Question

I have a templatized container class in C++ which is similar to a std::map (it’s basically a thread-safe wrapper around the std::map). I’d like to write a member function which dumps information about the entries in the map. Obviously, however, I don’t know the type of the objects in the map or their keys. The goal is to be able to handle the basic types (integers, strings) and also some specific class types that I am particularly interested in. For any other class, I’d like to at least compile, and preferably do something somewhat intelligent, such as print the address of the object. My approach so far is similar to the following (please note, I didn’t actually compile this or anything…):

template<typename Index, typename Entry>
class ThreadSafeMap
{
    std::map<Index, Entry> storageMap;
    ...
    dumpKeys()
    {
        for(std::map<Index, Entry>::iterator it = storageMap.begin();
            it != storageMap.end();
            ++it)
        {
            std::cout << it->first << " => " << it->second << endl;
        }
    }
    ...
}

This works for basic types. I can also write custom stream insertion functions to handle specific classes I’m interested in. However, I can’t figure out a good way to handle the default case where Index and/or Entry is an unhandled arbitrary class type. Any suggestions?

Answer 1

You can provide a templated << operator to catch the cases where no custom output operator is defined, since any more specialized versions will be preferred over it. For example:

#include <iostream>

namespace detail
{
    template<typename T, typename CharT, typename Traits>
    std::basic_ostream<CharT, Traits> &
    operator<<(std::basic_ostream<CharT, Traits> &os, const T &)
    {
        const char s[] = "<unknown-type>";
        os.write(s, sizeof(s));
        return os;
    }
}

struct Foo {};

int main()
{
    using namespace detail;
    std::cout << 2 << "\n" << Foo() << std::endl;
    return 0;
}

will output:

2
<unknown-type>

The detail namespace is there to keep this “default” output operator from interfering with code in places other than where it is needed. I.e. you should only use it (as in using namespace detail) in your dumpKeys() method.