So the program fails, and you haven't a clue as to why. Well, there's some tricks you can try.
Turn off optimizations
One thing you can do is turn off all the optimizations in the release version. Go to the Project | Settings for the release version, choose the C/C++ tab, select Optimizations in the combo box, and simply turn off everything. Then do Build | Rebuild All and try again. If the bug went away, then you have a clue. No, you still don't know if it was an optimization bug in the strict sense, but you now know that the bug in your program is a consequence of an optimization transformation, which can be as simple as an uninitialized stack variable whose non-initialized value is sensitive to the optimization of the code. Not a lot of help, but you now know something more than you did before.
Turn on symbols
You can debug a release version of the program; just go into the C/C++ tab, select the General category, and select Program Database for Edit and Continue. You must also select the Link tab, and under the General category, check the Generate Debug Information box. In particular, if you have turned off optimization you have the same debugging environment that you had for the debug version, except you are running with the non-debug MFC shared library, so you can't single-step into the library functions. If you have not turned optimizations off, there are ways in which the debugger will lie to you about variable values, because the optimizations may make copies of variables in registers and not tell the debugger. Debugging optimized code can be hard, because you really can't be sure of what the debugger is telling you, but you can be further ahead with symbols (and line backtrace) than without them. Note that statements can be reordered, pulled out of loops, never computed, etc. in an optimized version, but the goal is that the code is semantically identical to the unoptimized version. You hope. But the rearrangement of the code makes it very difficult sometimes for the debugger to tell the exact line on which the error occurred. Be prepared for this. Generally, you'll find the errors are so blindingly obvious once you know more-or-less where to look that more detailed debugger information is not critical.
Enable/Disable Optimizations Selectively
You can use the Project | Settings to change the characteristics of a project selectively, on a file-by-file basis. My usual strategy is to disable all optimizations globally (in the release version), then selectively turn them back on only in those modules that matter, one at a time, until the problem reappears. At that point, you've got a good idea where the problem is. You can also apply pragmas to the project for very close optimization control.
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