How should I use FormatMessage() properly in C++?

Question

Without:

• MFC
• ATL

How can I use FormatMessage() to get the error text for a HRESULT?

 HRES         


        

Answer 1

As pointed out in other answers:

• FormatMessage takes a DWORD result not a HRESULT (typically GetLastError()).
• LocalFree is needed to release memory that was allocated by FormatMessage

I took the above points and added a few more for my answer:

• Wrap the FormatMessage in a class to allocate and release memory as needed
• Use operator overload (e.g. operator LPTSTR() const { return ...; } so that your class can be used as a string

class CFormatMessage
{
public:
    CFormatMessage(DWORD dwMessageId,
                   DWORD dwLanguageId = MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT)) :
        m_text(NULL)
    {
        Assign(dwMessageId, dwLanguageId);
    }

    ~CFormatMessage()
    {
        Clear();
    }

    void Clear()
    {
        if (m_text)
        {
            LocalFree(m_text);
            m_text = NULL;
        }
    }

    void Assign(DWORD dwMessageId,
                DWORD dwLanguageId = MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT))
    {
        Clear();
        DWORD dwFlags = FORMAT_MESSAGE_FROM_SYSTEM
            | FORMAT_MESSAGE_ALLOCATE_BUFFER
            | FORMAT_MESSAGE_IGNORE_INSERTS,
        FormatMessage(
            dwFlags,
            NULL,
            dwMessageId,
            dwLanguageId,
            (LPTSTR) &m_text,
            0,
            NULL);
    }

    LPTSTR text() const { return m_text; }
    operator LPTSTR() const { return text(); }

protected:
    LPTSTR m_text;

};

Find a more complete version of the above code here: https://github.com/stephenquan/FormatMessage

With the above class, the usage is simply:

    std::wcout << (LPTSTR) CFormatMessage(GetLastError()) << L"\n";