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  • calling a function from a set of overloads depending on the dynamic type of an object

    - by Jasper
    I feel like the answer to this question is really simple, but I really am having trouble finding it. So here goes: Suppose you have the following classes: class Base; class Child : public Base; class Displayer { public: Displayer(Base* element); Displayer(Child* element); } Additionally, I have a Base* object which might point to either an instance of the class Base or an instance of the class Child. Now I want to create a Displayer based on the element pointed to by object, however, I want to pick the right version of the constructor. As I currently have it, this would accomplish just that (I am being a bit fuzzy with my C++ here, but I think this the clearest way) object->createDisplayer(); virtual void Base::createDisplayer() { new Displayer(this); } virtual void Child::createDisplayer() { new Displayer(this); } This works, however, there is a problem with this: Base and Child are part of the application system, while Displayer is part of the GUI system. I want to build the GUI system independently of the Application system, so that it is easy to replace the GUI. This means that Base and Child should not know about Displayer. However, I do not know how I can achieve this without letting the Application classes know about the GUI. Am I missing something very obvious or am I trying something that is not possible?

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  • Selecting first records of a type in a given period

    - by Emanuil Rusev
    I have a database table that stores user comments: comments(id, user_id, created_at) I want to get from it the number of users that have commented for the first time in the past week. Here's what I have so far: SELECT COUNT(DISTINCT `user_id`) FROM `comments` WHERE `created_at` BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW() This would give the number of users that have commented, but it would not take into consideration whether these comments are first for these users.

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  • Why do Java and C# not have implicit conversions to boolean?

    - by Shaun
    Since I started Java it's been very aggravating for me that it doesn't support implicit conversions from numeric types to booleans, so you can't do things like: if (flags & 0x80) { ... } instead you have to go through this lunacy: if ((flags & 0x80) != 0) { ... } It's the same with null and objects. Every other C-like language I know including JavaScript allows it, so I thought Java was just moronic, but I've just discovered that C# is the same (at least for numbers, don't know about null/objects): http://msdn.microsoft.com/en-us/library/c8f5xwh7(VS.71).aspx Microsoft changed it on purpose from C++, so why? Clearly I'm missing something. Why change (what I thought was) the most natural thing in the world to make it longer to type? What on Earth is wrong with it?

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  • C# Nullable Type question

    - by TatMing
    for example: int? taxid; if (ddlProductTax.SelectedValue == "") { taxid = null; } else { taxid = Convert.ToInt32(ddlProductTax.SelectedValue); } //Correct But int? taxid; taxid = (ddlProductTax.SelectedValue == "" ? null : Convert.ToInt32(ddlProductTax.SelectedValue)); //Error It error say and int32 cannot implicit convert. The ( ? truepart : falsepart); is not short of (if ..else..) ?

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  • C# Extension Method for String Data Type

    - by Jimbo
    My web application deals with strings that need to be converted to numbers alot - users often put commas, currency symbols etc. in these fields so what I want to do is create a string extension method that cleans the field up and converts it to a decimal. For example: decimal myNumber = "$1,250.85".ToDecimal(); Can anyone help with this? Thanks!

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  • enum type in C++

    - by httpinterpret
    This works: enum TPriority { EPriorityIdle = -100, EPriorityLow = -20, EPriorityStandard = 0, EPriorityUserInput = 10, EPriorityHigh = 20 }; TPriority priority = EPriorityIdle; But this doesn't work: TPriority priority = -100; Any reason?

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  • Type-safe generic data structures in plain-old C?

    - by Bradford Larsen
    I have done far more C++ programming than "plain old C" programming. One thing I sorely miss when programming in plain C is type-safe generic data structures, which are provided in C++ via templates. For sake of concreteness, consider a generic singly linked list. In C++, it is a simple matter to define your own template class, and then instantiate it for the types you need. In C, I can think of a few ways of implementing a generic singly linked list: Write the linked list type(s) and supporting procedures once, using void pointers to go around the type system. Write preprocessor macros taking the necessary type names, etc, to generate a type-specific version of the data structure and supporting procedures. Use a more sophisticated, stand-alone tool to generate the code for the types you need. I don't like option 1, as it is subverts the type system, and would likely have worse performance than a specialized type-specific implementation. Using a uniform representation of the data structure for all types, and casting to/from void pointers, so far as I can see, necessitates an indirection that would be avoided by an implementation specialized for the element type. Option 2 doesn't require any extra tools, but it feels somewhat clunky, and could give bad compiler errors when used improperly. Option 3 could give better compiler error messages than option 2, as the specialized data structure code would reside in expanded form that could be opened in an editor and inspected by the programmer (as opposed to code generated by preprocessor macros). However, this option is the most heavyweight, a sort of "poor-man's templates". I have used this approach before, using a simple sed script to specialize a "templated" version of some C code. I would like to program my future "low-level" projects in C rather than C++, but have been frightened by the thought of rewriting common data structures for each specific type. What experience do people have with this issue? Are there good libraries of generic data structures and algorithms in C that do not go with Option 1 (i.e. casting to and from void pointers, which sacrifices type safety and adds a level of indirection)?

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  • Inheritance and Implicit Type Casting

    - by Josué Molina
    Suppose I have the following three classes: class Animal {}; class Human : public Animal {}; class Dog : public Animal { public: void setOwner(Animal* owner) { this->owner = owner; } private: Animal* owner; }; Why is the following allowed, and what exactly is happening? Dog d; Human h; d.setOwner(&h); // ? At first, I tried to cast it like this d.setOwner(&(Animal)h), but the compiler gave me a warning, and I hit a run-time error. Edit: the warning the compiler gave me was "taking address of temporary". Why is this so?

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  • postgres - group by on multiple columns - master/detail type table

    - by smpillay
    I have a table order(orderid, ordernumber, clientid, orderdesc etc.,) and a corresponding status for that order on an order_status table ( statusid, orderid, statusdesc, statusNote, statustimestamp) say I have a record in order as below orderid orderumber clientid orderdesc 1111 00980065 ABC blah..blah.. and a corresponding status entries statusid orderid statusdesc statusNote statustimestamp 11 1111 recvd status blah yyyy-mm-dd:10:00 12 1111 clientproce status blah yyyy-mm-dd:11:00 13 1111 clientnotice status blah yyyy-mm-dd:15:00 14 1111 notified status blah yyyy-mm-dd:17:00 How can I get the following result (latest timestamp along with multiple columns) 1111 14 00980065 ABC blah..blah.. notified status blah yyyy-mm-dd:17:00

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  • How to Create A Document Type Definition

    - by DaveDev
    Hi Guys I've been creating a lot of my own custom attributes in my XHTML documents lately, and am aware that because they are custom attributes, they won't validate against the W3C standard. Isn't it true that I can specify my own DTD to make it validate? If so, can anyone tell me what's involved in doing this in an ASP.NET MVC app? Thanks Dave

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  • Casting to specific asp.net page type

    - by DEH
    I have an asp.net page with a code-behind class definition as follows: public partial class examplepage : System.Web.UI.Page I'd like to set a public property within the page that I can reference from other classes. My understanding is that if I cast to examplepage then I should be able to get at the public property that is specific to example page, as in: string test=((examplepage)HttpContext.Current.Handler).propertyX; However, when I try casting as above the compiler does not recognise examplepage. Can anyone tell me how I can cast? I have no specific namespaces defined. Thanks

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  • ASP.NET - How to edit 'bit' data type?

    - by Peter
    I am coding in Visual Basic. I am using a checkbox control. Now depending on its checked property I need to set/unset a bit column in a SQL Server database. Here's the code: Try conSQL.Open() Dim cmd As New SqlCommand("update Student set send_mail = " + _ sendemailCheckBox.Checked.ToString + " where student_id = '" _ + sidnolabel.Text + "'", conSQL) cmd.ExecuteNonQuery() Finally conSQL.Close() End Try The send_mail attribute is of bit datatype. This code is not working. How do I go about it?

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  • LINQ to SQL auto-generated type for stored procedure

    - by StuffHappens
    Hello. I have the following stored procedure ALTER PROCEDURE [dbo].Test AS BEGIN CREATE TABLE ##table ( ID1 int, ID2 int ) DECLARE @query varchar(MAX); INSERT INTO ##table VALUES(1, 1); SELECT * FROM ##table; END And I try to use it from C# code. I use LINQ to SQL as an O/RM. When I add the procedure to DataBaseContext it says that it can't figure out the return value of this procedure. How to modify the stored procedure so that I can use it with LINQ to SQL. Note: I need to have global template table!

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  • Why can I derived from a templated/generic class based on that type in C# / C++

    - by stusmith
    Title probably doesn't make a lot of sense, so I'll start with some code: class Foo : public std::vector<Foo> { }; ... Foo f; f.push_back( Foo() ); Why is this allowed by the compiler? My brain is melting at this stage, so can anyone explain whether there are any reasons you would want to do this? Unfortunately I've just seen a similar pattern in some production C# code and wondered why anyone would use this pattern.

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  • 9gag type of website

    - by awtasdfas
    for something like www.9gag.com , which open source mvc framework can be used? in particular the main page, with highly rated content is what i am looking for.. along with an option for users to thumbs up/heart every article/post/blog/vlog/podcast/link.

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  • Useless variable name in C struct type definition

    - by user1210233
    I'm implementing a linked list in C. Here's a struct that I made, which represents the linked list: typedef struct llist { struct lnode* head; /* Head pointer either points to a node with data or NULL */ struct lnode* tail; /* Tail pointer either points to a node with data or NULL */ unsigned int size; /* Size of the linked list */ } list; Isn't the "llist" basically useless. When a client uses this library and makes a new linked list, he would have the following declaration: list myList; So typing llist just before the opening brace is practically useless, right? The following code basically does the same job: typedef struct { struct lnode* head; /* Head pointer either points to a node with data or NULL */ struct lnode* tail; /* Tail pointer either points to a node with data or NULL */ unsigned int size; /* Size of the linked list */ } list;

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  • C++: Vector3 type "wall" ?

    - by anon
    Say I have: class Vector3 { float x, y, z; ... bunch of cuntions .. static operator+(const Vector3&, const Vector3); }; Now, suppose I want to have classes: Position, Velocity, that are exactly like Vector3 (basically, I want typedef Vector3 Position; typedef Vector3 Velocity; Except, given: Position position; Vector3 vector3; Velocity velocity; I want to make sure the following can't happen: position + vector3; vector3 + velocity; velocity + position; What is the best way to achieve this?

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  • Make conversion to a native type explicit in C++

    - by Tal Pressman
    I'm trying to write a class that implements 64-bit ints for a compiler that doesn't support long long, to be used in existing code. Basically, I should be able to have a typedef somewhere that selects whether I want to use long long or my class, and everything else should compile and work. So, I obviously need conversion constructors from int, long, etc., and the respective conversion operators (casts) to those types. This seems to cause errors with arithmetic operators. With native types, the compiler "knows" that when operator*(int, char) is called, it should promote the char to int and call operator*(int, int) (rather than casting the int to char, for example). In my case it gets confused between the various built-in operators and the ones I created. It seems to me like if I could flag the conversion operators as explicit somehow, that it would solve the issue, but as far as I can tell the explicit keyword is only for constructors (and I can't make constructors for built-in types). So is there any way of marking the casts as explicit? Or am I barking up the wrong tree here and there's another way of solving this? Or maybe I'm just doing something else wrong...

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