ShreevatsaR's comment: I do not much care for where the text of the footnote appears.  I do however insist on the footnote marks being attached to the relevant entries in the table, so that I can avoid having to reference "the second entry in the eighth line of the table", etc... 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