How to print boost::any to a stream?

Question

I have a Map std::map, which comes from the boost::program_options package. Now I would like to print the content o

Answer 1

The list of type switches proposed in other answers can be improved with a loop over a type list using Boost MPL (see documentation of mpl::for_each and mpl::vector). The following code defines an operator<< for any boost::any that is given in the type list SupportedTypes and throws an exception otherwise.

#include 
#include 
#include 

#include 

#include 
#include 
#include 

class StreamInserter
{
private:
    std::ostream& os_;
    const boost::any &v_;
    mutable bool has_printed_;

public:
    struct UnsupportedType {};

    StreamInserter(std::ostream& os, const boost::any &v)
        : os_(os), v_(v), has_printed_(false) {}

    template 
    void operator()(const T&) const
    {
        if (!has_printed_ && v_.type() == typeid(T))
        {
            os_ << boost::any_cast(v_);
            has_printed_ = true;
        }
    }

    void operator()(const UnsupportedType&) const
    {
        if (!has_printed_)
            throw std::runtime_error("unsupported type");
    }
};

std::ostream& operator<<(std::ostream& os, const boost::any& v)
{
    typedef boost::mpl::vector SupportedTypes;
    StreamInserter si(os, v);
    boost::mpl::for_each(si);
    return os;
}

int main(int, char**)
{
    std::cout << boost::any(42.0) << std::endl;
    std::cout << boost::any(42) << std::endl;
    std::cout << boost::any(42UL) << std::endl;
    std::cout << boost::any("42") << std::endl;
    std::cout << boost::any(std::string("42")) << std::endl;
    std::cout << boost::any(bool(42)) << std::endl; // throws exception
}