Unknown 3rd party code, need help and explanation

[lucky6969b]

Hello,
I am using boost property_tree ptree here

class _cochar {
private:
  BOOL m_bAutoDelete;
  LPSTR m_szBuffer;

public:
  _cochar(LPCWSTR wszText, BOOL bAutoDelete = TRUE)
  {
    m_bAutoDelete = bAutoDelete;
    _ASSERTE(wszText);
    int nLen = wcslen(wszText)+1;
    m_szBuffer = (LPSTR)::CoTaskMemAlloc(nLen * sizeof(CHAR));
    wcstombs(m_szBuffer, wszText, nLen);
  }
  
  ~_cochar()
  {
    if (m_bAutoDelete)
      ::CoTaskMemFree(m_szBuffer);
  }
  
  operator LPCSTR()
  {
    return (LPCSTR)m_szBuffer;
  }
};

ptree& frame = parent.add("FrameName", _cochar(n.c_str()));

In the second coding, I don't understand the error

Error	2	error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const _cochar' (or there is no acceptable conversion)	e:\programming\libraries\boost_1_54_0\boost\property_tree\stream_translator.hpp	33	1	SimExporter2

When I cancel the _cochar(), just n.c_str() in the second snippet, it works
Please help!
Thanks
Jack

[2kaud]

What types are parent.add() expecting?

[lucky6969b]

template<class Type>
        self_type &add(const path_type &path, const Type &value);

I believe it is expecting a template class type

[2kaud]

If you look at the boost source code and the line referenced, it is

s << e;

where e is effectively of type const Type &value. Hence, for the .add method to work, there must be an operator<< defined for the type used. In this case, there needs to be an operator<< defined for the _cochar class.

[lucky6969b]

Sorry 2kaud, at first blush, I seldom use operator overriding, could you give me an example on this?
Happy new year 2014
Thanks
Jack

[2kaud]

As you have defined the LPCSTR operator cast, it might be easier to use

ptree& frame = parent.add("FrameName", (LPCSTR)_cochar(n.c_str()));

as the boost code has

s << e

where s is a templated class of basic_ostream.

and a Happy New Year to you.

[lucky6969b]

Hi
It really works, thanks
But what is the principle behind? Why casting would work with s << e where s is a template class?
Does it bypass the operator method?
I'd like to know.
Thanks
Jack

[2kaud]

s << e has been defined for e of type LPCSTR. So in your parent.add(), as long as the second param is of type LPCSTR then the .add class function is fine. As your _cochar class defined the operator LPCSTR, then (LPCSTR)_cochar(n.cstr()) results in a LPCSTR type which is fine. Sometimes an operator cast is easier to implement than providing an alternative operator overloading function.

[lucky6969b]

Oh I see, so it's the last method LPCSTR() does the magic, Thanks so much
Jack

[Steve R Jones]

Testing.