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  • Who deletes the copied instance in + operator ? (c++)

    - by Dima
    Hello, I searched how to implement + operator properly all over the internet and all the results i found do the following steps : const MyClass MyClass::operator+(const MyClass &other) const { MyClass result = *this; // Make a copy of myself. Same as MyClass result(*this); result += other; // Use += to add other to the copy. return result; // All done! } I have few questions about this "process" : Isn't that stupid to implement + operator this way, it calls the assignment operator(which copies the class) in the first line and then the copy constructor in the return (which also copies the class , due to the fact that the return is by value, so it destroys the first copy and creates a new one.. which is frankly not really smart ... ) When i write a=b+c, the b+c part creates a new copy of the class, then the 'a=' part copies the copy to himself. who deletes the copy that b+c created ? Is there a better way to implement + operator without coping the class twice, and also without any memory issues ? thanks in advance

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  • Why friend function is preferred to member function for operator<<

    - by skydoor
    When you are going to print an object, a friend operator<< is used. Can we use member function for operator<< ? class A { public: void operator<<(ostream& i) { i<<"Member function";} friend ostream& operator<<(ostream& i, A& a) { i<<"operator<<"; return i;} }; int main () { A a; A b; A c; cout<<a<<b<<c<<endl; a<<cout; return 0; } One point is that friend function enable us to use it like this cout<<a<<b<<c What other reasons?

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  • operator overloading c++

    - by segfault
    When overloading operators, is it necessary to overload = <= and !=? It seems like it would be smart for c++ to call !operator= for !=, ! for operator<= and !< for operator=. Is that the case, or is it necessary to overload every function?

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  • Problems with first argument being string when overloading the + operator in C++

    - by Chris_45
    I have an selfmade Stringclass: //String.h String & operator = (const String &); String & operator = (char*); const String operator+ (String& s); const String operator+ (char* sA); . . //in main: String s1("hi"); String s2("hello"); str2 = str1 + "ok";//this is ok to do str2 = "ok" + str1;//but not this way //Shouldn't it automatically detect that one argument is a string and in both cases?

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  • IS NULL vs = NULL in where clause + SQL Server

    - by Nev_Rahd
    Hello How to check a value IS NULL [or] = @param (where @param is null) Ex: Select column1 from Table1 where column2 IS NULL => works fine If I want to replace comparing value (IS NULL) with @param. How can this be done Select column1 from Table1 where column2 = @param => this works fine until @param got some value in it and if is null never finds a record. How can this achieve?

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  • IS NULL vs = NULL in where clause + MSSQL

    - by Nev_Rahd
    Hello How to check a value IS NULL [or] = @param (where @param is null) Ex: Select column1 from Table1 where column2 IS NULL = works fine If I want to replace comparing value (IS NULL) with @param. How can this be done Select column1 from Table1 where column2 = @param = this works fine until @param got some value in it and if is null never finds a record. How can this achieve?

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  • Nullable types and ?? operator C# [en-US]

    - by ruimachado
    Nullable types vs Non-nullable types   While developing our C# projects its frequent the null comparison operation to avoid null exceptions. This simple operation is mainly coded using the "var x = null" code example inside an if clause. However not all types of variables are nullable, which means that setting a variable to null is not allowed in every cases, it depends on what kind of type are you defining. But what if there was an extension to your non-nullable type that would convert your variable types to nullable? This extension really exists. As I said before in C# you have nullable types which represent all the values of an underlying type, and an additional null value and can be declared easily using "T?", where T is the type of the variable and for example the normal int type cannot be null, so its a non-nullable type, however if you define a "int?" your variable can be null, what you do is convert a non-nullable type to a nullable type. Example: int x=null;     Not allowed     int? x=null;   Allowed     While using nullable types you can check if a variable is null the same way you do it with nullable types:     But what about setting a default value when a certain variable is null?   In this cases the c# .net framework let you set a default value when you try to assign a nullable type to a non-nullable type, using the ?? operator. If you don't use this operator you can still catch the InvalidOperationException which is throw in this cases. For example  without the ?? operator :     Using the ?? operator your code becomes cleaner and more easy to read and you get a bonus, you can set a default value for multiple variables using the ?? in a chain set.     That’s it,   Thanks, Rui Machado rpmachado.wordpress.com

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  • Can't operator == be applied to generic types in C#?

    - by Hosam Aly
    According to the documentation of the == operator in MSDN, For predefined value types, the equality operator (==) returns true if the values of its operands are equal, false otherwise. For reference types other than string, == returns true if its two operands refer to the same object. For the string type, == compares the values of the strings. User-defined value types can overload the == operator (see operator). So can user-defined reference types, although by default == behaves as described above for both predefined and user-defined reference types. So why does this code snippet fail to compile? void Compare<T>(T x, T y) { return x == y; } I get the error Operator '==' cannot be applied to operands of type 'T' and 'T'. I wonder why, since as far as I understand the == operator is predefined for all types? Edit: Thanks everybody. I didn't notice at first that the statement was about reference types only. I also thought that bit-by-bit comparison is provided for all value types, which I now know is not correct. But, in case I'm using a reference type, would the the == operator use the predefined reference comparison, or would it use the overloaded version of the operator if a type defined one? Edit 2: Through trial and error, we learned that the == operator will use the predefined reference comparison when using an unrestricted generic type. Actually, the compiler will use the best method it can find for the restricted type argument, but will look no further. For example, the code below will always print true, even when Test.test<B>(new B(), new B()) is called: class A { public static bool operator==(A x, A y) { return true; } } class B : A { public static bool operator==(B x, B y) { return false; } } class Test { void test<T>(T a, T b) where T : A { Console.WriteLine(a == b); } }

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  • inserting or updating values into null textboxes [closed]

    - by tanya
    this is my sql table structure create table cottonpurchase ( slipNo int null, purchasedate datetime, farmercode int , farmername varchar (100), villagename varchar (100), basicprice int null, premium int null, totalamountpaid int null, weight int null, totalamountbasic int null, totalamountpremium int null, Yeildestimates int null ) I want to fill in the null values in my sql with actual values and not leave them null but I want to do that from my winforms app form! But if I do that it just adds a new record when what I want is for it to update the record which has, for example, farmercode = 2, farmername = blaah, and so on. I also want to enter this in the same records as in farmercode = 2 and the corresponding corresponding basic price = ... , everytime i do it it ends up making a new record and not in the existing one.

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  • Sql Server Where Case Then Is Null Else Is Not Null

    - by Fabio Montezuma
    I have a procedure which receive a bit variable called @FL_FINALIZADA. If it is null or false I want to restrict my select to show only the rows that contain null DT_FINALIZACAO values. Otherwise I want to show the rows containing not null DT_FINALIZACAO values. Something like this: SELECT * FROM ... WHERE ... AND (OPE.DT_FINALIZACAO = CASE WHEN (@FL_FINALIZADA <> 1) THEN NULL END OR OPE.DT_FINALIZACAO IS NOT NULL) In this case I receive the message: None of the result expressions in a CASE specification can be NULL. How can I achieve this? Thanks in advance.

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  • overloaded stream insertion operator with a vector

    - by Julz
    hi, i'm trying to write an overloaded stream insertion operator for a class who's only member is a vector. i dont really know what i'm doing. (lets make that clear) it's a vector of "Points" which is a struct containing two doubles. i figure what i want is to insert user input (a bunch of doubles) into a stream that i then send to a modifier method? i keep working off other stream insertion examples such as... std::ostream& operator<< (std::ostream& o, Fred const& fred) { return o << fred.i_; } but when i try a similar..... istream & operator >> (istream &inStream, Polygon &vertStr) { inStream >> ws; inStream >> vertStr.vertices; return inStream; } i get an error "no match for operator etc etc. if i leave off the .vertices it compiles but i figure it's not right? (vertices is the name of my vector ) and even if it is right, i dont actually know what syntax to use in my driver to use it? also not %100 on what my modifier method needs to look like. here's my Polygon class //header #ifndef POLYGON_H #define POLYGON_H #include "Segment.h" #include <vector> class Polygon { friend std::istream & operator >> (std::istream &inStream, Polygon &vertStr); public: //Constructor Polygon(const Point &theVerts); //Default Constructor Polygon(); //Copy Constructor Polygon(const Polygon &polyCopy); //Accessor/Modifier methods inline std::vector<Point> getVector() const {return vertices;} //Return number of Vector elements inline int sizeOfVect() const {return (int) vertices.capacity();} //add Point elements to vector inline void setVertices(const Point &theVerts){vertices.push_back (theVerts);} private: std::vector<Point> vertices; }; #endif //Body using namespace std; #include "Polygon.h" // Constructor Polygon::Polygon(const Point &theVerts) { vertices.push_back (theVerts); } //Copy Constructor Polygon::Polygon(const Polygon &polyCopy) { vertices = polyCopy.vertices; } //Default Constructor Polygon::Polygon(){} istream & operator >> (istream &inStream, Polygon &vertStr) { inStream >> ws; inStream >> vertStr; return inStream; } any help greatly appreciated, sorry to be so vague, a lecturer has just kind of given us a brief example of stream insertion then left us on our own thanks. oh i realise there are probably many other problems that need fixing

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  • overloaded stream insetion operator with a vector

    - by julz666
    hi, i'm trying to write an overloaded stream insertion operator for a class who's only member is a vector. i dont really know what i'm doing. (lets make that clear) it's a vector of "Points" which is a struct containing two doubles. i figure what i want is to insert user input (a bunch of doubles) into a stream that i then send to a modifier method? i keep working off other stream insertion examples such as... std::ostream& operator<< (std::ostream& o, Fred const& fred) { return o << fred.i_; } but when i try a similar..... istream & operator >> (istream &inStream, Polygon &vertStr) { inStream >> ws; inStream >> vertStr.vertices; return inStream; } i get an error "no match for operator etc etc. if i leave off the .vertices it compiles but i figure it's not right? (vertices is the name of my vector ) and even if it is right, i dont actually know what syntax to use in my driver to use it? also not %100 on what my modifier method needs to look like. here's my Polygon class //header #ifndef POLYGON_H #define POLYGON_H #include "Segment.h" #include <vector> class Polygon { friend std::istream & operator >> (std::istream &inStream, Polygon &vertStr); public: //Constructor Polygon(const Point &theVerts); //Default Constructor Polygon(); //Copy Constructor Polygon(const Polygon &polyCopy); //Accessor/Modifier methods inline std::vector<Point> getVector() const {return vertices;} //Return number of Vector elements inline int sizeOfVect() const {return (int) vertices.capacity();} //add Point elements to vector inline void setVertices(const Point &theVerts){vertices.push_back (theVerts);} private: std::vector<Point> vertices; }; #endif //Body using namespace std; #include "Polygon.h" // Constructor Polygon::Polygon(const Point &theVerts) { vertices.push_back (theVerts); } //Copy Constructor Polygon::Polygon(const Polygon &polyCopy) { vertices = polyCopy.vertices; } //Default Constructor Polygon::Polygon(){} istream & operator >> (istream &inStream, Polygon &vertStr) { inStream >> ws; inStream >> vertStr; return inStream; } any help greatly appreciated, sorry to be so vague, a lecturer has just kind of given us a brief example of stream insertion then left us on our own thanks. oh i realise there are probably many other problems that need fixing

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  • Operator== in derived class never gets called.

    - by Robin Welch
    Can someone please put me out of my misery with this? I'm trying to figure out why a derived operator== never gets called in a loop. To simplify the example, here's my Base and Derived class: class Base { // ... snipped bool operator==( const Base& other ) const { return name_ == other.name_; } }; class Derived : public Base { // ... snipped bool operator==( const Derived& other ) const { return ( static_cast<const Base&>( *this ) == static_cast<const Base&>( other ) ? age_ == other.age_ : false ); }; Now when I instantiate and compare like this ... Derived p1("Sarah", 42); Derived p2("Sarah", 42); bool z = ( p1 == p2 ); ... all is fine. Here the operator== from Derived gets called, but when I loop over a list, comparing items in a list of pointers to Base objects ... list<Base*> coll; coll.push_back( new Base("fred") ); coll.push_back( new Derived("sarah", 42) ); // ... snipped // Get two items from the list. Base& obj1 = **itr; Base& obj2 = **itr2; cout << obj1.asString() << " " << ( ( obj1 == obj2 ) ? "==" : "!=" ) << " " << obj2.asString() << endl; Here asString() (which is virtual and not shown here for brevity) works fine, but obj1 == obj2 always calls the Base operator== even if the two objects are Derived. I know I'm going to kick myself when I find out what's wrong, but if someone could let me down gently it would be much appreciated.

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  • C++ Beginner - 'friend' functions and << operator overloading: What is the proper way to overload an

    - by Francisco P.
    Hello, everyone! In a project I'm working on, I have a Score class, defined below in score.h. I am trying to overload it so, when a << operation is performed on it, _points + " " + _name is returned. Here's what I tried to do: ostream & Score::operator<< (ostream & os, Score right) { os << right.getPoints() << " " << right.scoreGetName(); return os; } Here are the errors returned: 1>c:\users\francisco\documents\feup\1a2s\prog\projecto3\projecto3\score.h(30) : error C2804: binary 'operator <<' has too many parameters (This error appears 4 times, actually) I managed to get it working by declaring the overload as a friend function: friend ostream & operator<< (ostream & os, Score right); And removing the Score:: from the function declaration in score.cpp (effectively not declaring it as a member). Why does this work, yet the code describe above doesn't? Thanks for your time! Below is the full score.h /////////////////////////////////////////////////////////// // Score.h // Implementation of the Class Score // Created on: 10-Mai-2010 11:43:56 // Original author: Francisco /////////////////////////////////////////////////////////// #ifndef SCORE_H_ #define SCORE_H_ #include <string> #include <iostream> #include <iostream> using std::string; using std::ostream; class Score { public: Score(string name); Score(); virtual ~Score(); void addPoints(int n); string scoreGetName() const; int getPoints() const; void scoreSetName(string name); bool operator>(const Score right) const; ostream & operator<< (ostream & os, Score right); private: string _name; int _points; }; #endif

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  • Ultrawingrid - how to display #1/1/1800# as blank ( as if null )

    - by Charles Hankey
    Ultrawingrid 9.2 VS2008 .net 3.5 My wingrid uses a bindingsource. All datetimes which are null in SQL Server are delivered to the bindingsource as #1/1/1800# I would like Ultrawingrid to display this date as blank as it would a null from source. Also, if the date is null in the grid ( i.e. blanked out ) I would like to update the data source to the date #1/1/1800# ( the framework takes care of getting that date back into the backend as a null ) This seems like it should be a trivial matter but I can find no documentation on just where to intervene so the grid will see a particular date as a null and save a null as a particular date. This is the direction I've been headed but I don't think either is the right place and I can't even get the syntax to work in the BeforeRowUpdate as I cannot see how to set a value that is passed to the data binding without setting the value of control itself, which I think has to remain null in order to display as blank Private Sub ugPropMaster_BeforeRowUpdate(ByVal sender As Object, ByVal e As _ Infragistics.Win.UltraWinGrid.CancelableRowEventArgs) Handles _ ugPropMaster.BeforeRowUpdate If e.Row.Cells.Item("Exdate").Value Is Nothing Then e.Row.Cells("Exdate").Value = CDate(#1/1/1800#) End If End Sub Private Sub ugPropMaster_InitializeRow(ByVal sender As Object, ByVal e As _ Infragistics.Win.UltraWinGrid.InitializeRowEventArgs) Handles _ ugPropMaster.InitializeRow If CDate(e.Row.Cells.Item("Exdate").Value) = CDate(#1/1/1800#) Then e.Row.Cells.Item("Exdate").Value = Nothing End If End Sub Guidance much appreciated

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  • Managed bean property value not set to null

    - by Vladimir
    Hi! I'm new to JSF, so this question might be strange. I have an inputText component's value bound to managed bean's property of type Float. I need to set property to null when inputText field is empty, not to 0 value. It's not done by default, so I added converter with the following method implemented: public Object getAsObject(FacesContext arg0, UIComponent arg1, String arg2) throws ConverterException { if (StringUtils.isEmpty(arg2)) { return null; } float result = Float.parseFloat(arg2); if (result == 0) { return null; } return result; } I registered converter, and assigned it to inputText component. I logged arg2 argument, and also logged return value from getAsObject method. By my log I can see that it returns null value. But, I also log setter property on backing bean and argument is 0 value, not null as expected. To be more precise, it is setter property is called twice, once with null argument, second time with 0 value argument. It still sets backing bean value to 0. How can I set value to null? Thanks in advance.

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  • Lua Operator Overloading

    - by Pessimist
    I've found some places on the web saying that operators in Lua are overloadable but I can't seem to find any example. Can someone provide an example of, say, overloading the + operator to work like the .. operator works for string concatenation? EDIT 1: to Alexander Gladysh and RBerteig: If operator overloading only works when both operands are the same type and changing this behavior wouldn't be easy, then how come the following code works? (I don't mean any offense, I just started learning this language): printf = function(fmt, ...) io.write(string.format(fmt, ...)) end Set = {} Set.mt = {} -- metatable for sets function Set.new (t) local set = {} setmetatable(set, Set.mt) for _, l in ipairs(t) do set[l] = true end return set end function Set.union (a,b) -- THIS IS THE PART THAT MANAGES OPERATOR OVERLOADING WITH OPERANDS OF DIFFERENT TYPES -- if user built new set using: new_set = some_set + some_number if type(a) == "table" and type(b) == "number" then print("building set...") local mixedset = Set.new{} for k in pairs(a) do mixedset[k] = true end mixedset[b] = true return mixedset -- elseif user built new set using: new_set = some_number + some_set elseif type(b) == "table" and type(a) == "number" then print("building set...") local mixedset = Set.new{} for k in pairs(b) do mixedset[k] = true end mixedset[a] = true return mixedset end if getmetatable(a) ~= Set.mt or getmetatable(b) ~= Set.mt then error("attempt to 'add' a set with a non-set value that is also not a number", 2) end local res = Set.new{} for k in pairs(a) do res[k] = true end for k in pairs(b) do res[k] = true end return res end function Set.tostring (set) local s = "{" local sep = "" for e in pairs(set) do s = s .. sep .. e sep = ", " end return s .. "}" end function Set.print (s) print(Set.tostring(s)) end s1 = Set.new{10, 20, 30, 50} s2 = Set.new{30, 1} Set.mt.__add = Set.union -- now try to make a new set by unioning a set plus a number: s3 = s1 + 8 Set.print(s3) --> {1, 10, 20, 30, 50}

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  • How to change the meaning of pointer access operator

    - by kumar_m_kiran
    Hi All, This may be very obvious question, pardon me if so. I have below code snippet out of my project, #include <stdio.h> class X { public: int i; X() : i(0) {}; }; int main(int argc,char *arv[]) { X *ptr = new X[10]; unsigned index = 5; cout<<ptr[index].i<<endl; return 0; } Question Can I change the meaning of the ptr[index] ? Because I need to return the value of ptr[a[index]] where a is an array for subindexing. I do not want to modify existing source code. Any new function added which can change the behavior is needed. Since the access to index operator is in too many places (536 to be precise) in my code, and has complex formulas inside the index subscript operator, I am not inclined to change the code in many locations. PS : 1. I tried operator overload and came to conclusion that it is not possible. 2. Also p[i] will be transformed into *(p+i). I cannot redefine the basic operator '+'. So just want to reconfirm my understanding and if there are any possible short-cuts to achieve. Else I need fix it by royal method of changing every line of code :) .

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  • Throwing exception vs checking null, for a null argument

    - by dotnetdev
    What factors dictate throwing an exception if argument is null (eg if (a is null) throw new ArgumentNullException() ), as opposed to checking the argument if it is null beforehand. I don't see why the exception should be thrown rather than checking for null in the first place? What benefit is there in the throw exception approach? This is for C#/.NET Thanks

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  • Java - JSON Null Exception

    - by user1112111
    Hi, I'm using JSON to deserialize an input string that contains a null value for certain hashmap property. Does anyone have any clue why this exception occurs ? Is it possible that null is not accepted as a value Is this configurable somehow ? input sample: {"prop1":"val1", "prop2":123, "prop3":null} stacktrace: net.sf.json.JSONException: null object at net.sf.json.JSONObject.verifyIsNull(JSONObject.java:2856) at net.sf.json.JSONObject.isEmpty(JSONObject.java:2212) Thanks.

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  • How can a not null constraint be dropped?

    - by Tomislav Nakic-Alfirevic
    Let's say there's a table created as follows: create table testTable ( colA int not null ) How would you drop the not null constraint? I'm looking for something along the lines of ALTER TABLE testTable ALTER COLUMN colA DROP NOT NULL; which is what it would look like if I used PostgreSQL. To my amazement, as far as I've been able to find, the MySQL docs, Google and yes, even Stackoverflow (in spite of dozens or hundreds of NULL-related questions) don't seem to lead towards a single simple SQL statement which will do the job.

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  • Why cast null before checking if object is equal to null?

    - by jacerhea
    I was looking through the "Domain Oriented N-Layered .NET 4.0 Sample App" project and ran across some code that I do not understand. In this project they often use syntax like the following to check arguments for null: public GenericRepository(IQueryableContext context,ITraceManager traceManager) { if (context == (IQueryableContext)null) throw new ArgumentNullException("context", Resources.Messages.exception_ContainerCannotBeNull); Why would you cast null to the type of the object you are checking for null?

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  • How to check if a BOOL is null?

    - by Sheehan Alam
    Is there a way I can check to see if a value is NULL/Nil before assigning it to a BOOL? For example, I have a value in a NSDictionary that can be either TRUE/FALSE/NULL mySTUser.current_user_following = [[results objectForKey:@"current_user_following"]boolValue]; When the value is NULL I get the following error *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSNull boolValue]: unrecognized selector sent to instance I would like to be able to handle the NULL case.

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  • When is `x IS NOT NULL` not the same as `NOT(x IS NULL)`

    - by Mark Hurd
    For what x is The expression x IS NOT NULL is not equal to NOT(x IS NULL), as is the case in 2VL (quote from this answer, which is quoting Fabian Pascal Practical Issues in Database Management - A Reference for the Thinking Practitioner -- near the end of that answer) My guess is when x IS NULL is NULL, but I cannot guess when that would be (i.e. I haven't checked the SQL standard).

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