How to insert NULL character in std::wstringstream?

Question

I need to construct a multi-string (REG_MULTI_SZ) in C++ from a list of values represented in a std::vector<std::wstring>.

This is what first came to mind but was of course incorrect:

std::wstring GetMultiString(std::vector<std::wstring> Values)
{
    std::wstringstream ss;

    for (const std::wstring& Value : Values) {
        ss << Value;
        ss << L"\0";
    }

    ss << L"\0";

    return ss.str();
}

Upon inspection, the returned string didn't have any embedded NULL characters.

Boyhood

ss << L"\0"; is same as ss << L""; and does nothing. The both point to null-terminated char literals, how much nulls are there after the first one, doesn't matter – Peer

@273K Yeah, I am aware of that, but it is very easy to make a mistake hence the Q&A. I wonder if using C++ string literals (L"\0"s) would behave differently? – Boyhood

"As expected"? You wouldn't be asking the question if that was the expected result. – Weeks

Yes, L"\0"s is a string containing the zero char. – Peer

Answer 1

The solution is actually quite simple:

std::wstring GetMultiString(std::vector<std::wstring> Values)
{
    std::wstringstream ss;

    for (const std::wstring& Value : Values) {
        ss << Value;
        ss.put(L'\0');
    }

    ss.put(L'\0');

    return ss.str();
}

The reason why the original code failed to accomplish anything is that ss << L"\0"; is the same as ss << L""; and therefore does nothing.

You can also pass 0 instead of L'\0':

        ss.put(0);

Or use << and std::ends:

        ss << std::ends;

Or use << and C++ string literal operator""s:

        using namespace std::string_literals;
        // ...
        ss << L"\0"s;

Or even basic_ostream::write:

        const wchar_t Null = 0;
        // ...
        ss.write(&Null, 1); // sizeof(Null) / sizeof(wchar_t)

Boyhood

To make this answer better you should explain why the original code failed and how this approach fixes it. – Ladawnladd

The fix here is in single-quotes. You could use put or continue using operator<<; doesn't matter. – Outcross

alternatively, ss.write which is an unformatted write – Creole

@Ladawnladd I updated the question as per your suggestion. – Boyhood

Add that to the answer, not the question. The question states the problem and the answer explains it and how to fix it. – Ladawnladd

@Ladawnladd Reading comprehension faiil on my end, hopefully this fixes it. – Boyhood

@Outcross I prefer put() since it is more clear that you are putting a single character in the stream and you can't use wrong quotes. – Boyhood

Note that you don't have to use a character constant L'\0', you could just use 0. – Weeks

@MarkRansom Good point, but I don't think being more explicit hurts. – Boyhood

Answer 2

There's an obscure manipulator in the standard library for this too:

ss << std::ends;

Peristalsis