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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++ 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• can't compile min_element in c++

  - by Vincenzo

  This is my code: #include <algorithm> #include <vector> #include <string> using namespace std; class A { struct CompareMe { bool operator() (const string*& s1, const string*& s2) const { return true; } }; void f() { CompareMe comp; vector<string*> v; min_element(v.begin(), v.end(), comp); } }; And this is the error: error: no match for call to ‘(A::CompareMe) (std::string*&, std::string*&)’ test.cpp:7: note: candidates are: bool A::CompareMe::operator()(const std::string*&, const std::string*&) const I feel that there is some syntax defect, but can't find out which one. Please, help!

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• Comparing Java enum members: == or equals() ?

  - by Bears will eat you

  I know that Java enums are compiled to classes with private constructors and a bunch of public static members. When comparing two members of a given enum, I've always used .equals(), e.g. public useEnums(SomeEnum a) { if(a.equals(SomeEnum.SOME_ENUM_VALUE)) { ... } ... } However, I just came across come code that uses the equals operator == instead: public useEnums2(SomeEnum a) { if(a == SomeEnum.SOME_ENUM_VALUE) { ... } ... } I've been programming in Java for 5+ years, and I thought I understood difference between the two - but I'm still scratching my head at which one is more correct. Which operator is the one I should be using?

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• What does "===" mean?

  - by ShadowJack

  I run into that operator in web development code. What does it mean?

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• Writing C# Code Using SOLID Principles

  - by bipinjoshi

  Most of the modern programming languages including C# support objected oriented programming. Features such as encapsulation, inheritance, overloading and polymorphism are code level features. Using these features is just one part of the story. Equally important is to apply some object oriented design principles while writing your C# code. SOLID principles is a set of five such principles--namely Single Responsibility Principle, Open/Closed Principle, Liskov Substitution Principle, Interface Segregation Principle and Dependency Inversion Principle. Applying these time proven principles make your code structured, neat and easy to maintain. This article discusses SOLID principles and also illustrates how they can be applied to your C# code.http://www.binaryintellect.net/articles/7f857089-68f5-4d76-a3b7-57b898b6f4a8.aspx 

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• Clean file separators in Ruby without File.join

  - by kerry

  I love anything that can be done to clean up source code and make it more readable.  So, when I came upon this post, I was pretty excited.  This is precisely the kind of thing I love. I have never felt good about ‘file separator’ strings b/c of their ugliness and verbosity. In Java we have: 1: String path = "lib"+File.separator+"etc"; And in Ruby a popular method is: 1: path = File.join("lib","etc") Now, by overloading the ‘/’ operator on a String in Ruby: 1: class String 2: def /(str_to_join) 3: File.join(self, str_to_join) 4: end 5: end We can now write: 1: path = 'lib'/'src'/'main' Brilliant!

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• What are the differences between programming languages? [closed]

  - by Omega

  Once upon a time, I heard from someone the only difference between programming languages is the syntax I wanted to deny it - to say that there are other fundamental aspects that truly set a language apart from others than just syntax. But I couldn't... So, can you? Whenever I search Google for something like "differences between programming languages", the results tend to be debates between two specific languages (I'd like something more general) - however, some of the aspects that people seemed to debate the most were: Object-Oriented Method/Operator overloading (I actually see this rather related to syntax) Garbage-Collection (While it seems like a good difference, for some reason it doesn't seem that "fundamental") What important aspects other than syntax can you think of?

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