Base Metal Blog Weekend Edition, August 24-25, 2013

Assert Yourself Conditionally

How many times have you written this code:

void Fn(TYPE *val)
{
    ASSERT(val);
    if(val)
    {
        //Do some stuff
    }
}

Or worse, this code:

void Fn(TYPE *val)
{
    ASSERT(val && "You are about to crash!");
    val->Fn();
}
Now there's a better way!  Introducing the if_assert:

void Fn(TYPE *val)
{
    if_assert(val) 
    {
        //Do some stuff
    }
}

Which is just defined as:

#ifdef DEBUG
     #define if_assert(x) if(!!(x) || System::AssertFail(#x " " STRINGIFY(__FILE__) ":" STRINGIFY(__LINE__)))
#else
    #define if_assert(x) if(x)
#endif

Some points on that code:

• You'll need to provide your own bool System::AssertFail(const char *msg) function
• It should return false.
• My System::AssertFail(...) just prints the message passed to it and does a normal ASSERT(0)
• I have this defined as extern rather than inline.  That keeps the contents of System::AssertFail(...) out of the instruction cache unless the assert actually fails, which depending on the complexity of your ASSERT handler can be substantial.
• In non-DEBUG builds, this just reduces to if(x)
• The !!(x) basically means the same thing as (x), but the compiler doesn't complain about automatic conversion to bool
• Because C uses lazy evaluation, nothing after the || will get evaluated if (x) is true
• STRINGIFY just converts its argument to a string.  There is some magic happening there, though, because you want the compiler to convert __FILE__ to a string before the macro converts it to the string (otherwise you just get "__FILE__" instead of the actual file name), so:
• #define _STRINGIFY(x) #x
• #define STRINGIFY(x) _STRINGIFY(x)
• Unfortunately, this doesn't support declaring variables inside the if_assert:
• Do:
• TYPE *val = GetValue();
• if_assert(val) ...
• not:
• if_assert(TYPE *val = GetValue()) ...
• though if you're dong this, you probably don't need an ASSERT:
• if(TYPE *val = GetValue()) ...