Errors in C, C++, and other programming languages often result when a programmer fails to consider all possible data states.
Non-Compliant Code Example
This example fails to test for conditions where a is neither b nor c. This may be the correct behavior in this case, but failure to account for all the values of a may result in logic errors if a unexpectedly assumes a different value.
...
if (a == b) {
...
}
else if (a == c) {
...
}
...
Compliant Solution
This compliant solution explicitly checks for the unexpected condition and handles it appropriately.
...
if (a == b) {
...
}
else if (a == c) {
...
}
else {
assert( (a == b) || (a == c) );
abort();
}
...
Non-Compliant Code Example
This non-compliant code example fails to consider all possible cases. This may be the correct behavior in this case, but failure to account for all the values of widget_type may result in logic errors if widget_type unexpectedly assumes a different value. This is particularly problematic in C, because an identifier declared as an enumeration constant has type int. Therefore, a programmer can accidently assign an arbitrary integer value to an enum type as shown in this example.
...
enum WidgetEnum { WE_W, WE_X, WE_Y, WE_Z } widget_type;
widget_type = 45;
switch (widget_type) {
case WE_X:
...
break;
case WE_Y:
...
break;
case WE_Z:
...
break;
}
...
Implementation Details
Microsoft Visual C++ .NET with /W4 does not warn when assigning an integer value to an enum type, or when the switch statement does not contain all possible values of the enumeration.
Compliant Solution
This compliant solution explicitly checks for the unexpected condition by adding a default clause to the switch statement.
...
enum WidgetEnum { WE_X, WE_Y, WE_Z } widget_type;
widget_type = WE_X;
switch (widget_type) {
case WE_X:
...
break;
case WE_Y:
...
break;
case WE_Z:
...
break;
default:
assert(0);
abort();
break;
}
...
References
Hatton 95 Section 2.7.2 Errors of omission and addition