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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

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 is inverted 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

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) {
&nbsp;&nbsp;&nbsp; cout << "Set contains: " << \*islIter << endl;
}

Risk Assessment

Using an invalid ordering rule can lead to erratic behavior or infinite loops.

References

\[[Meyers 01|AA. C++ References#Meyers 01]\] Item 21: Always have comparison functions return false for equal values.
\[[Sutter 05|AA. C++ References#Sutter 05]\] Item 83: Use a checked STL implementation.
\[[ISO/IEC 14882-2003|AA. C++ References#ISO/IEC 14882-2003]\] Section 24: Iterators Library.

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