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Brew Extensions: How They Work and When to Use Them : Page 3

Unlocking the power of Brew's extensions can be somewhat tedious at first. Once you know the ins and outs, you can bring their extensive horsepower to bear with ease and precision.


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Adventures in C++
On to the bonus round! As the ARM compiler has gotten better at correctly interpreting C++ code, many Brew programmers have decided to take that route for handset development. This makes it impossible, however, to have custom extensions since C++ compilers do not play nice with vtables and the guts of the Brew extensions architecture. Let's take a look at how to make these two languages play nice with each other; along the way you'll make a basic C++ class and explore how to tie these divergent objects together.

Obviously, the Brew Extension is a C only affair. This forces you to 'wrap' all the C++ class methods in C style function calls. Let's see how this applies to the fictitious extension IReversal. IReversal.cpp contains a simple class with one function: Reverse. To get access to this class method from within the C extension, you're going to have to do some preprocessor wizardry. The following code shows the mess you'll have to declare in the header file (IReversal.h) to access the constructor, destructor, and method of the example C++ class:

#ifdef __cplusplus extern "C" { #endif boolean IReversalpp_New(IShell *pShell, void **ppCppObj); char * IReversalpp_Reverse(void *pObject, char *szString); void IReversalpp_Delete(void * pObject); #ifdef __cplusplus } #endif

What's going on in the above declaration? This file is going to be included in both a C and C++ compile which means it'll have to be palatable to both. If you're doing a C++ compile, the precompiler flag __cplusplus will be true which extern the declarations as "C" style functions. This tells the C++ compiler to apply C style preprocessor and linking rules to them. When the compiler comes through IReversal.h from your extension file IReversal.c, these extern commands will be ignored and the functions will be linked normally as C functions. Here's the trick: the functions themselves have to be within a C++ file so they have access to the C++ lexicon of class interactions. By now, you probably have a pretty good idea what these C functions are going to look like. They're small, so take a look at them one at a time. Again, they’d be defined in IReversal.cpp:


boolean IReversalpp_New(IShell *pShell, void **ppCppObj) { //Set up the new pointer and pass in the IShell object. *ppCppObj = (void *) new IReversalpp(pShell); if(*ppCppObj) return TRUE; else return FALSE; }

This function is really nothing more than a wrapper around the "new" function. It allocates and passes back a pointer to this new object. Remember that the C++ class must be stored by the C extension as a void pointer because a C compiler would have no idea what a "class" is. This is what the wrapper itself would look like:

char *IReversalpp_Reverse(void *pObject, char *szString) { IReversalpp pMe = (IReversalpp *) pObject; return pMe->reverse(szString); }

The above code simply casts the void pointer to your class object and calls into the reversal method. As for the actual implementation of the string reversal method, ask any CS 101 student to help you out with that one. Now, you can take a look at the final function in this simple example, the destructor:

void IReversalpp_Delete(void *pObject) { IReversalpp pMe = (IReversalpp *) pObject; delete pMe; }

Again, remember that your version of these should also be within the C++ file. It's what allows them to access C++ syntax while at the same time letting them link into the C code. Using the tricks I've outlined above, you should be on your way to writing an extension wrapping a C++ class. Armed with this knowledge, you could also write your extension in C++ (using any number of classes) and pass that functionally back through BREW's extension methodology.



Chris Haseman is an independent software engineer specializing in wireless development. He can be found riding his bike between coffee shops in San Francisco. He's the author of the book Android Essentials (published by Apress). In his spare time, he's a resident DJ at xtcradio.com and a martial arts instructor.
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