optional-parameters | 易学教程

How do I use parameter overloading or optional parameters in rust?

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问题 I am trying to write a print function for a binary tree and here is what I have so far: impl TreeNode { fn print(&self) { self.print(0); } fn print(&self, level: u8) { for _i in range(0,level) { print!("\t"); } match self.data { Some(x) => println!("{}",x), None => () }; match self.left { Some(ref x) => x.print(level+1), None => () }; match self.right { Some(ref x) => x.print(level+1), None => () }; } } I am getting the error: duplicate definition of value print . So I was wondering if there

Using C# delegates with methods with optional parameters [duplicate]

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问题 This question already has answers here : Optional parameters on delegates doesn't work properly [duplicate] (4 answers) Closed 5 years ago . Is there a chance to make this code work? Of course I can make second definition of Foo, but I think it'd be a little non-elegant ;) delegate int Del(int x); static int Foo(int a, int b = 123) { return a+b; } static void Main() { Del d = Foo; } 回答1: Your delegate asks for exactly one parameter , while your Foo() method asks for at most two parameters

Fortran 90 Presence Of Optional Arguments

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问题 I do not understand the behavior of the present() intrinsic function with pgf90 7.2. I wrote a 20 line sample program to test this, but the results still make no sense to me. Observe: subroutine testopt(one,two,three,four,five) implicit none integer, intent(in) :: one,two integer, intent(out) :: three integer, intent(in), optional :: four integer, intent(out), optional :: five three = one + two print *,"present check: ",present(four),present(five) if (present(four) .and. present(five)) then

copy constructor with default arguments

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问题 As far as I know, the copy constructor must be of the form T(const T&) or T(T&) . What if I wanted to add default arguments to the signature? T(const T&, double f = 1.0); Would that be standards compliant? 回答1: Yes. §[class.copy]/2: A non-template constructor for class X is a copy constructor if its first parameter is of type X& , const X& , volatile X& or const volatile X& , and either there are no other parameters or else all other parameters have default arguments [ Example: X::X(const X&)

Why do optional parameters in C# 4.0 require compile-time constants?

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问题 Also is there a way to use run-time values for optional method parameters? 回答1: Optional parameters are required to be constants because they are written out as values of an attribute. Hence they inherit all of the restrictions that an attribute value has. There is no way to directly encode a runtime value. However you can get close with the following pattern public void MyApi(SomeType type = null) { type = type ?? new SomeType(); ... } 回答2: Optional parameters are compiled into the assembly

C# “Constant Objects” to use as default parameters

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Is there any way to create a constant object(ie it cannot be edited and is created at compile time)? I am just playing with the C# language and noticed the optional parameter feature and thought it might be neat to be able to use a default object as an optional parameter. Consider the following: //this class has default settings private const SettingsClass DefaultSettings = new SettingsClass (); public void doSomething(SettingsClass settings = DefaultSettings) { } This obviously does not compile, but is an example of what I would like to do. Would it be possible to create a constant object

Can you use optional parameters in a WCF service method? [duplicate]

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This question already has answers here : Named and optional parameters, and WCF (3 answers) I've seen posts like this and this but they are each a few years old. Can I do something like this? [OperationContract] [FaultContract(typeof(MyCustomFault))] List<InventoryPart> SelectMany(string partialPartNumber, string division = null); You can't. There are many restrictions on WCF regarding the method signatures; some restrictions are because of the host mechanism, and others because of the WSDL/MEX. Despite the fact that WCF could potentially let you have default parameters in your service code

Set default value for DateTime in optional parameter [duplicate]

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问题 This question already has answers here : C# 4.0: Can I use a TimeSpan as an optional parameter with a default value? (8 answers) Closed 6 years ago . How can I set default value for DateTime in optional parameter? public SomeClassInit(Guid docId, DateTime addedOn = DateTime.Now???) { //Init codes here } 回答1: There is a workaround for this, taking advantage of nullable types and the fact that null is a compile-time constant. (It's a bit of a hack though, and I'd suggest avoiding it unless you

C# - Calling a struct constructor that has all defaulted parameters

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I ran into this issue today when creating a struct to hold a bunch of data. Here is an example: public struct ExampleStruct { public int Value { get; private set; } public ExampleStruct(int value = 1) : this() { Value = value; } } Looks fine and dandy. The problem is when I try to use this constructor without specifying a value and desiring to use the defaulted value of 1 for the parameter: private static void Main(string[] args) { ExampleStruct example1 = new ExampleStruct(); Console.WriteLine(example1.Value); } This code outputs 0 and does not output 1 . The reason is that all structs have

Why do optional parameters in C# 4.0 require compile-time constants?

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Also is there a way to use run-time values for optional method parameters? Optional parameters are required to be constants because they are written out as values of an attribute. Hence they inherit all of the restrictions that an attribute value has. There is no way to directly encode a runtime value. However you can get close with the following pattern public void MyApi(SomeType type = null) { type = type ?? new SomeType(); ... } Optional parameters are compiled into the assembly and as such (just like anything that is designated as const ) they must be a compile-time constant. And no, you