CODEWITHSUNDEEP
c++-cli
Thomas
Thomas

Reputation: 3358

c++/cli reference to property

Well, I haven't yet found something that says this is impossible, though I'm starting to think it might be. Can you make this work?

using namespace System;

template <typename T> 
void unset(Nullable<T>& var) { var = Nullable<T>(); }
void unset(String^% var) { var=nullptr; }

//this is really a C# class in my situation, so I can't change its types
public ref class Foo 
{
public:
   property Nullable<Decimal> Dec;
   property Nullable<int> Num;
   property String^ Str;
};

int main()
{
   Foo^ foo = gcnew Foo;
   foo->Dec = Decimal(1.2);
   foo->Num = 3;
   foo->Str = "hi";

   unset(foo->Dec);
   unset(foo->Num);
   unset(foo->Str);

   Console::WriteLine(foo->Dec);
   Console::WriteLine(foo->Num);
   Console::WriteLine(foo->Str);
}

Update: unset is called from a code-generating macro which is called on about 50 params. I'd prefer not to have to go make varieties of the macro for each type.

Upvotes: 0

Views: 803

Answers (2)

Thomas
Thomas

Reputation: 3358

For those who may end up here wondering how I got on with this, I ended up being lucky that the class I was working with was an LLBLGen Entity, so I was able to replace

unset(re->var);

with

{  SD::LLBLGen::Pro::ORMSupportClasses::IEntityField2^ f = re->Fields[#var]; \
   if (f->IsNullable) \
       f->CurrentValue = nullptr; }

Upvotes: 0

Hans Passant
Hans Passant

Reputation: 941635

It isn't possible. Setting a property requires calling the property setter function. There is no way to guess for the called method that it needs to call a function vs can assign the passed variable pointer. If you really want to do this then pass a delegate.

There is actually one .NET language that supports it, VB.NET generates code like this:

  T temp = obj->prop;
  func(temp)
  obj->prop = temp;

There is however a dreadful aliasing problem with that, quite undebuggable. This goes belly up in the (rare) case where func() also uses the property. This is otherwise the way you'd work around the limitation, explicitly in your own code.

Beware that your code is wrong, possibly intentional, you are passing a C++ & reference, not a managed % interior pointer. The compiler is going to bitch about that, you can't create references or pointers to managed objects. They move. Unless the reference is to a variable on the stack. It doesn't otherwise change the answer.

Upvotes: 3

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