A Handy Debugging Macro for Switch Statements

Many times when writing switch statements, your code will handle every possible case that the variable controlling the switch could take. For example, if you have a MFC dialog with a combo box or radio button group, with an integer member variable hooked up to hold the user's selection, you might have a switch that takes an action based on that variable's value.

In switches like that example, you know that under normal circumstances, the code will never reach the default section in the switch. If the code ever should hit the default section, it would indicate either a bug in either the switch or the UI handling code. This is the kind of thing that asserts are designed to catch - conditions that should never occur if the code is working correctly.

I created this debugging macro, which I imaginitavely called DEFAULT_UNREACHABLE, that you can put at the end of your switches, to make sure that any bugs that are introduced which would cause execution to pass through the switch unhandled, are caught instead. The macro lets you know that something is wrong by generating an assertion failure. The macro has three parts:

• Debug builds: Forces an assertion failure so MSVC will drop you in the debugger right at the spot where the bug showed up.
• Release builds for VC 6 and later: Contains an __assume statement as an optimization hint to the compiler. According to the VC docs, this reduces the size of the compiled code for the switch.
• Release builds for VC 5 and earlier: Does nothing.

You may be wondering, "Why doesn't the release version do anything? Isn't the end result the same as without the macro?" That is indeed true, but remember that the debug version is the workhorse here. When you test your debug builds, you will catch and fix the bugs flagged by the macro, making sure that the code never gets to the default sections. That means that the release builds don't need any code in the default sections, because (say it with me) the code will never get there.

Without further ado, here is the code for the macro, and a sample showing how to use it.

#ifdef _DEBUG
#define DEFAULT_UNREACHABLE default: ASSERT(0); break
#elif _MSC_VER >= 1200
#define DEFAULT_UNREACHABLE default: __assume(0); break
#else
#define DEFAULT_UNREACHABLE default: break
#endif;

Sample usage: Imagine a dialog with radio buttons for selecting the day of the week. There is a variable m_nDay hooked up to the radio buttons.

void CMyDialog::OnOK()
{
    UpdateData();

    switch ( m_nDay )
        {
        case 0: case 6:
            // do something for the weekend...

        break;

        case 1: case 2: case 3:
            // something else for Mon-Wed...

        break;

        case 4: case 5:
            // and something else for Thu-Fri

        break;

        DEFAULT_UNREACHABLE;
        }

I, as the author of the code, know that m_nDay should never contain a value other than 0 through 6, and if it ever does, it indicates a bug. So I put the DEFAULT_UNREACHABLE macro in the switch, and it will alert me if that situation ever happens.

As a final note, I should mention that the latest winnt.h in the SDK has a DEFAULT_UNREACHABLE macro, but it is not as good as the one here. The SDK version just reduces down to an __assume statement, and has no provision for aiding in debugging.