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.NET Forum / Languages / C# / March 2008Creating structs
I wish to send in a struct called Str as a parameter to a method. Do i have to create it first and then send it in or is it possible to create in upon construction? For a class i'd do this. doSomething (new MyClass (1)); How can i do this for a struct? -- Regards Konrad Viltersten -------------------------------- sleep - a substitute for coffee for the poor ambition - lack of sense to be lazy > I wish to send in a struct called Str > as a parameter to a method. Do i have > to create it first and then send it in or is it possible to create in > upon construction? I recommend that your first effort should always be to "just try it". :) > For a class i'd do this. > > doSomething (new MyClass (1)); > > How can i do this for a struct? Just as you do for a class. You can use "new" to "instantiate" the needed value for the struct to pass to the method. Just as with the example above, you'll only have access to that value from within the method unless the method copies the value somewhere else. But as long as that's okay, there's no problem (in fact, unlike with a class, you'd have to pass by reference for any changes within the method to be visible outside the method anyway, so if anything it's less likely to be a problem to pass a struct like that) Pete >> I wish to send in a struct called Str >> as a parameter to a method. Do i have [quoted text clipped - 17 lines] > method anyway, so if anything it's less likely to be a problem to pass a > struct like that) I'd like to send in an instance of the struct and pass in a value to a constructor. However, constructors are not allowed for structs. As far i see i can't do the following. doSomething (new MyStruct (1)); My compiler complains. -- Regards Konrad Viltersten -------------------------------- sleep - a substitute for coffee for the poor ambition - lack of sense to be lazy > I'd like to send in an instance of the struct and pass in a > value to a constructor. However, constructors are not > allowed for structs. What makes you say that? It's not true. > As far i see i can't do the following. > > doSomething (new MyStruct (1)); > > My compiler complains. Well, as far as the code you posted goes, of course it would. There's no declaration for MyStruct, nor for doSomething(). My compiler would complain too. Now, if you have some other code, the context in which the above line _would_ otherwise compile (that is, it exists in a larger setting in which MyStruct and doSomething() are actually legitimate, declared identifiers), you should show that code. Likewise, you should be more specific about what's not working than "my compiler complains". There's no way for anyone to just guess at what you're doing wrong. You have to _show_ us. But there's nothing fundamentally wrong with a line of code that looks like what you posted, assuming you've correctly declared the struct and the method being called. Pete >> I'd like to send in an instance of the struct and pass in a >> value to a constructor. However, constructors are not >> allowed for structs. > > What makes you say that? It's not true. To answer your question - the error message i get from the compiler. It read exactly: "Structs cannot contain explicit parameterless constructors" See, now read it omitting the word "parameterless". Note, you need to OMIT it. Otherwise, it might sound like if one actually could declare constructors for structs. :) In plain text - i pulled a donkey. Sorry for confusing both of us. Thanks for the help. -- Regards Konrad Viltersten -------------------------------- sleep - a substitute for coffee for the poor ambition - lack of sense to be lazy > "Structs cannot contain explicit parameterless constructors" A struct inherently contains a parameterless constructor, it must do so because if it didn’t then how would you be able to do something like the following: MyStruct[] myArray = new MyStruct[100]; If the struct didn’t have the parametrless constructor the array above could not possible be created. The question ofcourse would be, why won't the compiler allow you to define your own parameterless constructor and override the default one...... the answer to that is..... I duno! > [...] > The question ofcourse would be, why won't the compiler allow you to > define your own parameterless constructor and override the default > one...... the answer to that is..... I duno! I don't know the answer myself, but if I had to guess, it'd be that it's because the default constructor for a struct basically always fills the struct with zeros. This means that the compiler knows that it doesn't have to do anything special for an array of structs; it's just filled with zeros the way any memory allocated from the heap would be. Likewise for structs found as fields in classes (which also get filled with zeros, not counting instance-initialization -- i.e. parameterless constructor or field initializers -- that _is_ supported for classes and so which might occur). If the language supported defining your own parameterless constructor (or field initializers, which are also not allowed in structs), it would introduce a special case in which the compiler sometimes would have to generate a bunch of code to iterate over an array, or to add implicit calls to the instance-initialization code for class members that are structs. It makes sense to me that rather than introduce that complication, the language designers just decided to forego the utility of user-defined default constructors. It was a trade-off in which simplicity of the language and compiler won (which IMHO is generally the right way to go, though I'm sure not everyone agrees). Basically, value types remain nice and simple with minimal overhead and consistent, predictable behavior. If you want more complex behavior, you need a class. It seems like a reasonable rule to me. Pete > > [...] > > The question ofcourse would be, why won't the compiler allow you to [quoted text clipped - 6 lines] > have to do anything special for an array of structs; it's just filled with > zeros the way any memory allocated from the heap would be. Bingo. That's also the case when the struct is used as a member variable - types are initialized by just wiping out memory. One interesting (to me anyway) point is that from a *language* point of view all value types have a parameterless constructor - but from a *runtime* point of view, value types *never* have a parameterless constructor. (They're initialized with a different op-code.) You can see this by trying to fetch the constructors for a value type using reflection - you won't see the parameterless one.)  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk
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