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

A little late for this party, but anyone that may be interested can also use std::tuple and a std::for_each-like template that iterates over a tuple.

This is based on the answer from ingomueller.net in this thread.

I had a recent case where I created a property map (reading configuration values, mainly fundamental types, from an XML file and inserting them into an std::unordered_map, where the value type is of type any. For debugging purposes I wanted to be able to print the entire map with its keys and values along with the type of the value.

In that project I am not using Boost at all, I used my own any implementation, but its very similar to boost::any.

The insertion operator basically looks like this:

template 
inline std::basic_ostream&
operator<< (std::basic_ostream& os, const sl::common::any& v)
{
    // Types that we support with sl::common::any.
    std::tuple<
        float, double, bool, 
        int8_t, uint8_t, 
        int16_t, uint16_t,
        int32_t, uint32_t, 
        int64_t, uint64_t,
        std::wstring, const wchar_t*,
        StreamInserter::UnsupportedType> t;

    // Prepare ostream for printing a value of type any
    StreamInserter si(os, v);

    // Iterate over all types in tuple t. If the last type(UnsupportedType) is
    // reached, given v is unsupported.
    for_each(t, si);
    return os;
}

The for_each template looks like this (C++14):

template 
constexpr void for_each_impl(Tuple&& tuple, F&& f, std::index_sequence) {
    using swallow = int[];
    (void)swallow{1,
        (f(std::get(std::forward(tuple))), void(), int{})...
    };
}

template 
constexpr void for_each(Tuple&& tuple, F&& f) {
    constexpr std::size_t N = std::tuple_size>::value;
    for_each_impl(std::forward(tuple), std::forward(f),
                  std::make_index_sequence{});
}

With this just use the StreamInserter class or something similar shown in Ingos answer.

Hope this helps.