Q125309: PRB: AllocMemoryDebug Gives Assertion or Trace Error on New[0]

Article: Q125309
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51
Operating System(s): 
Keyword(s): kbnokeyword kbMFC kbVC100 kbVC150 kbVC600 kbDSupport kbGrpDSMFCATL kbNoUpdate
Last Modified: 29-APR-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51 
   - Microsoft Visual C++, 32-bit Editions, version 1.0 
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SYMPTOMS
========

If new is called to allocate an array whose size is determined by a variable
that has a value of zero (0), the program terminates with the following message
in a debug build:

  Assertion Failed! <name> Windows Application:File Afxmem.cpp, Line
  <line>

The <line> value depends on the version of MFC that you have:

- In MFC 2.0 <line> = 275

- In MFC 2.1 <line> = 267

- In MFC 2.5 <line> = 277

- In MFC 2.51 <line> = 286

In MFC 3.x, the code generates a TRACE message similar to this:

  Warning: Allocating zero length memory block.

RESOLUTION
==========

This behavior changed with the introduction of Visual C++ version 2.0. In the
MFC that shipped with version 2.0, a TRACE is output to the Debug Window in
Debug mode.

If you encounter the assertion failure, you can work around it by simply not
calling new for a zero-length array. Check for a zero value before calling new.
If you need to have a valid address for the memory block, allocate an array of
size 1 as in new[1].

STATUS
======

This behavior is by design. It is intended to help programmers detect logic
errors in programs.

MORE INFORMATION
================

The run-time operator new() can be called with the argument zero. A distinct
(non-null) pointer to an object will be returned. (See Section 5.3.3, third
paragraph of the ARM - M.A.Ellis, B.Stroustrup.) This is the behavior of Visual
C++ versions 2.x and 4.0. However, Visual C++ version 4.0 does not warn when
new() is called with a zero size. It is the programmer's responsibility to
ensure the pointer returned by new() is of the appropriate size.

Sample Code
-----------

  /* Compile options needed:
          Default MFC App.
  */ 

  int size=0;
  char * p = new char[size];

  // With MFC 2.51 or earlier, this causes an Assert to be thrown
  // With MFC 3.x this causes a TRACE warning

Additional query words: 1.00 1.50 2.00 2.10 2.50 2.51

======================================================================
Keywords          : kbnokeyword kbMFC kbVC100 kbVC150 kbVC600 kbDSupport kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : :1.0,1.5,1.51
Issue type        : kbprb

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