试图在MFC的CString :: Format中捕获异常

Question

I am working with a C++ project (that I was not the author of) that has a lot of MFC string formatting functions. Unfortunately, stuff like %d and %s are very close together (including the location of letters d and s on the keyboard) that one can be transposed with another. So I may at times witness a code line as such:

CString s;
s.Format(L"Value v=%s", 100);    //Should've been %d instead

This results in a hard crash of the process, that is very hard to locate & isolate in the final project. So I was thinking to wrap the Format function in my own override and catch the exception & log it before it is thrown as unhandled exception.

So I employed the following construct:

__try
{
    //Do the Format function here
}
__except(1)
{
    //Log the error, etc.
}

But unfortunately the construct above did not catch the exception from the first code chunk, so I got VS 2008 C++ debugger kick in and show this:

I then tried this:

try
{
    //Do the Format function here
}
catch(int e)
{
    //Do the logging
}

But that didn't catch it either.

So how can I catch that fault?

PS. And I have a second question. Is there an easy way to override an MFC function, like Format for instance?

Answer 1

MFC throws CException pointers, so you could try this:

try
{
    // Do the Format function here
}
catch(CException* e)
{
    // Do the logging then free the exception
    if (m_bThrowExceptionAgain)
        throw; // Do not delete e 
    else 
        e->Delete();
}

You have to delete the exception object once you have caught it as shown in the example. Also make sure you have C++ exceptions enabled in your compiler. See http://msdn.microsoft.com/en-us/library/0e5twxsh.aspx for more information.

Answer 2

As others have already said low-level exceptions (like access violations) are not the same as C++ exceptions. They fall under the term Structured Exception Handling and would require other means to catch, at least by default.

It's possible to change compiler settings (at least in Visual Studio) to make it wrap those exceptions into something that C++ try/catch statements can handle, but as I recall that loses the details of what the SEH exception was and where it came from.

One way or another you could probably get exceptions to work well enough to help track down these issues, but there is also another way: Use static code analysis.

While standard C++ compilers don't normally verify format/printf-style calls, there are various tools that will. In fact some recent versions/editions of Visual Studio come with a code analysis tool , although it may not have been available in VS 2008 which you mentioned. So it might be worthwhile for you to do some research and see if you can get a hold of some kind of code analysis tool which could then catch all the CString::Format mistakes during analysis/compile-time rather than run-time.

Answer 3

You can use _set_se_translator() to convert SEH exceptions like access violation to C++ exceptions which you can then catch with except() .

Some sample code: http://www.codeproject.com/Articles/422/SEH-and-C-Exceptions-catch-all-in-one