Question

I am using mingw g++ 4.6.1 with -O0, WinXP SP2.

Minimal working example is here.

g++ is configured with –disable-sjlj-exceptions –with-dwarf2.

GetLastError() returns 0 or 2 depeding on how the exception is thrown:

throw runtime_error(error_message());

bogus “error code: 0” is printed, and

const string msg = error_message();

throw runtime_error(msg);

prints “error code: 2” as expected.

First, I thought GetLastError() is invoked twice but debugging shows it is invoked exactly once, as expected.

What is going on?

Answer 1

It’s possible that the code that sets up a throw calls a Win32 API function inside itself somewhere, that resets the Last-Error value to 0. This may be happening before your call to error_message().

Calling GetLastError() does not automatically reset the Last-Error value to 0, so it is safe to call twice.

Whether your compiler/runtime generates code that calls a Win32 API function will be up to your specific runtime. In order to be safe and not depend on this, use the two-statement version:

const string msg = error_message();
throw runtime_error(msg);

Better yet, for future readers of your code it would be useful to call GetLastError() outside error_message():

const string msg = error_message(GetLastError());
throw runtime_error(msg);

This way, readers will see the GetLastError() call immediately after the corresponding Win32 API call, where it belongs.