Error C2664 'BOOL CryptBinaryToStringW(const BYTE *,DWORD,DWORD,LPWSTR,DWORD *)': cannot convert argument 4 from 'std::unique_ptr' to 'LPWSTR'

Question

I am getting the below error when compiling the C++ project.

Error C2664 'BOOL CryptBinaryToStringW(const BYTE *,DWORD,DWORD,LPWSTR,DWORD *)': cannot convert argument 4 from 'std::unique_ptr>' to 'LPWSTR'

at the below line of code:

CryptBinaryToString(reinterpret_cast<const BYTE*>(strData.c_str()), dwSize,
        dwOptions, pwszBuffer, &dwLength);

And also I am getting the below error:

Error C2679 binary '=': no operator found which takes a right-hand operand of type 'std::unique_ptr>' (or there is no acceptable conversion)

at the below line:

sBase64 = pwszBuffer;

Below is the complete code:

bool EMRReader::EncodeBase64(DWORD dwSize, const std::string& strData, wstring& sBase64)
{
    DWORD dwOptions = CRYPT_STRING_BASE64 | CRYPT_STRING_NOCRLF;
    DWORD dwLength = 0;

    BOOL bRet = CryptBinaryToString(reinterpret_cast<const BYTE*>(strData.c_str()), dwSize,
        dwOptions, 0, &dwLength);

    if (!bRet)
        return bRet;

    std::unique_ptr<std::wstring> pwszBuffer = std::make_unique<std::wstring>(dwLength + 1);
    if (!pwszBuffer)
        return FALSE;

    SecureZeroMemory(pwszBuffer.get(), (dwLength + 1) * sizeof(wchar_t));
    CryptBinaryToString(reinterpret_cast<const BYTE*>(strData.c_str()), dwSize,
        dwOptions, pwszBuffer, &dwLength);

    sBase64 = pwszBuffer;

    return TRUE;
}

Could anyone please help me to resolve these errors?

Answer 1

You're assigning a std::unique_ptr<wstring> object to a variable of type wstring that is not allowed. If you want to assign the value of pwszBuffer to a variable of type wstring, you should get the unique_ptr's value and then assign it to the variable.

You can get the value of std::unique_ptr by calling its * operator:

sBase64 = *pwszBuffer;

---

Edit: If you want to pass a std::unique_ptr to a function, you have two ways:

1. Pass it by reference:

void func(std::unique_ptr<std::wstring>& input_ptr) {
    // Do something...
}

and then use it simply:

std::unique_ptr<std::wstring> function_input;
func(function_input);

Or 2. If you want to pass it by value, move it:

void func(std::unique_ptr<std::wstring>& 
input_ptr) {
    // Do something...
}

and then pass it with std::move:

std::unique_ptr<std::wstring> function_input;
func(std::move(function_input));

You should be aware that in this case after moving the function_input, It owns nothing and holds a nullptr and you shouldn't use it out of the func.

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