Re: [Boost-users] [Fusion] Output of adapted ADT

On 4/8/2011 7:34 PM, Tore Halvorsen wrote:

On Fri, Apr 8, 2011 at 11:53 AM, TONGARI wrote:

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I'm trying to output a string from an adapted class, but I'm getting 1>main.cpp(33): error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'boost::fusion::extension::adt_attribute_proxy' (or there is no acceptable conversion) for the following code (boost 1_46_1, msvc2010)

Is there a header I'm missing? Very similar code works for BOOST_FUSION_DEFINE_STRUCT - and using ints or doubles also work...

In that case, boost::fusion::front returns not the real element, but a attribute_proxy that separates the use of getter& setter.

Here's a workaround: std::cout<< (std::string const&)boost::fusion::front(f)<< std::endl;

Thanks, but I was not really looking for a workaround. My original problem was actually trying to use an adapted ADT with spirit::karma, but thought I'd tackle this seemingly simpler issue first.

(same as above)

foo f("foo"); using namespace boost::spirit::karma;

generate(std::ostream_iterator<char>(std::cout), string<< eol, f);

... which fails with error C2039: 'const_iterator' : is not a member of 'boost::fusion::extension::adt_attribute_proxy'

So, I guess that adapted_adts really can't be used in the same situations as adapted structs?

Adapted classes are different beasts. Adapted classes may not expose their attributes by reference due to abstraction and information hiding. It's not uncommon for a class not to actually have an actual member variable. The best we can assume is that a class has a getter and setter member functions. When you access its member (e.g. using at_c or dereferencing an iterator), here's what happens: 1) If the adapted class is immutable, it returns the class' adapted get member function for the member. E.g. T get_mem() const; 2) If the adapted class is mutable (your case), then it returns a proxy, that when assigned to, calls the class' adapted set member function for the member. E.g. void set_mem(T const&); The proxy also has a conversion to T which then returns the class' adapted get member function for the member. That's the best we can do. Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com