[GCC-XML] Default parameter values in member functions of templated classes.

Brad King brad.king at kitware.com
Tue Feb 26 10:03:46 EST 2008

Roman Yakovenko wrote:
> On Tue, Feb 26, 2008 at 3:59 PM, Brad King <brad.king at kitware.com> wrote:
>> Roman Yakovenko wrote:
>>  > On Tue, Feb 26, 2008 at 8:27 AM, Bryan Ischo <bji-gccxml at ischo.com> wrote:
>>>>  Can you elaborate on the other limits in the expression-to-string
>>  >>  conversion that people have encountered?  It would help me to know about
>>  >>  these things instead of discovering them later on myself.
>>  >>
>>  >>  As to the utility of correct default attributes - I had wanted to use
>>  >>  the text in the default attribute directly in generated C++ code, but it
>>  >>  wasn't working for defaults like the problematic one I wrote about.  So
>>  >>  I scrapped that, and instead just give the user the text string of the
>>  >>  default value to do with what they will.  It's just for completeness of
>>  >>  my API (the xrtti "extended runtime typing for C++" system) that I want
>>  >>  to be able to provide programmatic representations of the defaults to
>>  >>  the API users, not because I have any particular use case in which valid
>>  >>  default values would be useful.  Perhaps I should submit a feature
>>  >>  request in the gccxml bug database for better expression-to-string
>>  >>  handling, especially with regards to default values?
>>  >
>>  >>From my experience only "very simple" default value expressions work.
>>  > "Very simple" defined as POD, default constructor of non template class.
>>  Roman is correct.  The default argument attribute is intended mostly for
>>  human reference and possibly documentation generation.  Implementing
>>  full support for default arguments would require full expression
>>  dumping, which then gets most of the way to function body support.  It
>>  is non-trivial and was not a design goal of gccxml.
>>  You don't actually need the default arguments to generate bindings.
>>  Just generate one binding for each function signature:
>>   void f(int, int = 1);
>>  gets wrapped with two interfaces.  Something like this:
>>   if(num_args == 1)
>>     {
>>     f(convert_argument<int>(args[0]));
>>     }
>>    else if(num_args == 2)
>>     {
>>     f(convert_argument<int>(args[0]), convert_argument<int>(args[1]));
>>     }
>>  Then just include in the generated interface documentation that f()'s
>>  second argument is optional and the default value is "<default-string>".
> I think that the better solution is to dump default value as is, not
> as gcc expression.

Unfortunately that information is not available at the point where
gccxml dumps.  You could try using the source code location that is
dumped to go back and parse out the default expression.  However, having
written automatic wrapper generation based on gccxml's output, I know
from experience that the approach above works fine.


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