USES_CONVERSION.

In ATL there are some helper macros defined such as W2A() which is used to convert widechar to ansi and A2W which is used to convert ansi to WideChar. Before using this macros you should initialize the conversion by calling USES_CONVERSION. See the code snippet below.

#include "atlconv.h"
...

// Ansi string.
char* ptrStr = "Example";

// Unicode string.
wchar_t* pUStr = 0;

// Use this macro for Initializing the conversion.
// Actually, some member variable initialization deep inside.
USES_CONVERSION;

// Convert Ansi to Unicode.
pUStr = A2W( ptrStr );

// Convert Unicode to Ansi.
ptrStr = W2A( pUStr );