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• Why are there two implementations of std::sort (with and without a comparator) rather than one implementation with a default template parameter?

  - by PolyVox

  In my code I'm adopting a design strategy which is similar to some standard library algorithms in that the exact behavior can be customized by a function object. The simplest example is std::sort, where a function object can control how the comparison is made between objects. I notice that the Visual C++ provides two implementations of std::sort, which naturally involves code duplication. I would have imagined that it was instead possible to have only one implementation, and provide a default comparator (using operator< ) as a default template parameter. What is the rational behind two separate versions? Would my suggestion make the interface more complex in some way? Or result in confusing error messages when the object does not provide operator Thanks, David

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• Not a statement?

  - by abelenky

  I have a simple little code fragment that is frustrating me: HashSet<long> groupUIDs = new HashSet<long>(); groupUIDs.Add(uid)? unique++ : dupes++; At compile time, it generates the error: Only assignment, call, increment, decrement, and new object expressions can be used as a statement HashSet.Add is documented to return a bool, so the ternary (?) operator should work, and this looks like a completely legitimate way to track the number of unique and duplicate items I add to a hash-set. When I reformat it as a if-then-else, it works fine. Can anyone explain the error, and if there is a way to do this as a simple ternary operator?

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• C++ function object terminology functor, deltor, comparitor, etc..

  - by Robert S. Barnes

  Is there a commonly accepted terminology for various types for common functors? For instance I found myself naturally using comparitor for comparison functors like this: struct ciLessLibC : public std::binary_function<std::string, std::string, bool> { bool operator()(const std::string &lhs, const std::string &rhs) const { return strcasecmp(lhs.c_str(), rhs.c_str()) < 0 ? 1 : 0; } }; Or using the term deltor for something like this: struct DeleteAddrInfo { void operator()(const addr_map_t::value_type &pr) const { freeaddrinfo(pr.second); } }; If using these kinds of shorthand terms is common, it there some dictionary of them all someplace?

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• C++ return type overload hack

  - by aaa

  I was bored and came up with such hack (pseudocode): 1 struct proxy { 2 operator int(); // int function 3 operator double(); // double function 4 proxy(arguments); 5 arguments &arguments_; 6 }; 7 8 proxy function(arguments &args) { 9 return proxy(args); 10 } 11 int v = function(...); 12 double u = function(...); is it evil to use in real code?

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• Fast comparison of char arrays?

  - by StackedCrooked

  I'm currently working in a codebase where IPv4 addresses are represented as pointers to u_int8. The equality operator is implemented like this: bool Ipv4Address::operator==(const u_int8 * inAddress) const { return (*(u_int32*) this->myBytes == *(u_int32*) inAddress); } This is probably the fasted solution, but it causes the GCC compiler warning: ipv4address.cpp:65: warning: dereferencing type-punned pointer will break strict-aliasing rules How can I rewrite the comparison correctly without breaking strict-aliasing rules and without losing performance points? I have considered using either memcmp or this macro: #define IS_EQUAL(a, b) \ (a[0] == b[0] && a[1] == b[1] && a[2] == b[2] && a[3] == b[3]) I'm thinking that the macro is the fastest solution. What do you recommend?

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• boost variant static_visitor problem picking correct function

  - by Steve

  I'm sure I'm having a problem with template resolution here, but I'm not sure why I'm having the problem. I have a static visitor I'm passing to boost variant where i've had to do template specialization for certain cases. The case for everything except for MyClass should throw in the static_visitor below. Unfortunately, when the visitor is applied to pull a MyClass out, it selects the most generic case rather than the exact match. I would type each case explicitly, but that will be rather long. So, why is the compiler resolving the most generic case over the exact match, and is there anyway to fix it template<> class CastVisitor<MyClass>:public boost::static_visitor<MyClass> { public: template<typename U> MyClass operator()(const U & i) const { throw std::exception("Unable to cast"); } MyClass operator()(const MyClass& i) { return i; } };

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• Copy Constructor in C++

  - by user265260

  i have this code #include <iostream> using namespace std; class Test{ public: int a; Test(int i=0):a(i){} ~Test(){ cout << a << endl; } Test(const Test &){ cout << "copy" << endl; } void operator=(const Test &){ cout << "=" << endl; } Test operator+(Test& p){ Test res(a+p.a); return res; } }; int main (int argc, char const *argv[]){ Test t1(10), t2(20); Test t3=t1+t2; return 0; } Output: 30 20 10 Why isnt the copy constructor called here?

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• What is the difference between the * and the & operators in c programming?

  - by Wesley

  I am just making sure I understand this concept correctly. With the * operator, I make a new variable, which is allocated a place in memory. So as to not unnecessarily duplicate variables and their values, the & operator is used in passing values to methods and such and it actually points to the original instance of the variable, as opposed to making new copies...Is that right? It is obviously a shallow understanding, but I just want to make sure I am not getting them mixed up. Thanks!

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• MySQL Query Where Column Like Column

  - by shmeeps

  I'm working on a small project that involves grabbing a list of contacts which are stored for each group. Essentially, the database is set up so that each group has a primary and secondary contact stored as, unsurprisingly, Group.Primary and Group.Secondary. The objective is to pull every Primary and Secondary contact for each Group and display them in a sortable table. I have the sortable table all worked out, but I have come across a small problem. Each primary and secondary field can have more than one contact separated by a comma. For instance, if Primary contained 123,256 , it would need to pull both Contacts with IDs 123 and 256. I had intended to use a query formatted like this: SELECT * FROM Group G, Contacts C WHERE G.Primary LIKE %C.ID% OR G.Secondary LIKE %C.ID% so that I could just skip the comma part, but I can't seem to find a working query for this. My question to you is, am I just overlooking something here? Is there a simple query that would let me do this? Or am I better off getting the groups and contacts separately, and combine the two later. I think the former is a little easier to understand when read, which is a plus as this is a shared project, but if that is not possible I will do the latter. This code is simplified, but it gets the point across.

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• Encrypt/ Decrypt text file in Delphi?

  - by Hemant Kothiyal

  Hi i would like to know best encryption technique for text file encryption and ecryption. My Scenario: I have software having two type of users Administartor and Operators. Our requirement is to encrypt text file when Administrator enter data using GUI and save it. That encrypted file would be input for Operator and they just need to select it and use that file. Here file should be automatically decrypt data for further calculation when Operator select those files. Please help me which encryption/ decryption technique should i use?

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• Why does this simple bash code give a syntax error?

  - by Tim

  I have the following bash code, which is copied and pasted from "bash cookbook" (1st edition): #!/bin/bash VERBOSE=0; if [[ $1 =-v ]] then VERBOSE=1; shift; fi When I run this (bash 4.0.33), I get the following syntax error: ./test.sh: line 4: conditional binary operator expected ./test.sh: line 4: syntax error near `=-v' ./test.sh: line 4: `if [[ $1 =-v ]]' Is this as simple as a misprint in the bash cookbook, or is there a version incompatibility or something else here? What would the most obvious fix be? I've tried various combinations of changing the operator, but I'm not really familiar with bash scripting.

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• C++ struct sorting error

  - by Betamoo

  I am trying to sort a vector of custom struct in C++ struct Book{ public:int H,W,V,i; }; with a simple functor class CompareHeight { public: int operator() (Book lhs,Book rhs) { return lhs.H-rhs.H; } }; when trying : vector<Book> books(X); ..... sort(books.begin(),books.end(), CompareHeight()); it gives me exception "invalid operator <" What is the meaning of this error? Thanks

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• C++ enforce conditions on inherited classes

  - by user231536

  I would like to define an abstract base class X and enforce the following: a) every concrete class Y that inherits from X define a constructor Y(int x) b) it should be possible to test whether two Y objects are equal. For a, one not very good solution is to put a pure virtual fromInt method in X which concrete class will have to define. But I cannot enforce construction. For b), I cannot seem to use a pure virtual method in X bool operator == (const X& other) const =0; because in overridden classes this remains undefined. It is not enough to define bool operator == (const Y& other) const { //stuff} because the types don't match. How do I solve these problems?

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• Not allow more than 5 digits after decimal. in on javascript "OnKeyUp"?

  - by James123

  I have a javascript code for textbox that will put commas on in digits like (11,23,233) mTextbox.Attributes.Add("OnKeyUp", "javascript:this.value=Comma(this.value);") function Comma(Num) { Num += ''; Num = Num.replace(',', ''); Num = Num.replace(',', ''); Num = Num.replace(',', ''); Num = Num.replace(',', ''); Num = Num.replace(',', ''); Num = Num.replace(',', ''); x = Num.split('.'); x1 = x[0]; x2 = x.length > 1 ? '.' + x[1] : ''; var rgx = /(\d+)(\d{3})/; while (rgx.test(x1)) x1 = x1.replace(rgx, '$1' + ',' + '$2'); return x1 + x2; } Now same here I need to restrict user to enter not morethan 5 digits after decimal (ex: Allow: 12,23,221.34323 Not Allow: 12,23,232.232423 I can change above javascript to work that?

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• Can I reproduce Scala's behavior for == ?

  - by JPP

  In Programming in Scala, I can read that the == operator behaves as if it was defined like this: final def == (that: Any): Boolean = if (null eq this) {null eq that} else {this equals that} But there must actually be compiler magic to avoid null pointer exceptions, right? Is there any way for me to replicate this behavior with pure Scala; i.e., have an operator/method return one thing if the receiver is null and another one if it isn't? What I mean is an actual implementation of null eq this. I suppose I can write a "pimp" and then define the method on the wrapper class, but is there a more direct way to do this?

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• How to replace the char '[' etc with '\[' using "sed" in a file ?

  - by Abhijeet

  I have a file say "file.txt" with following contents: Capsule arr**[**0**]** in state A rate_ul/dl=**(**2000000/7000000**)** Capsule RBx**[**0**]** in state ... ... using sed operator how can i replace all occurences of '[' with '[', '(' with '(', ']' with ']' and so on. Capsule arr**\[**0**\]** in state A rate_ul/dl=**\(**2000000/7000000**\)** Capsule RBx**\[**0**\]** in state ... ... Using the substitue operator in "gvim" I am able to achieve the same result. ie. if i use ":1,$ s/\[/\\[/g" in the vi editor in command mode I see all the '[' chars replaced with '['. However if I try to use the same substitue command in a shell script using a sed command, i am not able to achieve the same result. ie If i use the following command in a shell script I am not able to achieve the desired result: sed "s/\[/\\[/g" $temp_file2 > $temp_file1 where $temp_file2 conatins the lines with '[' characters and $temp_file1 should contain the replaced '\[' chars

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• logical or expression c++

  - by user1870181

  I have a problem using the Logical OR operator in C++. The problem is coming that the right-side expression is not evaluated if the left-side is true. I have two deque-s and I need to popLeft from them with a while, but if I can pop from the first deque, I don't pop from the second because is not evaluated, by the OR operator. How can I overcome this problem. Here is the piece of code: while( D.popLeft( k ) || E.popLeft( m ) ) { if( k < m ) { C.pushRight( k ); E.pushLeft( m ); } else { C.pushRight( m ); D.pushLeft( k ); } }

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• doubt in - Function Objects - c++

  - by Eternal Learner

  I have a class class fobj{ public: fobj(int i):id(i) {} void operator()() { std::cout<<"Prints"<<std::endl; } private: int id; }; template<typename T> void func(T type) { type(); } My Doubt is if I invoke func like Method 1: func(fobj(1); the message I wanted to print is printed. I was always thinking I needed to do something like Method 2: fobj Iobj(1); // create an instance of the fobj class func(Iobj); //call func by passing Iobj(which is a function object) How does Method 1 work? I mean what exactly happens? and how is a call made to the operator() in class fobj ?

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• Subtracting months/years from boost::posix_time::ptime

  - by Zack

  I have a boost::posix_time::ptime that points to March 31st 2010 like this: ptime p(date(2010, Mar, 31)); I would like to subtract a month (and possibly years) from this date. From the docs I see these two operators: ptime operator-(time_duration) and ptime operator-(days) but none of them can work with months/years. If I try and do: time_duration duration = hours(24 * 30); ptime pp = p - duration; I'm getting March 1st and if I'm trying: ptime pp = p - days(30); I'm still getting March 1st, while I'd like to get February 28th. How can I achieve my desired result? (I would like to get the desired result also when subtracting a month from March 28, 29, 30)

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• expected identifier or '(' before '{' token in Flex

  - by user1829177

  I am trying to use Flex to parse 'C' source code. Unfortunately I am getting the error "expected identifier or '(' before '{' token" on lines 1,12,13,14... . Any ideas why? %{ %} digit [0-9] letter [a-zA-Z] number (digit)+ id (letter|_)(letter|digit|_)* integer (int) character (char) comma [,] %% {integer} {return INT;} {character} {return CHAR;} {number} {return NUM;} {id} {return IDENTIFIER;} {comma} {return ',';} [-+*/] {return *yytext;} . {} %% main() { yylex(); } The corresponding flex file is as shown below: %{ #include <ctype.h> #include <stdio.h> #include "myhead.h" #include "mini.l" #define YYSTYPE double # undef fprintf %} %token INT %token CHAR %token IDENTIFIER %token NUM %token ',' %left '+' '-' %left '*' '/' %right UMINUS %% lines:lines expr '\n' {printf("%g\n",$2);} |lines '\n' |D | ; expr :expr '*' expr {$$=$1*$3;} |expr '/' expr {$$=$1/$3;} |expr '+' expr {$$=$1+$3;} |expr '-' expr {$$=$1+$3;} |'(' expr ')' {$$=$2;} |'-' expr %prec UMINUS {$$=-$2;} |IDENTIFIER {} |NUM {} ; T :INT {} |CHAR {} ; L :L ',' IDENTIFIER {} |IDENTIFIER {} ; D :T L {printf("T is %g, L is %g",$1,$2);} ; %% /*void yyerror (char *s) { fprintf (stderr, "%s\n", s); } */ I am compiling the generated code using the command: gcc my_file.c -ly

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• SQL Server 2008 - Update a temporary table

  - by user336786

  Hello, I have stored procedure in which I am trying to retrieve the last ticket completed by each user listed in a comma-delimited string of usernames. The user may not have a ticket associated with them, in this case I know that i just need to return null. The two tables that I am working with are defined as follows: User ---- UserName, FirstName, LastName Ticket ------ ID, CompletionDateTime, AssignedTo, AssignmentDate, StatusID TicketStatus ------------ ID, Comments I have created a stored procedure in which I am trying to return the last completed ticket for a comma-delimited list of usernames. Each record needs to include the comments associated with it. Currently, I'm trying the following: CREATE TABLE #Tickets ( [UserName] nvarchar(256), [FirstName] nvarchar(256), [LastName] nvarchar(256), [TicketID] int, [DateCompleted] datetime, [Comments] text ) -- This variable is actually passed into the procedure DECLARE @userList NVARCHAR(max) SET @userList='user1,user2,user2' -- Obtain the user information for each user INSERT INTO #Tickets ( [UserName], [FirstName], [LastName] ) SELECT u.[UserName], u.[FirstName], u.[LastName] FROM User u INNER JOIN dbo.ConvertCsvToTable(@userList) l ON u.UserName=l.item At this point, I have the username, first and last name for each user passed in. However, I do not know how to actually get the last ticket completed for each of these users. How do I do this? I believe I should be updating the temp table I have created. At the same time, id do not know how to get just the last record in an update statement. Thank you!

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• Returning objects with dynamic memory

  - by Caulibrot

  I'm having trouble figuring out a way to return an object (declared locally within the function) which has dynamic memory attached to it. The problem is the destructor, which runs and deletes the dynamic memory when the object goes out of scope, i.e. when I return it and want to use the data in the memory that has been deleted! I'm doing this for an overloaded addition operator. I'm trying to do something like: MyObj operator+( const MyObj& x, const MyObj& y ) { MyObj z; // code to add x and y and store in dynamic memory of z return z; } My destructor is simply: MyObj::~MyObj() { delete [] ptr; } Any suggestions would be much appreciated!

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• Using ANTLR with Left-Recursive Rules

  - by CNevin561

  Basically Ive written a Parse for a language with just basic arithmetic operators ( +, -, * / ) etc, but for the minus and plus cases, the Abstract Syntax Tree which is generated has parsed them as right associative when they need to be left associative. Having a googled for a solution, i found a tutorial that suggests rewriting the rule from: Expression ::= Expression <operator> Term | Term as Expression ::= Term <operator> Expression*. However in my head this seems to generate the tree the wrong way round. Any pointers on a way to resolve this issue?

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• Extracting specific words that end with .c and .h [on hold]

  - by Alberto Mederos

  I have a very big list of file names that end with a the following: .c .h .cpp and much more. I need to extract file names that end with .c and .h How do I do that? Also, how could I add quotation marks to the beginning and end of the word, followed with a comma? For example, if I have this in the list: mi_var.c How could I extract it from a very big list, and everything else that ends in .c and replace it to have quotation marks and a comma at the end? Like this: "mi_var.c", I'm new to this, any help is greatly appreciated. Here is part of the list: gsd5t_image.c, gsd5t_image_sqif.c, proc_arm.c, proc_cortex.c, proc_k32.c, proc_k32_entry.s, proc_k32_test.c, proc_k32_test_start.s, rom_sub_functions.s, rom_sub_functions_gcc.s, sqif_jump_table.s, sqif_jump_table_gcc.s, tracker_wrapper_functions.s, vector_M0.c, ptimer.c, ptimer_arm.c, ptimer_internal.h, ptimer_internal_arm.h, ptimer_internal_k32.h, ptimer_k32.c, RstMod_if.h, drvRstMod.h, tbus.dxy, tbus_common.c, tbus_common.h, act.c, act.h, act.msgs, act_if.c, act_if.h, sat_signal_processor.c, sat_signal_processor.h, ssp.dxy, ssp.msgs, ssp_acq_handlers.c, ssp_acq_handlers.h, ssp_atx_if.c, ssp_atx_if.h, ssp_bitsync_handlers.c, ssp_bitsync_handlers.h, ssp_cohver_handlers.c, ssp_cohver_handlers.h, ssp_cwscan_handlers.c, ssp_cwscan_handlers.h, ssp_track_handlers.c, ssp_track_handlers.h, ssp_atx_if_test_sort.c, ssp_hack.c, ssp_hack.h, ssp_suite.cpp, ssp_suite.h, ssptloop.c, ssptloop.h, sss.dxy, sss.msgs, sss_atx_if.c, sss_atx_if.h, strong_signal_scan.c, So how to extract certain names?

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• Why System.String's beahaves like a value type?

  - by Sorush Rabiee

  I want to write a 'Date' class that behaves like a Value Type. for example, Instead of writing a Clone method for setting properties safely, make the Date class to pass by value: public Date Birthday { get { return this.birthday; } set { this.birthday = value.Clone(); } //I want to write this.birthday = value; //without changing external value when this.Birthday changes } I know this is possible because System.String is a class and behaves like a value. for example: String s1 = "Hello"; String s2 = "Hi"; s1 = s2; s2="Hello"; Console.WriteLine(s1); //Prints 'Hi' First I thought writers of this class override '=' operator, but now I know that the '=' operator can not be overridden. so how they write String class?

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