how to convert const WCHAR * to const char *

You can use sprintf for this purpose:

const char output[256];
const WCHAR* wc = L"Hellow World" ;
sprintf(output, "%ws", wc );

you can also try this:

#include <comdef.h>  // you will need this
const WCHAR* wc = L"Hello World" ;
_bstr_t b(wc);
const char* c = b;
printf("Output: %s\n", c);

_bstr_t implements following conversion operators, which I find quite handy:

operator const wchar_t*( ) const throw( ); 
operator wchar_t*( ) const throw( ); 
operator const char*( ) const; 
operator char*( ) const;

EDIT: clarification with regard to answer comments: line const char* c = b; results in a narrow character copy of the string being created and managed by the _bstr_t instance which will release it once when it is destroyed. The operator just returns a pointer to this copy. Therefore, there is no need to copy this string. Besides, in the question, CString::GetBuffer returns LPTSTR (i.e. TCHAR*) and not LPCTSTR (i.e. const TCHAR*).

Another option is to use conversion macros:

USES_CONVERSION;
const WCHAR* wc = L"Hello World" ;
const char* c = W2A(wc);

The problem with this approach is that the memory for converted string is allocated on stack, so the length of the string is limited. However, this family of conversion macros allow you to select the code page which is to be used for the conversion, which is often needed if wide string contains non-ANSI characters.