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

WINDOWS        | WINDOWS NT

The information in this article applies to:

   The Microsoft Foundation Classes (MFC), included with:
     - Microsoft Visual C++ for Windows, versions 1.0, 1.5, and 1.51
     - Microsoft Visual C++, 32-bit Edition, version 1.0

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>

• In MFC 2.0 <line> = 275
• In MFC 2.1 <line> = 267
• In MFC 2.5 <line> = 277
• In MFC 2.51 <line> = 286

   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