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  • typeof === "undefined" vs. != null

    - by Thor Thurn
    I often see JavaScript code which checks for undefined parameters etc. this way: if (typeof input !== "undefined") { // do stuff } This seems kind of wasteful, since it involves both a type lookup and a string comparison, not to mention its verbosity. It's needed because 'undefined' could be renamed, though. My question is: How is that code any better than this approach: if (input != null) { // do stuff } As far as I know, you can't redefine null, so it's not going to break unexpectedly. And, because of the type-coercion of the != operator, this checks for both undefined and null... which is often exactly what you want (e.g. for optional function parameters). Yet this form does not seem widespread, and it even causes JSLint to yell at you for using the evil != operator. Why is this considered bad style?

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  • C++ Why is the copy constructor implicitly called?

    - by ShaChris23
    Why is the Child class's copy constructor called in the code below? I mean, it automatically converts Base to Child via the Child copy constructor. The code below compiles, but shouldn't it not compile since I haven't provided bool Child::operator!=(Base const&)? class Base { }; class Child : public Base { public: Child() {} Child(Base const& base_) : Base(base_) { std::cout <<"should never called!"; } bool operator!=(Child const&) { return true; } }; void main() { Base base; Child child; if(child != base) std::cout << "not equal"; else std::cout << "equal"; }

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  • C# XOR on two byte variables will not compile without a cast

    - by Ash
    Why does the following raise a compile time error: 'Cannot implicitly convert type 'int' to 'byte': byte a = 25; byte b = 60; byte c = a ^ b; This would make sense if I were using an arithmentic operator because the result of a + b could be larger than can be stored in a single byte. However applying this to the XOR operator is pointless. XOR here it a bitwise operation that can never overflow a byte. using a cast around both operands works: byte c = (byte)(a ^ b);

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  • non-scalar type requested

    - by lego69
    can somebody please help me with an error conversion from `A' to non-scalar type `B' requested I have class A and derived from it B, but I have problems with these rows: A a(1); A *pb = new B(a); B b = *pb; //here I have an error thanks in advance for any help class A { protected: int player; public: A(int initPlayer = 0); A(const A&); A& operator=(const A&); virtual ~A(){}; virtual void foo(); void foo() const; operator int(); }; class B: public A { public: B(int initPlayer): A(initPlayer){}; ~B(){}; virtual void foo(); };

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  • C# Type Casting at Runtimefor Array.SetValue

    - by sprocketonline
    I'm trying to create an array using reflection, and insert values into it. I'm trying to do this for many different types so would like a createAndFillArray function capable of this : Type t1 = typeof(A); Type t2 = typeof(B); double exampleA = 22.5; int exampleB = 43; Array arrA = createAndFillArray(t1, exampleA); Array arrB = createAndFillArray(t2, exampleB); private Array createAndFillArray(Type t, object val){ Array arr = Array.CreateInstance( t, 1); //length 1 in this example only, real-world is of variable length. arr.SetValue( val, 0 ); //this causes the following error: "System.InvalidCastException : Object cannot be stored in an array of this type." return arr; } with the class A being as follows: public class A{ public A(){} private double val; public double Value{ get{ return val; } set{ this.val = value; } } public static implicit operator A(double d){ A a = new A(); a.Value = d; return a; } } and class B being very similar, but with int: public class B{ public B(){} private double val; public double Value{ get{ return val; } set{ this.val = value; } } public static implicit operator B(double d){ B b = new B(); b.Value = d; return b; } } I hoped that the implicit operator would have ensured that the double be converted to class A, or the int to class B, and the error avoided; but this is obviously not so. The above is used in a custom deserialization class, which takes data from a custom data format and fills in the corresponding .Net object properties. I'm doing this via reflection and at runtime, so I think both are unavoidable. I'm targeting the C# 2.0 framework. I've dozens, if not hundreds, of classes similar to A and B, so would prefer to find a solution which improved on the createAndFillArray method rather than a solution which altered these classes.

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  • Using a class with const data members in a vector

    - by Max
    Given a class like this: class Foo { const int a; }; Is it possible to put that class in a vector? When I try, my compiler tells me it can't use the default assignment operator. I try to write my own, but googling around tells me that it's impossible to write an assignment operator for a class with const data members. One post I found said that "if you made [the data member] const that means you don't want assignment to happen in the first place." This makes sense. I've written a class with const data members, and I never intended on using assignment on it, but apparently I need assignment to put it in a vector. Is there a way around this that still preserves const-correctness?

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  • BASH: Checking for environment variables

    - by Hamza
    Hi folks, I am trying to check the value of an environment variable and depending on the value do certain things and it works fine as long as the variable is set. When it isn't though I get a whole bunch of errors (as BASH is trying to compare the string I specify with an undefined variable, I guess) I tried implementing an extra check to prevent it happening but no luck. The block of code I am using is: #!/bin/bash if [ -n $TESTVAR ] then if [ $TESTVAR == "x" ] then echo "foo" exit elif [ $TESTVAR == "y" ] then echo "bar" exit else echo "baz" exit fi else echo -e "TESTVAR not set\n" fi And this the output: $ export TESTVAR=x $ ./testenv.sh foo $ export TESTVAR=y $ ./testenv.sh bar $ export TESTVAR=q $ ./testenv.sh baz $ unset TESTVAR $ ./testenv.sh ./testenv.sh: line 5: [: ==: unary operator expected ./testenv.sh: line 9: [: ==: unary operator expected baz My question is, shouldn't 'unset TESTVAR' nullify it? It doesn't seem to be the case... Thanks.

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  • C++ - Print Out Objects From Set

    - by John Smith
    If I have a C++ set, declaration set personList; with iterator, set::const_iterator location; how can I print out the contents of the set? They are all person objects, and I have overridden the operator<< for Person. The line that errors is: cout << location and it's in a basic for loop. Netbeans gives the following error: proj.cpp:78: error: no match for ‘operator<<’ in ‘std::cout << location’ so it looks like it wants an override for the iterator's <<. Basically, I am taking objects that used to be stored in an array format, but are now in a set. What is the equivalent to cout << array[i] for sets?

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  • C++ enumaration

    - by asli
    Hi,my question is about enumaration,my codes are : #include<iostream> using namespace std; int main() { enum bolumler{programcilik,donanim,muhasebe,motor,buro} bolum; bolum = donanim; cout<<bolum<<endl; bolum+=2; /* bolum=motor */ cout<<bolum; return 0; } The output should be 1 3 but according to these codes the error is: error C2676: binary '+=' : 'enum main::bolumler' does not define this operator or a conversion to a type acceptable to the predefined operator Error executing cl.exe. 111.obj - 1 error(s), 0 warning(s) Can you help me ?The other question is what can I do if I want to see the output like that "muhasebe"?

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  • Library for Dataflow in C

    - by msutherl
    How can I do dataflow (pipes and filters, stream processing, flow based) in C? And not with UNIX pipes. I recently came across stream.py. Streams are iterables with a pipelining mechanism to enable data-flow programming and easy parallelization. The idea is to take the output of a function that turns an iterable into another iterable and plug that as the input of another such function. While you can already do this using function composition, this package provides an elegant notation for it by overloading the operator. I would like to duplicate a simple version of this kind of functionality in C. I particularly like the overloading of the operator to avoid function composition mess. Wikipedia points to this hint from a Usenet post in 1990. Why C? Because I would like to be able to do this on microcontrollers and in C extensions for other high level languages (Max, Pd*, Python). * (ironic given that Max and Pd were written, in C, specifically for this purpose – I'm looking for something barebones)

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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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  • 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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  • 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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  • 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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  • 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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  • 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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  • 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++ 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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  • 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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