From MSDN docs −
Use of two sequential underscore characters ( __ ) at the beginning of an identifier, or a single leading underscore followed by a capital letter, is reserved for C++ implementations in all scopes. You should avoid using one leading underscore followed by a lowercase letter for names with file scope because of possible conflicts with current or future reserved identifiers.
So you should avoid using names like −
__foo, __FOO, _FOO
And names like the following should not be used in the global namespace −
Other than this, there are some more prefixes like LC_, SIG_, and suffixes like _t should not be used as they're also reserved for the implementation.
So you can create variables that contain the underscore between the name or ends with an underscore.