Interesting behavior of String^

Question

Consider this snippet:

Platform::String^ str2tchar(std::string str)

{
    Platform::String^ res = ""; // 1
    res += (wchar_t) (str.at(0)); //2
    return res;
}

It crashes at 2.  The debugger shows, res is a null pointer. Had I had res = "x" at line 1, then at line 2, it is not null and it does not crash.

Furthermore, at line 2, if I have, res += "xx", (with res = "" at 1), it does not crash. Having that cast (wchar_t) "+=" got fooled on what object is being added.

Yes, I am using UNICODE mode.

Baffled that at line 1, res = "" shows the res as nullptr, even though I assign an empty string to res, (not a nullptr).

Has anyone seen this behavior?

The debugger takes you to Object.cpp:434

    CPPCLI_FUNC Platform::String^ __stdcall __abi_ObjectToString(Platform::Object^ o, bool useIPrintable)
    {
        __abi_IUnknown* pUnk = reinterpret_cast<__abi_IUnknown*>(o);

        HRESULT hr;

        if (useIPrintable)
        {
            Platform::Details::IPrintable^ printable;
            ::Platform::Guid gdIPrintable(__uuidof(Platform::Details::IPrintable^));
434:     hr = pUnk->__abi_QueryInterface(gdIPrintable, (void**)&printable);
            if (SUCCEEDED(hr))
            {
                return printable->ToString();
            }
        }

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sua

Answer 1

Yes this behavior was discussed some at:

http://social.msdn.microsoft.com/Forums/en-US/winappswithnativecode/thread/bd1d7cc0-e9f0-4cda-ad23-053d88b7ee2f

 It looks like you might be trying to convert from std::string to Platform::String.  Perhaps the Code below will help, although it will only work for single character ascii.

std::wstring to_wstring( const std::string & txt )
{
	std::wstring wstr;
	wstr.resize( txt.size() + 1 );	// Allocate space for trailing 0
	
	size_t charsConverted;	// Convert mult-byte-str to wide-char-string
	errno_t err= ::mbstowcs_s( &charsConverted, (wchar_t * )wstr.data(), wstr.size(), txt.data(), txt.size() );
	wstr.pop_back();	// Discard trailing \0

	return wstr;
}



//	std::string		kansas( "kansas" );
//	MyTextBlock->Text= to_PlatString( kansas );
//
Platform::String^ to_PlatString( std::string stdstr ) 
{	
	std::wstring	stdwstr= to_wstring( stdstr );
	return ref new String( stdwstr.c_str() );
}

You would think that there was a built in conversion for std::string to Platform::String available in WinRT. And perhaps there is, but I don't know where.
 

Answer 2

Could you please file a Visual Studio Connect bug on this issue for us to investigate? The += operator you are using isn't documented for the Platform::String.

Thanks!

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David Lamb

Answer 3

Ah. That was the subtle reason that exposed it. Thanks.

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sua

Answer 4

This behavior is seen in Build 8400 in my environment, and was not seen in Build 8250.

Somewhere after 8250, it was introduced.

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sua