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C++ function with a generic and optional class


C++ function with a generic and optional class

By : user2185837
Date : November 23 2020, 04:01 AM
will be helpful for those in need template argument deduction don't perform implicit conversions.
The issue here is that you passing a pointer to function to fun which is double (*) (std::vector, c_myclass) and the function parameter is expecting std::function, OptionalClass)>, and is using a template parameter that at that moment is unknown.
code :
galg(std::function(f), par, extra)
template <class T> struct identity {using type = T;};
template <class T> using identity_t = typename identity<T>::type;

template <class OptionalClass>
double galg(identity_t<std::function<double(std::vector<double>,OptionalClass)>> func, std::vector<double>& params, OptionalClass extra);

galg(fun, par, extra);


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Can I make a generic optional, defaulting to a certain class?

Can I make a generic optional, defaulting to a certain class?


By : krishna
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , I don't think there's much you can do about it, to be honest. You could make Foo doubly generic:
code :
public class Foo<TData, TArgs> where TArgs : FooEventArgs<TData>
{
    public delegate void EventHandler<TArgs>(object sender, TArgs e);
    public event EventHandler<TArgs> Changed;
}
public class Foo : Foo<object, FooEventArgs>
public interface IFoo
{
    // Members which don't depend on the type parameter
}

public interface IFoo<T> : IFoo
{
    // Members which all use T
}
return optional from generic function in c++

return optional from generic function in c++


By : Mace
Date : March 29 2020, 07:55 AM
seems to work fine First of all, boost::optional or std::optional were made exactly for that case. Is there a reason why you don't want to use them? Because most solutions will either mimic std::optional or they are a poor replacement.
Pass an output parameter by reference.
code :
template <class T>
bool search( T arg, T & out );
template <class T>
const T & search( T arg );
template <class T>
const T * search( T arg );
template <class T>
struct DummyValue;

template <>
struct DummyValue<int>
{
    static constexpr const int value = -1;
    // Or:
    // static int makeDummyValue();
};
// And so on ...
How to declare optional class property with generic `Results`

How to declare optional class property with generic `Results`


By : Joshua Holloway
Date : March 29 2020, 07:55 AM
help you fix your problem Unfortunately, you will probably have to turn Results into [Car] in order for this to work. The built-in Swift collections are covariant, but any user-defined Swift generic type (including user-defined collections) is invariant. There's no way right now to specify otherwise.
iOS 12 SDK Generic function returns Optional.some(nil)

iOS 12 SDK Generic function returns Optional.some(nil)


By : saeba lee
Date : March 29 2020, 07:55 AM
may help you . This is due to an intentional change (#13910) where the compiler is now more conservative with unwrapping an optional value that's being cast to a generic placeholder type. Now, the results you get are more consistent with those that you would get in a non-generic context (see SR-8704 for further discussion).
For example:
code :
// note the constraint `where Key : ExpressibleByStringLiteral` is needlessly restrictive.
extension Dictionary where Key : ExpressibleByStringLiteral {
  func find<T>(key: Key) -> T? {
    return self[key] as? T
  }
}

let dict: [String: Any] = ["foo": "bar"]

let genericBar: Any? = dict.find(key: "bar")
print(genericBar as Any) // in Swift 4.1: nil, in Swift 4.2: Optional(nil)

// `T` in the above example is inferred to be `Any`.
// Let's therefore substitute `as? T` with `as? Any`.
let nonGenericBar = dict["bar"] as? Any
print(nonGenericBar as Any) // in both versions: Optional(nil)
extension Dictionary {
  func find<T>(key: Key) -> T? {
    return (self[key] as? T?) ?? nil
  }
}
extension Dictionary {
  func find<T>(key: Key) -> T? {
    guard let value = self[key] else { return nil }
    return value as? T
  }
}
extension Dictionary {
  func find<T>(key: Key) -> T? {
    return self[key].flatMap { $0 as? T }
  }
}
Compilation error with Optional in generic class

Compilation error with Optional in generic class


By : Tharaa Browner
Date : March 29 2020, 07:55 AM
may help you . I have an unexplained compilation error with guava Optional I have this test for demonstrate : , In the line
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