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Associative containers place a strict weak ordering requirement on their key comparison predicates [ISO/IEC 14882-2014]. A strict weak ordering has the following properties:

• for all x: x < x == false (irreflexivity)
• for all x, y: if x < y then !(y < x) (asymmetry)
• for all x, y, z: if x < y && y < z then x < z (transitivity)

Providing an invalid ordering predicate for an associative container (e.g., sets, maps, multisets, and multimaps), or as a comparison criterion with the sorting algorithms, can result in erratic behavior or infinite loops [Meyers 01]. When an ordering predicate is required for an associative container or a generic standard template library algorithm, the predicate must meet the requirements for inducing a strict weak ordering.

Noncompliant Code Example

In this noncompliant code example, the std::set object is created with a comparator that does not adhere to the strict weak ordering requirement. Specifically, it fails to return false for equivalent values. As a result, the behavior of iterating over the results from std::set::equal_range results in unspecified behavior.

#include <functional>
#include <iostream>
#include <set>

void f() {
  std::set<int, std::less_equal<int>> s{5, 10, 20};  
  for (auto r = s.equal_range(10); r.first != r.second; ++r.first) {
    std::cout << *r.first << std::endl;
  }
}

Compliant Solution

This compliant solution uses the default comparator with std::set instead of providing an invalid one.

#include <iostream>
#include <set>

void f() {
  std::set<int> s{5, 10, 20};  
  for (auto r = s

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 r.first; sleIter \!= psler.second; \++sleIterr.first) {
&nbsp;&nbsp;&nbsp; cout << "Set containsstd::cout " << \*sleIterr.first << std::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) {
&nbsp;&nbsp;&nbsp; cout << "Set contains: " << \*islIter << endl;
}

References

Noncompliant Code Example

In this noncompliant code example, the objects stored in the std::set have an overloaded operator< implementation, allowing the objects to be compared with std::less. However, the comparison operation does not provide a strict weak ordering. Specifically, two sets, x and y, whose i values are both 1, but have differing j values can result in a situation where comp(x, y) and comp(y, x) are both false, failing the asymmetry requirements.

#include <iostream>
#include <set>

class S {
  int i, j;

public:
  S(int i, int j) : i(i), j(j) {}
  
  friend bool operator<(const S &lhs, const S &rhs) {
    return lhs.i < rhs.i && lhs.j < rhs.j;
  }
  
  friend std::ostream &operator<<(std::ostream &os, const S& o) {
    os << "i: " << o.i << ", j: " << o.j;
    return os;
  }
};

void f() {
  std::set<S> t{S(1, 1), S(1, 2), S(2, 1)};
  for (auto v : t) {
    std::cout << v << std::endl;
  }
}

Compliant Solution

This compliant solution uses std::tie() to properly implement the strict weak ordering operator< predicate.

#include <iostream>
#include <set>
#include <tuple>
 
class S {
  int i, j;
 
public:
  S(int i, int j) : i(i), j(j) {}
  
  friend bool operator<(const S &lhs, const S &rhs) {
    return std::tie(lhs.i, lhs.j) < std::tie(rhs.i, rhs.j);
  }
  
  friend std::ostream &operator<<(std::ostream &os, const S& o) {
    os << "i: " << o.i << ", j: " << o.j;
    return os;
  }
};

void f() {
  std::set<S> t{S(1, 1), S(1, 2), S(2, 1)};  
  for (auto v : t) {
    std::cout << v << std::endl;
  }
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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