| AngelScript

Property accessors

Many times when working with properties it is necessary to make sure specific logic is followed when accessing them. An example would be to always send a notification when a property is modified, or computing the value of the property from other properties. By implementing property accessor methods for the properties this can be implemented by the class itself, making it easier for the one who accesses the properties.

In AngelScript property accessors are declared with the following syntax:

      class MyObj
      {
        // A virtual property with accessors
        int prop
        {
          get const
          {
            // The actual value of the property could be stored
            // somewhere else, or even computed at access time.
            return realProp;
          }
          set
          {
            // The new value is stored in a hidden parameter appropriately called 'value'.
            realProp = value;
          }
        }

        // The actual value can be stored in a member or elsewhere.
        // It is actually possible to use the same name for the real property, if so is desired.
        private int realProp;
      }

Behind the scene the compiler transforms this into two methods with the name of the property and the prefixes `get_` and `set_`. The following generates the equivalent code, and is perfectly valid too:

      class MyObj
      {
        int get_prop() const { return realProp; }
        void set_prop(int value) { realProp = value; }
        private int realProp;
      }

If you implement the property accessors by explicitly writing the two methods you must make sure the return type of the get accessor and the parameter type of the set accessor match, otherwise the compiler will not know which is the correct type to use.

For interfaces the first alternative is usually the preferred way of declaring the property accessors, as it gets quite short and easy to read.

      interface IProp
      {
        int prop { get const; set; }
      }

You can also leave out either the get or set accessor. If you leave out the set accessor, then the property will be read-only. If you leave out the get accessor, then the property will be write-only.

Property accessors can also be implemented for global properties, which follows the same rules, except the functions are global.

When the property accessors have been declared it is possible to access them like ordinary properties, and the compiler will automatically expand the expressions to the appropriate function calls, either `set_` or `get_` depending on how the property is used in the expression.

      void Func()
      {
        MyObj obj;

        // Set the property value just like a normal property.
        // The compiler will convert this to a call to set_prop(10000).
        obj.prop = 10000;

        // Get the property value just a like a normal property.
        // The compiler will convert this to a call to get_prop().
        assert( obj.prop == 1000 );
      }

Observe that as property accessors are actually a pair of methods rather than direct access to the value, some restrictions apply as to how they can be used in expressions. Expressions that need to read and write the value at the same time is usually not possible, e.g. the increment operator ++, or compound assignments. In such cases the expression must be expanded so that the read and write operation are performed separately, e.g. the increment operator must be rewritten as follows:

      a = a + 1;

Property accessors can be used to emulate a single property or an array of properties accessed through the index operator. Property accessors for indexed access work the same way as ordinary property accessors, except that they take an index argument. The get accessor should take the index argument as the only argument, and the set accessor should take the index argument as the first argument, and the new value as the second argument.

      string firstString;
      string secondString;

      // A global indexed get accessor
      string get_stringArray(int idx)
      {
        switch( idx )
        {
        case 0: return firstString;
        case 1: return secondString;
        }
        return "";
      }

      // A global indexed set accessor
      void set_stringArray(int idx, const string &in value)
      {
        switch( idx )
        {
        case 0: firstString = value; break;
        case 1: secondString = value; break;
        }
      }

      void main()
      {
        // Setting the value of the indexed properties
        stringArray[0] = "Hello";
        stringArray[1] = "World";

        // Reading the value of the indexed properties
        print(StringArray[0] + " " + stringArray[1] + "\n");
      }

The application can optionally turn off support for property accessors, so you need to verify your application's manual to determine if this is supported for your application or not.