[C++-sig] Determine C++ type held by object? (original) (raw)

David Abrahams dave at boost-consulting.com
Tue Dec 13 23:21:20 CET 2005

Alex Mohr <amohr at pixar.com> writes:

extract() is awesome for getting C++ objects out of python objects. However, extract goes through the converters. I would like a way to determine whether an object is holding a particular C++ type without going through converters.

What do you mean by "going through converters," and why do you want to avoid it?

Anyone know of a way to do this?

For those interested, here's why I would like it: I am wrapping a boost::any-like object. 1. I want 'any' objects in python that are passed as arguments to wrapped C++ functions to convert to their held type if necessary. I have this working just fine with a custom converter. Call this converter #1. 2. I want to be able to call wrapped C++ functions that take 'any' objects with arbitrary python objects. That is, I want objects of all types to convert to 'any'. I have attempted to write this converter. Call it converter #2. Here's the problem. In #1's convertible() function, I have to check if the python object is holding an 'any', and if so, get it out and check if the type it's holding is a match. I use extract for this. However, #1's call to extract invokes converter #2. Converter #2 always says it can convert to 'any', and it does so by actually constructing an 'any' object holding what it was given.

Why? You only need to check the convertible function. That would be

  extract<any>().check()

-- Dave Abrahams Boost Consulting www.boost-consulting.com

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