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  • Best way to check for nullable bool in a condition expression (if ...)

    - by FireSnake
    I was wondering what was the most clean and understandable syntax for doing condition checks on nullable bools. Is the following good or bad coding style? Is there a way to express the condition better/more cleanly? bool? nullableBool = true; if (nullableBool ?? false) { ... } else { ... } especially the if (nullableBool ?? false) part. I don't like the if (x.HasValue && x.Value) style ... (not sure whether the question has been asked before ... couldn't find something similar with the search)

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  • C# Constructor Problem When Using Generics

    - by Jimbo
    Please see an example of my code below: public class ScrollableCheckboxList { public List<ScrollableCheckboxItem> listitems; public void ScrollableCheckboxList<TModel>(IEnumerable<TModel> items, string valueField, string textField, string titleField) where TModel : class { listitems = new List<ScrollableCheckboxItem>(); foreach (TModel item in items) { Type t = typeof(TModel); PropertyInfo[] props = new [] { t.GetProperty(textField), t.GetProperty(valueField), t.GetProperty(titleField) }; listitems.Add(new ScrollableCheckboxItem { text = props[0].GetValue(item, null).ToString(), value = props[1].GetValue(item, null).ToString(), title = props[2].GetValue(item, null).ToString() }); } } } The code produces the following error: 'ScrollableCheckboxList': member names cannot be the same as their enclosing type This clearly means that there is a method in the class that has the same name as the class, but usually insinuates that the method is trying to return something (which is not allowed) In my case, all I have done is declare a constructor - why would this be a problem?

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  • Type signature "Maybe a" doesn't like "Just [Event]"

    - by sisif
    I'm still learning Haskell and need help with the type inference please! Using packages SDL and Yampa I get the following type signature from FRP.Yampa.reactimate: (Bool -> IO (DTime, Maybe a)) and I want to use it for: myInput :: Bool -> IO (DTime, Maybe [SDL.Event]) myInput isBlocking = do event <- SDL.pollEvent return (1, Just [event]) ... reactimate myInit myInput myOutput mySF but it says Couldn't match expected type `()' against inferred type `[SDL.Event]' Expected type: IO (DTime, Maybe ()) Inferred type: IO (DTime, Maybe [SDL.Event]) In the second argument of `reactimate', namely `input' In the expression: reactimate initialize input output process I thought Maybe a allows me to use anything, even a SDL.Event list? Why is it expecting Maybe () when the type signature is actually Maybe a? Why does it want an empty tuple, or a function taking no arguments, or what is () supposed to be?

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  • C# Using Reflection to Get a Generic Object's (and its Nested Objects) Properties

    - by Jimbo
    This is a scenario created to help understand what Im trying to achieve. I am trying to create a method that returns the specified property of a generic object e.g. public object getValue<TModel>(TModel item, string propertyName) where TModel : class{ PropertyInfo p = typeof(TModel).GetProperty(propertyName); return p.GetValue(item, null); } The code above works fine if you're looking for a property on the TModel item e.g. string customerName = getValue<Customer>(customer, "name"); However, if you want to find out what the customer's group's name is, it becomes a problem: e.g. string customerGroupName = getValue<Customer>(customer, "Group.name"); Hoping someone can give me some insight on this way out scenario - thanks.

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  • Storing a type in C++

    - by perimosocordiae
    Is it possible to store a type name as a C++ variable? For example, like this: type my_type = int; // or string, or Foo, or any other type void* data = ...; my_type* a = (my_type*) data; I know that 99.9% of the time there's a better way to do what you want without resorting to casting void pointers, but I'm curious if C++ allows this sort of thing.

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  • Calling C++/CLI Method with System::DateTime parameter requires 'ValueType' as a parameter?

    - by David Ipsen
    I'm attempting to call a method written in C++/CLI from C#. The C++/CLI code is used to update a TIMESTAMP column in an Oracle database, given a record ID and the System::DateTime object which is the .NET compatible data type for Oracle's TIMESTAMP type. The method I am calling has the following prototype: bool ChangeJobUpdateDate (int jobIdIn, System::DateTime^ updateDateIn) I've added a reference to this DLL project in a test project that I made; I'm writing the tests in C#. However, when I try to call this method from the C# unit test project, the function appears to have the following method declaration (via intellisense): bool ChangeJobUpdateDate (int jobIdIn, ValueType updateDateIn) I'm admittedly not that familiar with C++/CLI, so is there something I'm missing?

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  • (newbie) type signature "Maybe a" doesn't like "Just [Event]"

    - by sisif
    i'm still learning Haskell and need help with the type inference please! using packages SDL and Yampa i get the following type signature from FRP.Yampa.reactimate: (Bool -> IO (DTime, Maybe a)) and i want to use it for: myInput :: Bool -> IO (DTime, Maybe [SDL.Event]) myInput isBlocking = do event <- SDL.pollEvent return (1, Just [event]) ... reactimate myInit myInput myOutput mySF but it says Couldn't match expected type `()' against inferred type `[SDL.Event]' Expected type: IO (DTime, Maybe ()) Inferred type: IO (DTime, Maybe [SDL.Event]) In the second argument of `reactimate', namely `input' In the expression: reactimate initialize input output process i thought "Maybe a" allows me to use anything, even a SDL.Event list? why is it expecting "Maybe ()" when the type signature is actually "Maybe a"? why does it want an empty tuple, or a function taking no arguments, or what is () supposed to be?

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  • How to change a vairable type in C#?

    - by Mosho Mulan
    I wanted to use something like this: if(x==5) { var mydb= ........ ; } else { var mydb = ........ ; } but it didn't work because I can't declare a variable inside if statement. So I tried to do this: var mydb; if (x==5) { mydb= ............. ; } else { mydb=.............; } but id didn't work either because I had to initialize the variable (mydb). So the question is: I don't necessarily know the type of the variable, can I declare it anyway and then change the type inside the if statement?

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  • C# Type comparison

    - by Sean.C
    This has me pooped, is there any reason the following: public abstract class aExtension { public abstract bool LoadExtension(Constants c); // method required in inherit public abstract string AppliesToModule // property required in inherit { get; } public abstract string ExtensionName // property required in inherit { get; } public abstract string ExtensionDescription // property required in inherit { get; } } public class UK : aExtension { public override bool LoadExtension(Constants c) { return true; } public override string AppliesToModule { get { return "string"; } } public override string ExtensionName { get { return "string"; } } public override string ExtensionDescription { get { return "string"; } } } would return false for the following expressions: bool a = t.IsAssignableFrom(aExtension)); bool b = t.BaseType.IsAssignableFrom(aExtension)); bool c = typeof(aExtension).IsAssignableFrom(t); bool d = typeof(aExtension).IsAssignableFrom(t.BaseType); bool e = typeof(aExtension).IsSubclassOf(t); bool f = typeof(aExtension).IsSubclassOf(t.BaseType); bool g = t.IsSubclassOf(typeof(aExtension)); bool h = t.BaseType.IsSubclassOf(typeof(LBT.AdMeter.aExtension)); bool i = t.BaseType.Equals(typeof(aExtension)); bool j = typeof(aExtension).Equals(t.BaseType); T is the reflected Type from the calss UK. Stange thing is i do the exact same thing just on an external assembly in the same application and it works as expected...

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  • Strange type in c++

    - by Cemre
    I have a method with the prototype: bool getAssignment(const Query& query, Assignment *&result); I am a bit confused about the type of the second param (Assignment *&result) since I don't think I have seen something like that before. It is used like: Assignment *a; if (!getAssignment(query, a)) return false; Is it a reference to a pointer or the other way around ? or neither ? Any explanation is appreciated. Thanks.

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  • Haskell type signature with multiple type somethings (predicates?, for example Eq a =>)

    - by Andrew
    I'm not sure if type predicates is the right term, in fact I've never learned the word for this, so an edit to correct would be helpful - I'm referring to when you give the tipe of function f :: a -> b and you want to say a is a Eq and you say f :: Eq a => a -> b, the name for Eq a => - this is the thing i called a type predicate. My question, though, is how to have multiple of these, so if A is an Eq and B is a Num, I could say either f :: Eq a => a -> b or f :: Num b => a -> b. So, how can I have Eq a => and Num b => at the same time? f :: Eq a => Num b => a -> b, f :: Eq a -> Num b => a -> b, and f :: Eq a, Num b => a -> b all didn't do what I wanted.

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  • Casting in mixed type calculations in C?

    - by yCalleecharan
    Hi, If I define these variables: double x0, xn, h; int n; and I have this mathematical expression: h = (xn - x0)/n; Is it necessary that I cast n into double prior doing the division for maximum accuracy like in h = (xn - x0)/ (double) n; I wrote a program to check the above but both expressions give the same answers. I understand that C will promote the integer to double type as variables xn and x0 are of type double but strangely enough in a book, the second expression with casting was emphasized. Thanks a lot...

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  • Nullable values in C++

    - by DanDan
    I'm creating a database access layer in native C++, and I'm looking at ways to support NULL values. Here is what I have so far: class CNullValue { public: static CNullValue Null() { static CNullValue nv; return nv; } }; template<class T> class CNullableT { public: CNullableT(CNullValue &v) : m_Value(T()), m_IsNull(true) { } CNullableT(T value) : m_Value(value), m_IsNull(false) { } bool IsNull() { return m_IsNull; } T GetValue() { return m_Value; } private: T m_Value; bool m_IsNull; }; This is how I'll have to define functions: void StoredProc(int i, CNullableT<int> j) { ...connect to database ...if j.IsNull pass null to database etc } And I call it like this: sp.StoredProc(1, 2); or sp.StoredProc(3, CNullValue::Null()); I was just wondering if there was a better way than this. In particular I don't like the singleton-like object of CNullValue with the statics. I'd prefer to just do sp.StoredProc(3, CNullValue); or something similar. How do others solve this problem?

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  • singledispatch in class, how to dispatch self type

    - by yanxinyou
    Using python3.4. Here I want use singledispatch to dispatch different type in __mul__ method . The code like this : class Vector(object): ## some code not paste @functools.singledispatch def __mul__(self, other): raise NotImplementedError("can't mul these type") @__mul__.register(int) @__mul__.register(object) # Becasue can't use Vector , I have to use object def _(self, other): result = Vector(len(self)) # start with vector of zeros for j in range(len(self)): result[j] = self[j]*other return result @__mul__.register(Vector) # how can I use the self't type @__mul__.register(object) # def _(self, other): pass # need impl As you can see the code , I want support Vector*Vertor , This has Name error Traceback (most recent call last): File "p_algorithms\vector.py", line 6, in <module> class Vector(object): File "p_algorithms\vector.py", line 84, in Vector @__mul__.register(Vector) # how can I use the self't type NameError: name 'Vector' is not defined The question may be How Can I use class Name a Type in the class's method ? I know c++ have font class statement . How python solve my problem ? And it is strange to see result = Vector(len(self)) where the Vector can be used in method body .

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