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-
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Compliant Code Example
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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 inverted and the subsequent loop fails to terminate.
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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;
}
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Compliant Solution
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Provide an ordering rule that defines a strict weak ordering.
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