Contracts or Exceptions?

[Ali Çehreli]

On 03/29/2011 03:40 PM, Kai Meyer wrote:

 > I was given two words of advice on exceptions:
 > "Use exceptions for the exceptional"
 > "Use exceptions only for the exceptional"

Those advices are given by wise people: they are wise only because they 
leave the definition as vague as "exceptional." :)

And what do we do for the "not so exceptional"? Do we return error 
codes? So the function implementation will be complicated and the caller 
code will be complicated.

Exceptions are a great tool to eliminate the need for error codes.

Here is what I follow:

- Functions have specific tasks to do; if those tasks cannot be 
accomplished, the function must throw.

In some cases the function can continue, but that behavior must be 
documented. For example, if an HTML library function is responsible for 
making HTML headers, of which only the levels in the range of 1-6 are 
valid, that function may throw when the level is outside of the valid 
range, for in that case it cannot "make an HTML header"; or it can 
document that if the level is outside of the range, 1 or 6 will be used.

- Catch exceptions only when there is a sensible thing to do at that 
level: log an error, skip that operation, go back to the user with an 
error code, take corrective action, etc.

Disclaimer: That is what I follow in C++ code. I don't have experience 
with exception safety in D. I don't know issues that may be specific to D.

Ali