{"id":109922,"date":"2024-06-20T07:00:00","date_gmt":"2024-06-20T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109922"},"modified":"2024-06-20T09:35:36","modified_gmt":"2024-06-20T16:35:36","slug":"20240620-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240620-00\/?p=109922","title":{"rendered":"How to convert between different types of counted-string string types"},"content":{"rendered":"

Last time, we learned about the sadness of treating counted strings as null-terminated strings<\/a>. How can we escape this cycle of depression?<\/p>\n

Almost all of the counted-string types have a way of creating an instance with a pointer and a count. The trick is to use it.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Class<\/th>\nCreator<\/th>\n<\/tr>\n
std::wstring<\/code><\/td>\nstd::wstring(p, n)<\/code><\/td>\n<\/tr>\n
HSTRING<\/code><\/td>\nWindowsCreateString(p, n, &s)<\/code><\/td>\n<\/tr>\n
Platform::String<\/code><\/td>\nref new String(p, n)<\/code><\/td>\n<\/tr>\n
winrt::hstring<\/code><\/td>\nwinrt::hstring(p, n)<\/code><\/td>\n<\/tr>\n
BSTR<\/code><\/td>\nSysAllocStringLen(p, n)<\/code><\/td>\n<\/tr>\n
_bstr_t<\/code><\/td>\n_bstr_t(SysAllocStringLen(p, n), FALSE)<\/code><\/td>\n<\/tr>\n
CComBSTR<\/code><\/td>\nCComBSTR(n, p)<\/code><\/td>\n<\/tr>\n
CStringW<\/code><\/td>\nCStringW(p, n)<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

In most cases, the creator is fairly obvious. The one weirdo is _bstr_t<\/code>, which has no built-in constructor for counted strings. Instead, you have to use the underlying Sys\u00adAlloc\u00adString\u00adLen<\/code> function to create the counted string, and then pass it to the constructor in attach mode (bCopy = FALSE<\/code>).<\/p>\n

Of course, once you get the counted string in, you need a way to get it back out.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Class<\/th>\nPointer and length<\/th>\n<\/tr>\n
std::wstring<\/code><\/td>\ns.c_str(), s.size()<\/code><\/td>\n<\/tr>\n
HSTRING<\/code><\/td>\nWindowsGetStringRawBuffer(s, &n), n<\/code><\/td>\n<\/tr>\n
Platform::String<\/code><\/td>\ns->Data(), s->Length()<\/code><\/td>\n<\/tr>\n
winrt::hstring<\/code><\/td>\ns.c_str(), s.size()<\/code><\/td>\n<\/tr>\n
BSTR<\/code><\/td>\ns, SysStringLen(s)<\/code><\/td>\n<\/tr>\n
_bstr_t<\/code><\/td>\nstatic_cast<wchar_t*>(s), s.length()<\/code><\/td>\n<\/tr>\n
CComBSTR<\/code><\/td>\nstatic_cast<BSTR>(s), s.Length()<\/code><\/td>\n<\/tr>\n
CStringW<\/code><\/td>\nstatic_cast<wchar_t const*>(s), s.GetLength()<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The static_cast<\/code>s in the final three rows are typically not necessary if the compiler knows that it has to produce a wchar_t const*<\/code>.<\/p>\n

Unfortunately, combining these two tables gives you a horrible n<\/var> \u00d7 n<\/var> matrix of possibilities. Fortunately, you can reduce this to linear complexity by using std::wstring_view<\/code> as a common currency.<\/p>\n

std::wstring_view to_wstring_view(std::wstring const& s)\r\n{\r\n    return s;\r\n    \/\/ or: return { s.c_str(), s.size() };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(HSTRING s)\r\n{\r\n    UINT32 size;\r\n    auto p = WindowsGetStringRawBuffer(s, &size);\r\n    return { p, size };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(Platform::String^ const& s)\r\n{\r\n    return { s->Data(), s->Length() };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(winrt::hstring const& s)\r\n{\r\n    return { s.c_str(), s.size() };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(wchar_t*) = delete;\r\n\r\nstd::wstring_view bstr_to_wstring_view(BSTR s)\r\n{\r\n    return { s, SysStringLen(s) };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(_bstr_t const& s)\r\n{\r\n    return { s, s.length() };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(CComBSTR const& s)\r\n{\r\n    return { s, s.Length() };\r\n}\r\n\r\nstd::wstring_view to_wstring_view(CStringW const& s)\r\n{\r\n    return { s, s.GetLength() };\r\n}\r\n<\/pre>\n
For std::wstring<\/code>, we can take advantage of the pre-existing wstring_view<\/code> conversion operator.<\/p>\n
Note that the BSTR<\/code> version has to be named bstr_to_wstring_view<\/code> because BSTR<\/code> is a type alias for wchar_t*<\/code>, so just calling it to_wstring_view<\/code> would make it too easy to pass wchar_t*<\/code>s that aren’t really BSTR<\/code>s. We delete the to_wstring_view(wchar_t*)<\/code> overload so that the compiler won’t choose to convert the pointer to a std::wstring<\/code> and then use the std::wstring<\/code> overload, which would result in a dangling pointer.\u00b9<\/p>\n
The other half is creating the counted string from a wstring_view<\/code>.<\/p>\n
template<typename T>\r\nT size_check(std::wstring_view v)\r\n{\r\n    if (v.size() > (std::numeric_limits<T>::max)()) {\r\n        throw std::bad_alloc();\r\n    }\r\n    return static_cast<T>(v.size());\r\n}\r\n\r\ntemplate<typename T>\r\nT from_wstring_view(std::wstring_view v) = delete;\r\n\r\ntemplate<>\r\nstd::wstring from_wstring_view<std::wstring>(std::wstring_view v)\r\n{\r\n    return std::wstring(v.data(), v.size());\r\n}\r\n\r\ntemplate<>\r\nHSTRING from_wstring_view(std::wstring_view v)\r\n{\r\n    HSTRING s;\r\n    THROW_IF_FAILED(\r\n        WindowsCreateString(v.data(),\r\n                            size_check<UINT32>(v)));\r\n    return s;\r\n}\r\n\r\ntemplate<>\r\nPlatform::String^ from_wstring_view<Platform::String^>(std::wstring_view v)\r\n{\r\n    return ref new Platform::String(\r\n        v.data(),\r\n        size_check<UINT32>(v));\r\n}\r\n\r\ntemplate<>\r\nwinrt::hstring from_wstring_view<winrt::hstring>(std::wstring_view v)\r\n{\r\n    return winrt::hstring(v.c_str(), size_check<uint32_t>(v));\r\n}\r\n\r\ntemplate<>\r\nBSTR from_wstring_view<BSTR>(std::wstring_view v)\r\n{\r\n    return ::SysAllocStringLen(v.data(), size_check<UINT>(v));\r\n}\r\n\r\ntemplate<>\r\n_bstr_t from_wstring_view<_bstr_t>(std::wstring_view v)\r\n{\r\n    return _bstr_t(\r\n        ::SysAllocStringLen(v.data(), size_check<UINT>(v)),\r\n        FALSE);\r\n}\r\n\r\ntemplate<>\r\nCComBSTR from_wstring_view<CComBSTR>(std::wstring_view v)\r\n{\r\n    return CComBSTR(size_check<int>(v), v.data());\r\n}\r\n\r\ntemplate<>\r\nCStringW from_wstring_view<CStringW>(std::wstring_view v)\r\n{\r\n    return CStringW(v.data(), size_check<int>(v));\r\n}\r\n<\/pre>\n
You can now convert from one type to another through the pair of functions to_wstring_view<\/code> and from_wstring_view<\/code>.<\/p>\n
\/\/ _bstr_t to winrt::hstring\r\nauto s = from_wstring_view<winrt::hstring>(to_hstring_view(b));\r\n<\/pre>\n
You could wrap this in an adapter class, with a bunch of custom conversion operators:<\/p>\n
struct counted_string_converter\r\n{\r\n    std::wstring_view v;\r\n\r\n    template<typename T>\r\n    counted_string_converter(T const& source) :\r\n        v(to_wstring_view(source)) {}\r\n\r\n    counted_string_converter(std::wstring_view source) :\r\n        v(source) {}\r\n\r\n    static counted_string_converter from_bstr(BSTR source)\r\n    {\r\n        return counted_string_converter(\r\n            bstr_to_wstring_view(source));\r\n    }\r\n\r\n    template<typename T>\r\n    operator T() const {\r\n        return from_wstring_view<T>(v);\r\n    }\r\n};\r\n\r\nwinrt::hstring GetName();\r\n\r\nvoid example()\r\n{\r\n    std::wstring s = counted_string_converter(GetName());\r\n}\r\n<\/pre>\n
The danger with the counted_string_converter<\/code> is that you might try to store it in an auto<\/code>, which could produce dangling pointers.<\/p>\n
winrt::hstring GetName();\r\n\r\nvoid example()\r\n{\r\n    \/\/ Oops: Reference to destroyed temporary\r\n    auto s = counted_string_converter(GetName());\r\n}\r\n<\/pre>\n
All of this is really awkward, and the simplest approach might be to create custom case-by-case conversions. After all, it’s unlikely that any one program is going to need all n<\/var> \u00d7 n<\/var> conversions.<\/p>\n
Bonus chatter<\/b>: The to_wstring_view<\/code> is handy if you want to insert these counted strings into a C++ stream.<\/p>\n
BSTR s = \u27e6something\u27e7\r\n\r\n\/\/ This stops at the embedded null.\r\nstd::wcout << s;\r\n\r\n\/\/ This prints the whole string.\r\nstd::wcout << bstr_to_wstring_view(s);\r\n<\/pre>\n
\u00b9 If you really want to construct a wstring_view<\/code> from a pointer to a null-terminated C-style string, then just use the wstring_view<\/code> constructor.<\/p>\n","protected":false},"excerpt":{"rendered":"
Looking for constructors that take a character count.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-109922","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Looking for constructors that take a character count.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109922","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=109922"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109922\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=109922"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109922"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109922"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}