Question

I’m implementing a stream insertion operator for a class of mine. I’d like my class to work with both narrow and wide streams. I’m using a template to allow this kind of behavior — and everything is independent of which stream type is actually used, with the exception of character literals. If it’s a wide string, the character literals need L prepended to the literal, otherwise they do not.

Is there a way to key this sort of thing to the template parameter so that I don’t need to duplicate so much code on this?

(I would prefer to avoid performing narrow-to-wide character or wide-to-narrow character conversions at runtime if possible.)

Example of what I currently have — it’s a template but it won’t work with narrow character streams because of the wide character literals:

template <typename charT, typename traits>
std::basic_ostream<charT, traits>& operator<<(
    std::basic_ostream<charT, traits>& lhs,
    const Process& rhs
    )
{
    lhs << L"Process (0x" << std::setw(8) << std::hex
        << std::setfill(L'0') << rhs.GetId() << L") ";
    lhs << rhs.GetName() << std::endl;
    lhs << L"Command Line: " << rhs.GetCmdLine() << std::endl;
    const std::vector<Thread>& threads = rhs.GetThreads();
    for (std::vector<Thread>::const_iterator it = threads.begin(); 
        it != threads.end(); ++it)
    {
        lhs << L" --> " << *it << std::endl;
    }
    const std::map<void *, Module>& modules = rhs.GetModules();
    for (std::map<void *, Module>::const_iterator it = modules.begin(); 
        it != modules.end(); ++it)
    {
        lhs << L" --> " << it->second << std::endl;
    }
    return lhs;
}

Answer 1

If you don’t want the runtime overhead, I think that, although ugly, a macro can help you in this case.

template <typename T>
inline const T* select(const char* narrow, const wchar_t* wide);

template <>
inline const char* select<char>(const char* narrow, const wchar_t* /*wide*/)
{
    return narrow;
}

template <>
inline const wchar_t* select<wchar_t>(const char* /*narrow*/, const wchar_t* wide)
{
    return wide;
}

#define doselect(T, str) select<T>(str, L ## str)

template <typename charT, typename traits>
std::basic_ostream<charT, traits>& operator<<(
    std::basic_ostream<charT, traits>& lhs,
    const Process& rhs
    )
{
    lhs << doselect(charT, "Process (0x") << std::setw(8) << std::hex
        << std::setfill(charT('0')) << rhs.GetId() << doselect(charT, ") ");
    lhs << rhs.GetName() << std::endl;
    lhs << doselect(charT, "Command Line: ") << rhs.GetCmdLine() << std::endl;
    const std::vector<Thread>& threads = rhs.GetThreads();
    for (std::vector<Thread>::const_iterator it = threads.begin(); 
        it != threads.end(); ++it)
    {
        lhs << doselect(charT, " --> ") << *it << std::endl;
    }
    const std::map<void *, Module>& modules = rhs.GetModules();
    for (std::map<void *, Module>::const_iterator it = modules.begin(); 
        it != modules.end(); ++it)
    {
        lhs << doselect(charT, " --> ") << it->second << std::endl;
    }
    return lhs;
}

You can probably expand doselect with another nice macro, to further reduce code duplication. i.e. doselect2(" --> ") would automatically expand to doselect(charT, " --> ").