How Do I Safely Compare Pointers for Equality and Ordering in C ?

Comparing Pointers for Equality: a Walkthrough of the C Standard

In C , comparing pointers for equality can be straightforward, but it requires adherence to specific rules outlined in the language standard.

Equality Operator (==, !=)

For pointers of the same type, equality comparison using == or != yields intuitive results. Two pointers are equal if they are both null, both point to the same function, or both point to the same address.

Relational Operators (<, >, <=, >=)

Pointers of the same type can also be compared using relational operators. If they point to the same object or function, they compare as expected: p<=q and p>=q yield true, while p<q and p>q yield false.

However, if the pointers point to different objects or functions, or if one of them is null, the results of relational comparisons are unspecified. This means that their behavior is implementation-dependent.

Exceptions to the Rule

Array Elements: If pointers point to elements of the same array, the pointer to the object with a higher index compares greater.

Union Members: If pointers point to members of the same union, they compare equal after conversion to void*.

Other Types of Pointers:

• Pointers to non-static member functions: The ordering result is unspecified.
• Pointers to non-static data members of the same object with different access control: The ordering result is unspecified.
• Pointers to non-static data members of the same union object: They compare equal after conversion to void*.

Caveats

• Pointers to different global variables cannot be compared using relational operators.
• Pointers to local variables cannot be compared after the scope of the variable has ended.

Bonus: Standard Library Helper

The standard library provides the template functions std::greater, std::less, std::greater_equal, and std::less_equal, which can be used to order any type of pointer, even when built-in operators cannot.

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