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Posted 11 May 2005
Licenced CPOL

C++/CLI Properties - Syntactic sugar for accessor methods

, 11 May 2005
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Looks at the syntax for the declaration and use of properties in C++/CLI


Properties are entities that behave like fields but are internally handled by getter and setter accessor functions. They can be scalar properties (where they behave like a field) or indexed properties (where they behave like an array). In the old syntax, we had to specify the getter and setter methods directly in our code to implement properties - wasn't all that well received as you might guess. In C++/CLI, the syntax is more C#-ish (purely coincidental I am sure) and is easier to write and understand. This article goes through the syntax for both scalar and indexed properties with examples where suitable.

Scalar properties

    int _StudRank = 0;
    property int StudRank
        int get()
            return _StudRank;
        void set(int x)
            _StudRank = x;

The <code lang=mc++>get method is called when the property is read and the <code lang=mc++>set method is called when the property is written into, thereby giving us an option for data-hiding within the class. The compiler emits methods <code lang=mc++>get_StudRank and <code lang=mc++>set_StudRank as the getter and setter methods - but it won't let you access these functions directly - you'll have to use the property name (just like you'd use a field). You can optionally omit either the <code lang=mc++>get method (in which case you have a write-only property) or the <code lang=mc++>set method (a read-only property).

Trivial properties

For trivial properties, the compiler emits the getter and setter methods for you :-

property String^ TrivialString;

A private field of the same type as the property is generated in the IL called <code lang=mc++>'<backing_store>TrivialString' - note how quotes are used, so that there's no great risk of any CLI language allowing a variable of the same name thereby causing a collision. Bare-minimum getter and setter methods are generated - the getter simply returns this private field and the setter sets the passed in value to the private field. Trivial properties are useful when you have a situation where you might need to customize a field's accessor functions in future, but at present you have no specific customization to do. Using trivial properties eliminates the need to use a public field for now and having to later convert it to a property - this could cause problems as it changes the very signature of the class - for example reflection treats fields and properties separately.

Properties and inheritance

Properties can be virtual (which results in virtual getter and setter methods) though the derived class needs to have a property with the same name and the same type. Below sample should make it clear :-

<pre lang=mc++>ref class Base { public: ref struct SBaseData { public: SBaseData() { x = 0; } SBaseData(int n) { x = n; } int x; }; private: SBaseData^ _bd; public: Base() { _bd = gcnew SBaseData(); } virtual property SBaseData^ Data { SBaseData^ get() { return _bd; } void set(SBaseData^ val) { _bd->x = val->x; } } };

Notice the property definition in the above code (bolded out for your convenience). Now here's the derived class :-

<pre lang=mc++>ref class Derived : Base { public: ref struct SDerivedData : Base::SBaseData { public: SDerivedData(int n1, int n2) : SBaseData(n1) { y = n2; } SDerivedData() : SBaseData() { y = 0; } int y; }; private: SDerivedData^ _bd; public: Derived() { _bd = gcnew SDerivedData(); } virtual property SBaseData^ Data { SBaseData^ get() override = Base::Data::get { return _bd; } void set(SBaseData^ val) override = Base::Data::set { try { _bd->x = val->x; _bd->y = safe_cast<SDerivedData^>(val)->y; } catch(InvalidCastException ^) { //log an error } } } };

Notice how we've explicitly used the <code lang=mc++>override keyword for both the get and the set (if we don't then <code lang=mc++>new is assumed). Also note the use of the <code lang=mc++>safe_cast (chances are low that this method gets called for a non-<code lang=mc++>Derived object and if at all so, it'll be due to a programming error).

Static properties

<pre lang=mc++>private: static int _InstanceCount = 0; public: static property int InstanceCount { int get() { return _InstanceCount; } }

Properties can be static (in which case the generated accessor methods are static too). Off-topic perhaps, but note how static fields can be directly initialized (pretty handy).

What if I have a <code lang=mc++>get_ or <code lang=mc++>set_ method matching the name of a property?

You'll get a C3675 error :-)

<pre lang=mc++>/*** This won't compile - Error C3675 static int get_InstanceCount() { return 0; } ***/

It's very strict about this, in fact it won't even let you have a method with different return type and arguments.

<pre lang=mc++>char get_InstanceCount(char) { return 0; } // won't compile either

Indexed properties

Indexed properties allow array like access on an object and there's also support for a default indexed property - essentially a nameless property which lets you directly use <code lang=mc++>[] on the object. Below example features both named and default index properties. I believe C#ers call indexed properties as indexors so perhaps you might see these two words used interchangeably.

<pre lang=mc++>ref class R { private: Hashtable^ h; public: R() { h = gcnew Hashtable(); } //Named property property int Age[String^] { protected: int get(String^ s) { if(h->ContainsKey(s)) { for each(DictionaryEntry de in h) { if(s->CompareTo(de.Key) == 0) return (int)de.Value; } } return 0; } void set(String^ s, int age) { h->Add(s,age); } } //Default property property int default[String^] { int get(String^ s) { return Age[s]; } void set(String^ s, int age) { Age[s] = age; } } };

Notice how I've specified the named property accessor methods as <code lang=mc++> protected. In fact you can have different access levels for <code lang=mc++>get and <code lang=mc++>set methods if you want to. For the default property, specify <code lang=mc++> default as the name of the property. The default property gets compiled to a property called <code lang=mc++>Item in the resulting IL and the class is given the custom attribute <code lang=mc++>System::Reflection::DefaultMemberAttribute as follows - <code lang=mc++>System::Reflection::DefaultMemberAttribute("Item").

You can use it like :-

<pre lang=mc++>R^ r = gcnew R(); r["Nish"] = 27; r["Smitha"] = 15; r["Chris"] = 21; r["Don"] = 87; Console::WriteLine(r["Nish"]); Console::WriteLine(r["George"]); Console::WriteLine(r["Smitha"]); Console::WriteLine(r["Don"]); Console::WriteLine(r["Bert"]);


While properties are basically syntactic sugar for accessor functions, the new syntax is definitely a far improved one over the old syntax. I personally wish that they hadn't decided to compile default indexed properties into a property called <code lang=mc++>Item - this prevents you from using <code lang=mc++>Item as a named indexed property - they could easily have generated a GUID or something as the name for the default indexed property and allowed us to use <code lang=mc++>Item for our own purposes. Considering that MSIL uses the <code lang=mc++>DefaultMemberAttribute attribute to identify the default indexed property, I am pretty sure they were forced to do it this way to be as compatible with C# as possible. Feedback is appreciated including heavily critical ones.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Nish Nishant
United States United States
Nish Nishant is a Principal Software Architect based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored C++/CLI in Action for Manning Publications in 2005, and had previously co-authored Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on and another 250+ blog articles on his WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : If you are interested in hiring Nish as a consultant, you can reach him via his google email id voidnish.

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Comments and Discussions

QuestionHow do I define the default indexer property in a cpp file? Pin
GreenKnight26-Oct-10 5:45
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AnswerRe: How do I define the default indexer property in a cpp file? Pin
Nish Nishant26-Oct-10 6:05
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QuestionPossibility to define property in .cpp file? Pin
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AnswerRe: Possibility to define property in .cpp file? Pin
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GeneralGood articles on C++/CLI Pin
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Question"Is Much Easier to Write and Understand" - say what? Pin
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GeneralC++/CLI Properties implmentaion in cpp Pin
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GeneralRe: C++/CLI Properties implmentaion in cpp Pin
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