why using 'assert' in a project? (and why using it so many times)

objective-c assert design-by-contract
Paul picture Paul · Oct 6, 2011 · Viewed 8.5k times · Source

i was reading through the sample code ListAdder, and there are many asserts right after the variable, or used in almost every method, for example :

self.formatter = [[[NSNumberFormatter alloc] init] autorelease]; assert(self.formatter != nil);

or :

- (UITableViewCell *)tableView:(UITableView *)tv cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
   #pragma unused(tv)
   #pragma unused(indexPath)
   UITableViewCell *    cell;

   assert(tv == self.tableView);
   assert(indexPath != NULL);
   assert(indexPath.section < kListAdderSectionIndexCount);
   assert(indexPath.row < ((indexPath.section == kListAdderSectionIndexNumbers) ? [self.numbers count] : 1));

I was wondering, what's the point to do that?

Thanks

Answer

Prof. Falken picture Prof. Falken · Oct 6, 2011

It is an implementation of Design by Contract, or DbC.

Objective C has no native support for the pre-conditions, post-conditions and invariants of DbC, but especially post- and preconditions can be implemented pretty well with macros.

Here are some other approaches to implement DbC in Objective C:

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