error C3984: a non-value type cannot have any public data members

Question

Hello community,

I am new to Visual C++, and trying to learn the rules.  In my project I have a simple class, which is really more of a struct, but with some constructors for ease of use. However I am getting errors with my public members.

#include "Object.h"

namespace Gaia
{
	[Windows::Foundation::Metadata::WebHostHidden]
	public ref class Message sealed :
		public Object
	{
	public:

		Message() : Source(""), Type(""), Data(nullptr) {}
		Message(Platform::String^ source, Platform::String^ type) : Source(source), Type(type), Data(nullptr) {}
		Message(Platform::String^ source, Platform::String^ type, Gaia::Object^ data) : Source(source), Type(type), Data(data) {}
		Message(Message^ msg) : Source(msg->Source), Type(msg->Type), Data(msg->Data) {}

		virtual Platform::String^ GetType() { return "Message"; }

		Platform::String^ Source;
		Platform::String^ Type;
		Gaia::Object^ Data;
	};
}

The following errors are shown:

Error 1 error C3984: 'Gaia::Message' : a non-value type cannot have any public data members 'Source' e:\misc\gaia\message.h 29 1 Gaia

Error 2 error C3984: 'Gaia::Message' : a non-value type cannot have any public data members 'Type' e:\misc\gaia\message.h 30 1 Gaia

Error 3 error C3984: 'Gaia::Message' : a non-value type cannot have any public data members 'Data' e:\misc\gaia\message.h 31 1 Gaia

How do I get around this?  Am I not allowed to have public members?  What is meant by a "non-value" type?

---

Land Of Simeria - http://www.stormsonggames.com/los

Answer 1

C++/Cx doesn't support public fields, so you need to make these properties instead.  In this case, they can be done as trivial properties:

property Platform::String^ Source;
property Platform::String^ Type;
property Gaia::Object^ Data;

For details, see Properties: http://msdn.microsoft.com/en-us/library/windows/apps/hh755807.aspx

---

Reed Copsey, Jr. - http://reedcopsey.com
If a post answers your question, please click "Mark As Answer" on that post and "Mark as Helpful".

Answer 2

Thanks Reed.

I also had to initialize the properties inside my constructors, as properties cannot be initialized in a member list.

#include "Object.h"

namespace Gaia
{
	[Windows::Foundation::Metadata::WebHostHidden]
	public ref class Message sealed :
		public Object
	{
	public:

		Message() { Source = ""; Type = ""; Data = nullptr; }
		Message(Platform::String^ source, Platform::String^ type) { Source = source; Type = type; Data = nullptr; }
		Message(Platform::String^ source, Platform::String^ type, Gaia::Object^ data) { Source = source; Type = type; Data = data; }
		Message(Message^ msg) { Source = msg->Source; Type = msg->Type; Data = msg->Data; }

		virtual Platform::String^ GetType() { return "Message"; }

		property Platform::String^ Source;
		property Platform::String^ Type;
		property Gaia::Object^ Data;
	};
}

---

Land Of Simeria - http://www.stormsonggames.com/los

Answer 3

Reading a couple of your posts it sounds like you are coming to C++ and C++/CX at the same time.  There can be some challenges in learning them both together.  Not the least of which is that C++/CX imposes some additional rules and restrictions that C++ does not.  It also has additional features that raw C++ does not.  (Having said that, I would still much rather use C++/CX instead of C++/CLI).

One thing that I do feel is very important when learning is understanding exactly what applies when authoring a WinRT class versus a native class.  What you attempted to do would have been valid C++.

Answer 4

Hi,

I will move this thread to Building Metro style apps with C++ forum. This forum only talk about native codes.

Best regards,
Jesse

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Jesse Jiang [MSFT]
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