Microsoft KB Archive/167733

= Operator new does not throw a bad_alloc exception on failure in Visual C++ =

Article ID: 167733

Article Last Modified on 1/9/2006

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APPLIES TO

 The C Run-Time (CRT), when used with:  Microsoft Visual C++ 5.0 Enterprise Edition

 Microsoft Visual C++ 6.0 Enterprise Edition

 Microsoft Visual C++ 5.0 Professional Edition

 Microsoft Visual C++ 6.0 Professional Edition</li></ul>

 Microsoft Visual C++ 6.0 Standard Edition</li></ul>

 Microsoft Visual C++ 2005 Express Edition</li></ul>

 Microsoft Visual C++ .NET 2003 Standard Edition</li></ul>

 Microsoft Visual C++ .NET 2002 Standard Edition</li></ul> </li></ul>

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This article was previously published under Q167733

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SYMPTOMS
Operator new does not throw a bad_alloc exception when it fails. It simply returns a null pointer.

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RESOLUTION
Operator new does call the new handler function after it fails to procure the requested block of memory, but before it returns the null pointer. An application could install a new handler to throw a bad_alloc exception as follows: #include #include <new.h>  int my_new_handler(size_t) { throw std::bad_alloc; return 0; }  int main  { _PNH _old_new_handler; _old_new_handler = _set_new_handler(my_new_handler); /* ... application processing ... */      _set_new_handler(_old_new_handler); return 0; } To call new handler when malloc fails to obtain the requested block of memory, use the _set_new_mode function.

To install the new handler before your global objects are initialized, create a class that sets the new handler in its constructor and installs the old new handler in its destructor. Create a global object of that type and use the init_seg pragma to force this global object to be initialized before any of your global objects. The example below demonstrates this. It also demonstrates the use of _set_new_mode to cause a failed malloc call to generate an exception. Note that to do this, the code below must reside in its own source file. You cannot change the initializations segment more than once per translation unit (source file) with the pragma init_seg. #include #include <new.h>  #pragma init_seg(lib) int my_new_handler(size_t) { throw std::bad_alloc; return 0; }  struct my_new_handler_obj{ _PNH _old_new_handler; int _old_new_mode; _tag_g_new_handler_obj { _old_new_mode = _set_new_mode(1); _old_new_handler = _set_new_handler(my_new_handler); }  ~_tag_g_new_handler_obj { _set_new_handler(_old_new_handler); _set_new_mode(_old_new_mode); }

} _g_new_handler_obj; Operator new, as implemented by Visual C++ 5.0, ignores the function exception specification. So new(std::nothrow) still generates an exception if your new handler is installed to throw an exception as the examples above demonstrate. To change this behavior, override operator new as follows: void *__cdecl operator new(size_t cb, const std::nothrow_t&) throw {   char *p; try { p = new char[cb]; }   catch (std::bad_alloc) { p = 0; }   return p;   }

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STATUS
This is a bug in Microsoft's implementation of operator new as this in not in conformance with the ANSI C++ Standard.

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