Providing an invalid ordering rule for an associative container or as a comparison criterion with the sorting algorithms can result in erratic behavior or infinite loops. (See [Meyers01] §21 for examples.)
Non-compliant Code Example 1
In this non-compliant example, the *IntSetLE* type defines a set with *less_equal* specified as the ordering rule. Less than or equal is not a valid ordering rule because it violates the requirement to provide a "strict weak ordering" over the objects compared. In particular, this ordering rule fails to return false for equal values. As a result, the iterator pair returned by the *equal_range()* method are inversed and the subsequent loop fails to terminate.
typedef set<int, less_equal<int > > IntSetLE;
IntSetLE::const_iterator sleIter;
IntSetLE sle;
sle.insert(5);
sle.insert(10);
sle.insert(20);
pair<IntSetLE::const_iterator, IntSetLE::const_iterator> psle;
psle = sle.equal_range(10);
for (sleIter = psle.first; sleIter \!= psle.second; \++sleIter){
cout << "Set contains: " << \*sleIter << endl;
}
Compliant Solution 1
Provide an ordering rule that defines a strict weak ordering.
typedef set<int, less<int> > IntSetLess;
IntSetLess::const_iterator islIter;
IntSetLess isl;
isl.insert(5);
isl.insert(10);
isl.insert(20);
pair<IntSetLess::const_iterator, IntSetLess::const_iterator> pisl;
pisl = isl.equal_range(10);
for (islIter = pisl.first; islIter \!= pisl.second; \++islIter) {
cout << "Set contains: " << \*islIter << endl;
}
References
- Meyers 01\ Item 21: Always have comparison functions return false for equal values.
- Sutter 05\ Item 83: Use a checked STL implementation.
- ISO/IEC 14882-2003\ Section 24: Iterators Library.