oaidl.h(319): error C2057: expected constant expression

Question

I've recently switched from Visual Studio 2010 to Visual Studio 2012. The project I'm working on uses the BitwiseEnums library from MiLi . I'm including just the BitwiseEnums library as per the instructions here , so the only file being added to my project is bitwise_enums.h .

In VS2010 I had no issues. In VS2012 I'm getting the following error message whenever I #include bitwise_enums.h and try to compile:

1>c:\program files (x86)\windows kits\8.0\include\um\oaidl.h(319): 
error C2057: expected constant expression

When I check the external dependencies for my project, it does list the file oaidl.h. Looking inside this file, I find the following statement which ends on line 319.

typedef /* [v1_enum] */ 
enum tagSF_TYPE
    {
        SF_ERROR    = VT_ERROR,
        SF_I1   = VT_I1,
        SF_I2   = VT_I2,
        SF_I4   = VT_I4,
        SF_I8   = VT_I8,
        SF_BSTR = VT_BSTR,
        SF_UNKNOWN  = VT_UNKNOWN,
        SF_DISPATCH = VT_DISPATCH,
        SF_VARIANT  = VT_VARIANT,
        SF_RECORD   = VT_RECORD,
        SF_HAVEIID  = ( VT_UNKNOWN | VT_RESERVED ) 
    }   SF_TYPE; // Line 319

My questions are:

1. What is oaidl.h and what is causing it to be included in my project?
2. What does the error message mean, and how can I fix it?
3. What could be a reason for this error only occurring in VS2012?

Answer 1

I've managed to narrow this down to naming conflict of some kind between bitwise_enums.h and Windows.h, which I was also including in the project. I noticed that I didn't get the error if I included the entire MiLi library, because mili.h places everything inside a namespace using #define NAMESPACE_BEGIN and #define NAMESPACE_END , whereas defining them in the way suggested here , means that the contents of bitwise_enums.h end up polluting the global namespace. I am still not altogether clear on why this causes a problem, as I can't seem to locate the clash, nor am I clear on why it worked in VS2010. However, I was able to solve it by altering the #define instructions so that they create a namespace inside bitwise_enums.h:

#define NAMESPACE_BEGIN namespace Mili
#define NAMESPACE_END }
#include <bitwise_enums.h>
#undef NAMESPACE_BEGIN
#undef NAMESPACE_END

Another solution is to include mili.h and follow the second suggestion at this link , but since I only need one library, I prefer not to have to have lots of lines individually excluding the libraries I don't want.

Hopefully this answer will be useful if anyone in future ends up in the same obscure situation as myself, using that specific method of including a single library from MiLi, whilst also including Windows.h in VS2012 (probably an unlikely combination)!