Converters

In some cases, you want to supply a user-defined conversion. An example of this is in the definition of the javascript string object:

Say you want to expose a point class to javascript, you would define the class as:

    void string::init(context* c)
    {
	class_<string,object,std::string> cls("String",c);
	//Type conversion
	cls.converter(operator const char*);
	cls.converter(arguments<const char*>());
    }

We tell the system that string can be implicitly converted to const char* and equally important, that const char* can be used implicitly to construct a new string object. (The syntax for specifying converters may change)

Wrapping

You have two sorts of conversion: One is the implicit conversion:

    //A function takes a length argument and displays it with the correct unit
    void print(const Length& l);
    //Given that Length has a constructor converter from double, the following is legal JavaScript
    print(5.5);

This is the form of conversion that the converter function is meant for.
In the other case, you have the following:
 

    //A function takes a length argument and displays it with the correct unit
    double square(const double& arg);
    //When exposing this to javascript, we need to use a wrapper type, as primitives can not be
    //represented directly in CliPP.
    var a=square(5.6);

We do not want a to be a Length variable, even though Length has a constructor converter from double
This is the reason for the wrap syntax:

    void number::init(context* c)
    {
	class_<number,object> cls("Number",c);
	//Type wrapping
	cls.wrap(type<double>());
    }

The above statement signals that all double values returned by the system shall be translated to a number object. The above statement will only compile if double is convertible to number (via a constructor) and vice versa, that number is convertible to double (via a implicit type converting function)

You can also supply a third argument to the class_ template definition that will be automatically wrapped in the above manner.

Copyright © 2003-2005 Peder Holt

Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.