Question

I have trying to make wrapper for winapi function GetWindowText.
Function returns std::wstring but i don’t know how to handle where error happen. I return NULL but i know it’s wrong.

std::wstring GetWindowText(HWND handle)
{
    const int size = 1024;
    TCHAR wnd_text[size] = {0};

    HRESULT hr = ::GetWindowText(handle,
                    wnd_text, size);
    if(SUCCEEDED(hr))
        return std::wstring(wnd_text);
    else
        return NULL;    
}

Answer 1

As an alternative to exceptions you could also return the string by reference in the argument list and indicate sucess by returning true or false i.e.

bool GetWindowText(HWND handle, std::wstring& windowText)
{
    const int size = 1024;
    TCHAR wnd_text[size] = {0};

    HRESULT hr = ::GetWindowText(handle,
                    wnd_text, size);
    if(SUCCEEDED(hr))
    {
        windowText = wnd_text;
        return true;
    }
    else
        return false;    
}

Another alternative which avoids the reference argument is to return an instance of a class that wraps a value but also lets you know whether a value is present e.g.

class ValueWrapper
{
public:
    ValueWrapper() : present( false ) {}
    ValueWrapper( const std::wstring& s ) : value( s ), present( true ) {}

    bool isPresent() const { return present; }
    const std::wstring& getValue() const { return value; }

private:
    std::wstring value;
    bool present;
};

Note that you can template this wrapper pretty easily. Your function would then be

ValueWrapper GetWindowText(HWND handle)
{
    const int size = 1024;
    TCHAR wnd_text[size] = {0};

    HRESULT hr = ::GetWindowText(handle,
                    wnd_text, size);
    if(SUCCEEDED(hr))
        return ValueWrapper( wnd_text );
    else
        return ValueWrapper();
}