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 xx < x == false (irreflexivity)
  • for all x, y: if x < y then !(y < x) (asymmetry)
  • for all xyz: 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.equal_range(10); r.first != r.second; ++r.first) {
    std::cout << *r.first << std::endl;
  }
}

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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

CTR57-CPP

Low

Probable

High

P2

L3

Automated Detection

Tool

Version

Checker

Description

Helix QAC

2023.3

C++3293
Parasoft C/C++test

2023.1

CERT_CPP-CTR57-a

For associative containers never use comparison function returning true for equal values

Polyspace Bug Finder

R2023b

CERT C++: CTR57-CPPChecks for predicate lacking strict weak ordering (rule partially covered).

Related Vulnerabilities

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

Related Guidelines

Bibliography

[ISO/IEC 14882-2014]

Subclause 23.2.4, "Associative Containers"

[Meyers 2001]Item 21, "Always Have Comparison Functions Return False for Equal Values"
[Sutter 2004]Item 83, "Use a Checked STL Implementation"



1 Comment

  1. The list of requirements at the top do not correctly specify a strict weak ordering. In fact, these only specify a strict partial ordering (note that asymetry is already implied by irreflexivity and transitivity). One also needs transitivity of incomparability.

    The explanation for the second non-compliant example is also wrong. The relation is assymetric but fails the transitivity of incomparability. Consider P = (0, 2), Q = (2, 0) and R = (1, 3). Observe that P and Q are incomparable (neither P < Q nor Q < P), and Q and R are incomparable. However, P < R and hence, P and R are comparable.