* ping *

Hi Steven,

I have a const problem. Consider the following classes:

struct Something { Something(const char* n): myName(n){} std::string const& getName()const { return myName; } std::string myName; };

struct Container { typedef std::vector< AnyNameable > PublicData;

Container() { Something s1("bla"); Something s2("blabla"); myData.push_back(s1); myData.push_back(s2); }

PublicData getData()const { PublicData res; for (Data::const_iterator it = myData.begin(); it != myData.end() ; ++it) { res.push_back(AnyNameable(*it)); } return res; } private: typedef std::vector< Something > Data; Data myData; };

Container is simply a class returning its "Something", hidden behind AnyNameable (nice way to realize C++ Coding Standards Item 42 BTW).

I have a simple test function: void test() { Container c; Container::PublicData data = c.getData(); for (Container::PublicData::const_iterator it = data.begin(); it != data.end() ;++it) { AnyNameable a(*it); std::cout << "data: " << a.getName() << std::endl; // auxiliary question: why does this not work if I use const _self&? //std::cout << "data: " << (*it).getName() << std::endl; } }

int main() { test(); return 0; }

Now comes the interesting part. If I define AnyNameable using the concept interface, all goes (almost) well:

template<class C, class T> struct getName { static T apply(C const& cont) { return cont.getName(); } }; namespace boost { namespace type_erasure { template<class C, class T, class Base> struct concept_interface< ::getName<C, T>, Base, C> : Base { T getName() { return call(::getName<C, T>(), *this); } }; } }

typedef any< mpl::vector< getName<_self,std::string const&>, relaxed_match, copy_constructible<>, typeid_<> >, const _self& > AnyNameable;

This compiles and outputs as expected.

Problem 1: This will not compile std::cout << "data: " << (*it).getName() << std::endl;

Hmmm ok not so bad. I'll gladly pay the extra line. (interestingly, it would compile and work without const)

Problem 2: if I use the MEMBER macro:

BOOST_TYPE_ERASURE_MEMBER((has_getName), getName, 0); typedef any< mpl::vector< has_getName<std::string const&()>, relaxed_match, copy_constructible<>, typeid_<> >, const _self& > AnyNameable;

Then the code does not compile, unless I accept to give up all of my consts. On one hand, I will find it a easier job to sell type_erasure with the macro, OTOH, I will not give up my consts ;-)

Would it be possible to have the macro AND const-correctness?

Attached, a small test with the complete code.

Thanks, Christophe

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