---
title: "__uuidof Operator | Microsoft Docs"
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  - "__LIBID_"
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---
# __uuidof Operator
**Microsoft Specific**  
  
 Retrieves the GUID attached to the expression.  
  
## Syntax  
  
```  
  
      __uuidof (  
   expression   
)  
```  
  
## Remarks  
 The *expression* can be a type name, pointer, reference, or array of that type, a template specialized on these types, or a variable of these types. The argument is valid as long as the compiler can use it to find the attached GUID.  
  
 A special case of this intrinsic is when either **0** or **NULL** is supplied as the argument. In this case, `__uuidof` will return a GUID made up of zeros.  
  
 Use this keyword to extract the GUID attached to:  
  
-   An object by the [uuid](../cpp/uuid-cpp.md) extended attribute.  
  
-   A library block created with the [module](../windows/module-cpp.md) attribute.  
  
> [!NOTE]
>  In a debug build, `__uuidof` always initializes an object dynamically (at runtime). In a release build, `__uuidof` can statically (at compile time) initialize an object.  
  
## Example  
 The following code (compiled with ole32.lib) will display the uuid of a library block created with the module attribute:  
  
```  
// expre_uuidof.cpp  
// compile with: ole32.lib  
#include "stdio.h"  
#include "windows.h"  
  
[emitidl];  
[module(name="MyLib")];  
[export]  
struct stuff {  
   int i;  
};  
  
int main() {  
   LPOLESTR lpolestr;  
   StringFromCLSID(__uuidof(MyLib), &lpolestr);  
   wprintf_s(L"%s", lpolestr);  
   CoTaskMemFree(lpolestr);  
}  
```  
  
## Comments  
 In cases where the library name is no longer in scope, you can use __LIBID\_ instead of `__uuidof`. For example:  
  
```  
StringFromCLSID(__LIBID_, &lpolestr);  
```  
  
 **END Microsoft Specific**  
  
## See Also  
 [Expressions with Unary Operators](../cpp/expressions-with-unary-operators.md)   
 [Keywords](../cpp/keywords-cpp.md)