INFO: Can't Watch Variables in Debugger - Nine Possible Causes

ID: Q121367

The information in this article applies to:

The Visual Workbench Integrated Debugger
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0
- Microsoft CodeView for MS-DOS, versions 4.00, 4.01, 4.10
- Microsoft CodeView for Windows, versions 4.00, 4.01, 4.10

SUMMARY

This article gives nine reasons why you may be having trouble watching variables. It covers each of the following nine symptoms:

Watch on Global Const Variable Gives CXX0030 or "Cannot watch variable"

Watch on Base Class Member Gives CXX0030 or -??? * = {...}

CV1255 or "Unable to expand expression"

Watch on Structure Data Member Produces Incorrect Result

Can't View Data When Watching Or Quick Watching Static Data Variable

Expansion of *Reference* to Object May Consist of Only a Blank Line

CV1255 When Trying to Watch Elements of a Structure (MASM 6.10a)

Arrays Expanded Incorrectly and Values Displayed Are Wrong

Watch on Struct Member Gives CXX0030 If Offset >= 32K (C++ only)

MORE INFORMATION

Watch on Global Const Variable Gives CXX0030 or "Cannot watch variable"

When debugging a C++ program, watching a global const variable in the Watch window of Codeview or the Watch window of Visual Workbench IDE Debugger yields the following error:

CXX0030: Error: expression cannot be evaluated

Using the QuickWatch tool button in Visual Workbench IDE Debugger gives a dialog box that says:

Cannot watch 'variable'.

This is by design but is under review for future releases of Microsoft debug products. In Microsoft C++, storage of global constants is optimized for the purpose of saving space, so no debugging information is generated.

For more information, please see the following article in the Microsoft Knowledge Base:

Q115347 PRB: Debugger Cannot Watch Global Constants in C++ Programs

Watch on Base Class Member Gives CXX0030 or -??? * = {...}

An attempt to place a watch on a base class member variable within a derived class may fail. Microsoft Codeview for MS-DOS, Codeview for Windows, and the Visual Workbench IDE debugger may display one of the following messages in the Watch or Locals window:

CXX0030: Error: expression cannot be evaluated

-or-

-??? * = {...} which expands into the CXX0030 message

-or-

Just blank lines

If you get these error messages, add a default constructor for the base class. For information on this problem, please see the following article in the Microsoft Knowledge Base:

Q102212 BUG: Visual C++ Debugger Unable to Watch Base Class Members

IMPORTANT NOTE: If you get blank lines when expanding a member variable of a base class within a derived class, put a watch (or a Quick Watch) on the entire derived class object. Then expand it to see the base class members. This is often the case with base class member variables that are CStrings, and occurs even when there is a default constructor.

CV1255 or "Unable to expand expression"

If forward class references are added to STDAFX.H and a project is compiled with /Zi /Yu"stdafx.h", attempts to expand objects (of the type forward referenced in STDAFX.H) in the locals or watch window do nothing. Instead, the status bar states "Unable to expand expression" in 32-bit Visual C++ or "CV1255:Error: no data members" in 16-bit Visual C++.

If the forward references are removed from STDAFX.H, the objects can be expanded. The problem can also be corrected by rebuilding the project using either /YX or /Yc /Yd. In spite of the fact that the manuals state that /Yd is obsolete when used with /Zi, this is actually not the case.

For additional information, please see the following articles in the Microsoft Knowledge Base:

Q131147 BUG: CXX0033 Error in OMF Type from Forward Class Declaration

Watch on Structure Data Member Produces Incorrect Result

Setting a watch on a data member of a structure whose tag name is the same as the struct variable name produces an incorrect result. This problem occurs with C++ programs in the Visual Workbench IDE Debugger for Windows, Codeview for Windows, and Codeview for MS-DOS. It also occurs with C programs in the Visual Workbench Debugger for Windows NT and Codeview for Win32s.

For more information, please see the following article in the Microsoft Knowledge Base:

Q106394 BUG: Struct Data Member Incorrectly Displayed in Watch Window

Can't View Data When Watching or Quick Watching a Static Data Variable

When trying to watch or quick watch a static data variable in the context of a static member function, or a static data member in a class, you may not be able to view the data. If this is the case, you can refer to the static data by prefixing it with the class name (for example, CLASS::staticdata).

When trying to watch or quick watch a DLL exported static data member from the debug session of the .EXE, you may not be able to view the data. This is due to a bug in the debugger. The only workaround is to watch the data member in the DLL debug session using the syntax suggested above.

Expansion of Reference to an Object May Consist of Only a Blank Line

When trying to expand a reference to an object in the Watch or Locals window, the expansion may consist of only a blank line. This is due to a bug in both CodeView and the Visual C++ IDE debugger (16-bit version only). To work around the problem, cast the reference (in the watch window) as a pointer to the class. For example, if you are trying to expand a reference defined as "CRect &rect," in the watch window, type "(CRect*)rect" (without the quotation marks). Then expand it.

CV1255 When Trying to Watch Elements Of a Structure (MASM 6.10a)

Using MASM version 6.10a, if you create a program with a structure and use the OPTION OLDSTRUCTS directive, you may get the error: CV1255 when trying to watch elements of the structure. There is no workaround for this version other than not using the OLDSTRUCTS option.

Array Expanded Incorrectly and Values Displayed Are Wrong

In C++ source files, passing an array as an argument to a function and then trying to watch that array in the function causes CodeView and the Visual WorkBench Debugger (16-bit) to expand the array incorrectly.

To work around the problem, declare the function argument as a pointer rather than an array. For example, change a declaration which uses array syntax:


void Function( int Array[5] )

to a declaration which uses pointer syntax:


void Function( int * Array )

For more information, please see the following article in the Microsoft Knowledge Base:

Q112984 BUG: Debugger Expands Arrays Passed to Functions Incorrectly

Watch on Struct Member Gives CXX0030 If Offset >= 32K (C++ only)

If a C++ application has a global struct containing member arrays that exceed 32K total, expanding a member of the struct in the watch window will give CXX0030 if the starting address of the member is over 0x7FFF from the beginning of the struct. This is a bug in Visual C++ version 1.5x (16-bit).

The following code reproduces the problem:


// struc32k.CPP <- MUST use C++ compiler
struct test
{
  char a[16384];
  char b[16384];     // if changed to b[16383],
  char c[10000];     // expanding c works correctly
} teststruct;

void main( void )
{
 teststruct.c[0] = 'A';   // break here, set watch on teststruct,
                          // expand c
}

NOTE: There are no problems if this is a .C file.

For more information, please see the following article in the Microsoft Knowledge Base:

Q117150 BUG: Watch on Struct Member Gives CXX0030 if Offset >= 32K

Additional query words: 9.00

Keywords : kbtshoot kbCodeView kbDebug kbide kbVC100 kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : WIN3X:1.0,1.5,4.0,4.01,4.1;MS- DOS:4.0,4.01,4.1;WINNT:1.0,2.0,2.1,4.0,5.0
Platform : MS-DOS NT WINDOWS
Issue type : kbinfo