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  • How do I indicate that a class doesn't support certain operators?

    - by romeovs
    I'm writing a class that represents an ordinal scale, but has no logical zero-point (eg time). This scale should permit addition and substraction (operator+, operator+=, ...) but not multiplication. Yet, I always felt it to be a good practice that when one overloads one operator of a certain group (in this case the math operators), one should also overload all the others that belong to that group. In this case that would mean I should need to overload the multiplication and division operators also, because if a user can use A+B he would probable expect to be able the other operators. Is there a method that I can use to throw an error for this at compiler time? The easiest method would be just no to overload the operators operator*, ... yet it would seem appropriate to add a bit more explaination than operator* is not know for class "time". Or is this something that I really should not care about (RTFM user)?

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  • What is the ISO C++ way to directly define a conversion function to reference to array?

    - by ben
    According to the standard, a conversion function has a function-id operator conversion-type-id, which would look like, say, operator char(&)[4] I believe. But I cannot figure out where to put the function parameter list. gcc does not accept either of operator char(&())[4] or operator char(&)[4]() or anything I can think of. Now, gcc seems to accept (&operator char ())[4] but clang does not, and I am inclined to not either, since it does not seem to fit the grammar as I understand it. I do not want to use a typedef because I want to avoid polluting the namespace with it.

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  • Script throwing unexpected operator when using mysqldump

    - by Astron
    A portion of a script I use to backup MySQL databases has stopped working correctly after upgrading a Debian box to 6.0 Squeeze. I have tested the backup code via CLI and it works fine. I believe it is in the selection of the databases before the backup occurs, possibly something to do with the $skipdb variable. If there is a better way to perform the function then I'm will to try something new. Any insight would be greatly appreciated. $ sudo ./script.sh [: 138: information_schema: unexpected operator [: 138: -1: unexpected operator [: 138: mysql: unexpected operator [: 138: -1: unexpected operator Using bash -x script here is one of the iterations: + for db in '$DBS' + skipdb=-1 + '[' test '!=' '' ']' + for i in '$IGGY' + '[' mysql == test ']' + : + '[' -1 == -1 ']' ++ /bin/date +%F + FILE=/backups/hostname.2011-03-20.mysql.mysql.tar.gz + '[' no = yes ']' + /usr/bin/mysqldump --single-transaction -u root -h localhost '-ppassword' mysql + /bin/tar -czvf /backups/hostname.2011-03-20.mysql.mysql.tar.gz mysql.sql mysql.sql + rm -f mysql.sql Here is the code. if [ $MYSQL_UP = "yes" ]; then echo "MySQL DUMP" >> /tmp/update.log echo "--------------------------------" >> /tmp/update.log DBS="$($MYSQL -u $MyUSER -h $MyHOST -p"$MyPASS" -Bse 'show databases')" for db in $DBS do skipdb=-1 if [ "$IGGY" != "" ] ; then for i in $IGGY do [ "$db" == "$i" ] && skipdb=1 || : done fi if [ "$skipdb" == "-1" ] ; then FILE="$DEST$HOST.`$DATE +"%F"`.$db.mysql.tar.gz" if [ $ENCRYPT = "yes" ]; then $MYSQLDUMP -u $MyUSER -h $MyHOST -p"$MyPASS" $db > $db.sql && $TAR -czvf - $db.sql | $OPENSSL enc -aes-256-cbc -salt -out $FILE.enc -k $ENC_PASS && rm -f $db.sql else $MYSQLDUMP --single-transaction -u $MyUSER -h $MyHOST -p"$MyPASS" $db > $db.sql && $TAR -czvf $FILE $db.sql && rm -f $db.sql fi fi done fi

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  • Guru of the Week 2 no match for the operator==

    - by Adam
    From Guru of the Week 2. We have the function: string FindAddr(const list<Employee> l, string name) { for( list<Employee>::const_iterator i = l.begin(); i != l.end(); i++) { if( *i == name ) // here will be compilation error { return (*i).addr; } } return ""; } I added dummy Employee class to that: class Employee { string n; public: string addr; Employee(string name) : n(name) {} Employee() {} string name() const { return n; } operator string() { return n; } }; And got compilation error: error: no match for ‘operator==’ in ‘i.std::_List_iterator<_Tp>::operator* [with _Tp = Employee]() == name’ It works only if add operator== to Employee. But, Herb Sutter wrote that: The Employee class isn't shown, but for this to work it must either have a conversion to string or a conversion ctor taking a string. But Employee has a conversion function and conversion constructor as well. GCC version 4.4.3. Compiled normally, g++ file.cpp without any flags. There should be implicit conversion and it should work, why it doesn't?

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  • operator << : std::cout << i << (i << 1);

    - by Oops
    Hi, I use the stream operator << and the bit shifting operator << in one line. I am a bit confused, why does code A) not produce the same output than code B)? A) int i = 4; std::cout << i << " " << (i << 1) << std::endl; //4 8 B) myint m = 4; std::cout << m << " " << (m << 1) << std::endl; //8 8 class myint: class myint { int i; public: myint(int ii) { i = ii; } inline myint operator <<(int n){ i = i << n; return *this; } inline operator int(){ return i; } }; thanks in advance Oops

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  • Setting height of div equal to containing div

    - by Syom
    i have the following simple script <div id="content"> <div id="left"> <div id="menu"> <ul> <li>menu</li> </ul> </div> <div id="left_ad"> </div> </div> <div id="middle"> some text here... </div> <div id="right"> <div id="right_ad"> <div id="ad2"> </div> </div> </div> </div> css: #content { margin: 0; padding: 0; overflow: hidden; position: relative; } #left { float: left; width:25%; margin-left:0; margin-right:0; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #middle { background-image:url("http://localhost/kino/img/theme.png"); background-repeat:no-repeat; float: left; width:50%; margin-left:0; margin-right:0; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #right { float: left; width:25%; margin-left:0; margin-right:-1px; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #left_ad { background-image:url("http://localhost/kino/img/lens1.png"); background-repeat:no-repeat; min-height:498px; margin-left: auto; margin-right: auto; margin-top:31px; padding:0; width: 188px; position: relative; height: 100%; } as you see, left, middle and right have the same size of heigth, which is equal to max of them. and now, what is the question, i want left_ad to have the same heigth too. how can i set the heigth of left_id is equal to left? thanks

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  • setting heigth of div equal to containing div

    - by Syom
    i have the following script <div id="content"> <div id="left"> <div id="menu"> <ul> <li>menu</li> </ul> </div> <div id="left_ad"> </div> </div> <div id="middle"> some text here... </div> <div id="right"> <div id="right_ad"> <div id="ad2"> </div> </div> </div> </div> css: #content { margin: 0; padding: 0; overflow: hidden; position: relative; } #left { float: left; width:25%; margin-left:0; margin-right:0; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #middle { background-image:url("http://localhost/kino/img/theme.png"); background-repeat:no-repeat; float: left; width:50%; margin-left:0; margin-right:0; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #right { float: left; width:25%; margin-left:0; margin-right:-1px; margin-top:0; margin-bottom:-5000px; padding-left:0; padding-right:0; padding-top:0; padding-bottom:5000px; } #left_ad { background-image:url("http://localhost/kino/img/lens1.png"); background-repeat:no-repeat; min-height:498px; margin-left: auto; margin-right: auto; margin-top:31px; padding:0; width: 188px; position: relative; height: 100%; } as you see, left, middle and right are have the same size of heigth, which is equal to max of them. and now, what is the question, i want left_ad to have the same heigth too. how can i set the heigth of left_id is equal to left? thanks

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  • Custom types as key for a map - C++

    - by Appu
    I am trying to assign a custom type as a key for std::map. Here is the type which I am using as key. struct Foo { Foo(std::string s) : foo_value(s){} bool operator<(const Foo& foo1) { return foo_value < foo1.foo_value; } bool operator>(const Foo& foo1) { return foo_value > foo1.foo_value; } std::string foo_value; }; When used with std::map, I am getting the following error. error C2678: binary '<' : no operator found which takes a left-hand operand of type 'const Foo' (or there is no acceptable conversion) c:\program files\microsoft visual studio 8\vc\include\functional 143 If I change the struct like the below, everything worked. struct Foo { Foo(std::string s) : foo_value(s) {} friend bool operator<(const Foo& foo,const Foo& foo1) { return foo.foo_value < foo1.foo_value; } friend bool operator>(const Foo& foo,const Foo& foo1) { return foo.foo_value > foo1.foo_value; } std::string foo_value; }; Nothing changed except making the operator overloads as friend. I am wondering why my first code is not working? Any thoughts?

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  • Python: Slicing a list into n nearly-equal-length partitions

    - by Drew
    I'm looking for a fast, clean, pythonic way to divide a list into exactly n nearly-equal partitions. partition([1,2,3,4,5],5)->[[1],[2],[3],[4],[5]] partition([1,2,3,4,5],2)->[[1,2],[3,4,5]] (or [[1,2,3],[4,5]]) partition([1,2,3,4,5],3)->[[1,2],[3,4],[5]] (there are other ways to slice this one too) There are several answers in here http://stackoverflow.com/questions/1335392/iteration-over-list-slices that run very close to what I want, except they are focused on the size of the list, and I care about the number of the lists (some of them also pad with None). These are trivially converted, obviously, but I'm looking for a best practice. Similarly, people have pointed out great solutions here http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python for a very similar problem, but I'm more interested in the number of partitions than the specific size, as long as it's within 1. Again, this is trivially convertible, but I'm looking for a best practice.

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  • defining < operator for map of list iterators

    - by Adrian
    I'd like to use iterators from an STL list as keys in a map. For example: using namespace std; list<int> l; map<list<int>::const_iterator, int> t; int main(int argv, char * argc) { l.push_back(1); t[l.begin()] = 5; } However, list iterators do not have a comparison operator defined (in contrast to random access iterators), so compiling the above code results in an error: /usr/include/c++/4.2.1/bits/stl_function.h:227: error: no match for ‘operator<’ in ‘__x < __y’ If the list is changed to a vector, a map of vector const_iterators compiles fine. What is the appropriate way to define the operator < for list::const_iterator?

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  • Algorithm for finding the smallest power of two that's greater or equal to a given value

    - by Boyan
    I need to find the smallest power of two that's greater or equal to a given value. So far, I have this: int value = 3221; // 3221 is just an example, could be any number int result = 1; while (result < value) result <<= 1; It works fine, but feels kind of naive. Is there a better algorithm for that problem? EDIT. There were some nice Assembler suggestions, so I'm adding those tags to the question.

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  • Allow member to be const while still supporting operator= on the class

    - by LeopardSkinPillBoxHat
    I have several members in my class which are const and can therefore only be initialised via the initialiser list like so: class MyItemT { public: MyItemT(const MyPacketT& aMyPacket, const MyInfoT& aMyInfo) : mMyPacket(aMyPacket), mMyInfo(aMyInfo) { } private: const MyPacketT mMyPacket; const MyInfoT mMyInfo; }; My class can be used in some of our internally defined container classes (e.g. vectors), and these containers require that operator= is defined in the class. Of course, my operator= needs to do something like this: MyItemT& MyItemT::operator=(const MyItemT& other) { mMyPacket = other.mPacket; mMyInfo = other.mMyInfo; return *this; } which of course doesn't work because mMyPacket and mMyInfo are const members. Other than making these members non-const (which I don't want to do), any ideas about how I could fix this?

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  • Why does `intval(1990)` equal `1989`?

    - by George Edison
    Boy, this one is really weird. I expect the following code to print 1990, but it prints 1989! $val = '$19.9'; $val = preg_replace('/[^\d.]/','',$val); $val = intval($val * 100); echo $val; Why on earth is this happening? Edit: and this code: $val = '$19.9'; $val = preg_replace('/[^\d.]/','',$val); echo $val . "<br>"; $val = $val * 100; echo $val . "<br>"; $val = intval($val); echo $val; Prints: 19.9 1990 1989 Why does intval(1990) equal 1989???

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  • Concatenation Operator

    - by Chaitanya
    This might be a silly question but it struck me, and here i ask. <?php $x="Hi"; $y=" There"; $z = $x.$y; $a = "$x$y"; echo "$z"."<br />"."$a"; ?> $z uses the traditional concatenation operator provided by php and concatenates, conversely $a doesn't, My questions: by not using the concatenation operator, does it effect the performance? If it doesn't why at all have the concatenation operator. Why have 2 modes of implementation when one does the work?

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  • Concatenation Operator - PHP

    - by Chaitanya
    This might be a silly question but it struck me, and here i ask. <?php $x="Hi"; $y=" There"; $z = $x.$y; $a = "$x$y"; echo "$z"."<br />"."$a"; ?> $z uses the traditional concatenation operator provided by php and concatenates, conversely $a doesn't, My questions: a. by not using the concatenation operator, does it effect the performance? b. If it doesn't why at all have the concatenation operator. c. Why have 2 modes of implementation when one does the work?

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  • Understanding pattern matching with cons operator

    - by Mathias
    In "Programming F#" I came across a pattern-matching like this one (I simplified a bit): let rec len list = match list with | [] -> 0 | [_] -> 1 | head :: tail -> 1 + len tail;; Practically, I understand that the last match recognizes the head and tail of the list. Conceptually, I don't get why it works. As far as I understand, :: is the cons operator, which appends a value in head position of a list, but it doesn't look to me like it is being used as an operator here. Should I understand this as a "special syntax" for lists, where :: is interpreted as an operator or a "match pattern" depending on context? Or can the same idea be extended for types other than lists, with other operators?

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  • Different results when applying function to equal values

    - by Johannes Stiehler
    I'm just digging a bit into Haskell and I started by trying to compute the Phi-Coefficient of two words in a text. However, I ran into some very strange behaviour that I cannot explain. After stripping everything down, I ended up with this code to reproduce the problem: let sumTup = (sumTuples°concat) frequencyLists let sumFixTup = (138, 136, 17, 204) putStrLn (show ((138, 136, 17, 204) == sumTup)) putStrLn (show (phi sumTup)) putStrLn (show (phi sumFixTup)) This outputs: True NaN 0.4574206676616167 So although the sumTupand sumFixTup show as equal, they behave differently when passed to phi. The definition of phi is: phi (a, b, c, d) = let dividend = fromIntegral(a * d - b * c) divisor = sqrt(fromIntegral((a + b) * (c + d) * (a + c) * (b + d))) in dividend / divisor

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  • Nothing = String.Empty (Why are these equal?)

    - by Ek0nomik
    I find it very puzzling that null equals String.Empty in VB.NET. There must be an explanation for it that I'm not understanding. Somehow, when the equality of String.Empty (which has an object type of string) is tested against Nothing (which has no type) the test returns true. It just seems like a turn around in logic to me. In SQL if I were to compare an empty char or varchar against a null value, the test wouldn't return true. A null value is not the same as an empty string in this case. Yet in VB.NET they are equal. I'd love it if someone could provide a good explanation around this.

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