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

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.

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.

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

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

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.