How would you convert a std::string to BSTR*?

Question

How would you convert a std::string to BSTR*?

STDMETHODIMP CMyRESTApp::rest(BSTR data, BSTR* restr)
{
    RESTClient restclient;
    RESTClient::response resp = restclient.get(data);

    Log("Response Status code: %s", resp.code);
    Log("Response Body: %s", resp.body);

    *restr = // here
    return S_OK;
}

I need convert the resp.body and this then to be returned for the *restr here.

Knighterrant

Depends. What is a BSTR*? Tag the library you're using. – Bisitun

Answer 1

An ATL based approach is to use ATL::CComBSTR and then a Detach() (or CopyTo(...)) the resultant CComBSTR to the BSTR*

Something like:

CComBSTR temp(stlstr.c_str());
*restr = temp.Detach();

Else in general for std::basic_string you can use the Win32 API Sys* family of functions, such as SysAllocStringByteLen and SysAllocString;

// For the `const char*` data type (`LPCSTR`);
*restr = SysAllocStringByteLen(stlstr.c_str(), stlstr.size());

// More suitable for OLECHAR
*restr = SysAllocString(stlwstr.c_str());

OLECHAR depends on the target platform, but generally it is wchar_t.

Given your code, the shortest snippet could just be;

*restr = SysAllocStringByteLen(resp.body.c_str(), resp.body.size());

Note these Windows API functions use the "usual" windows code page conversions, please see further MSDN documentation on how to control this if required.

Nitroso

Regarding the "depends on underlying char type" comment: the question specifically asks about converting from a std::string, not from a std::wstring or from an arbitrary std::basic_string<T>. – Prescience

@jamesdlin. Correct, this was a general comment. I'll clear that up in the answer. – Nitroso

I have some concern about this code: CComBSTR temp(stlstr.c_str());. While I know there is a CComBSTR constructor taking a const char*, so this code would certainly compile, I'm not sure what kind of "code page" conversion this constructor implements. For example, I don't believe it converts from UTF-8 (possible content of the std::string) to UTF-16 (for the BSTR). Those kind of unclear ambiguous conversions can cause subtle bugs with international text. I'd suggest using code that does explicit conversions from whatever format the std::string uses to UTF-16 for the BSTR. – Irfan

Answer 2

std::string is made by chars; BSTR is usually a Unicode UTF-16 wchar_t-based string, with a length prefix.

Even if one could use a BSTR as a simple way to marshal a byte array (since the BSTR is length-prefixed, so it can store embedded NULs), and so potentially a BSTR could be used also to store non-UTF-16 text, the usual "natural" behavior for a BSTR is to contain a Unicode UTF-16 wchar_t-string.

So, the first problem is to clarify what kind of encoding the std::string uses (for example: Unicode UTF-8? Or some other code page?). Then you have to convert that string to Unicode UTF-16, and create a BSTR containing that UTF-16 string.

To convert from UTF-8 (or some other code page) to UTF-16, you can use the MultiByteToWideChar() function. If the source std::string contains a UTF-8 string, you can use the CP_UTF8 code page value with the aforementioned API.

Once you have the UTF-16 converted string, you can create a BSTR using it, and pass that as the output BSTR* parameter.

The main Win32 API to create a BSTR is SysAllocString(). There are also some variants in which you can specify the string length.

Or, as a more convenient alternative, you can use the ATL's CComBSTR class to wrap a BSTR in safe RAII boundaries, and use its Detach() method to pass the BSTR as an output BSTR* parameter.

CComBSTR bstrResult( /* UTF-16 string from std::string */ );
*restr = bstrResult.Detach();

---

Bonus reading:

Eric's Complete Guide To BSTR Semantics

Irfan

Another alternative is the _bstr_t Class, available as a Compiler COM Support Class in Visual Studio, if you do not want a dependency on ATL. – Merri

Answer 3

This is very much possible :

std::string singer("happy new year 2016");
_bstr_t sa_1(singer.c_str()); //std::string to _bstr_t
_bstr_t sa_2("Goodbye 2015"); 
std::string kapa(sa_2); //_bstr_t to std::string

Logarithmic

And how you convert _bstr_t to BSTR? question is asking std::string to BSTR – Discommodity

Answer 4

size_t sztBuffer = (resp.body.length() + 1) * sizeof(wchar_t);
wchar_t* pBuffer = new wchar_t[resp.body.length() + 1];
ZeroMemory(&pBuffer[0], sztBuffer);
MultiByteToWideChar(CP_ACP, 0, resp.body.c_str(), resp.body.length(), pBuffer, sString.length());    
SysAllocString((OLECHAR*)pBuffer);
delete[] pBuffer;

Do not forget to deallocate it afterward.

Surrogate

That last paragraph is very unhelpful, it is not the contract. – Lucienlucienne

I do not see how this can work if resp.body is a std::string (rather than a std::wstring), which is what the question is asking about. – Prescience

You are right of course. To use this approach with std::string it must be converted to Unicode first. – Surrogate

The code is using a std::string. No need to not use a std::vector<wchar_t>, instead of the clunky array. Plus, your math for calculating the required buffer size is wrong. Let MultiByteToWideChar do the math for you. – Merri