How Reliable is Using `==` to Compare Pointers for Object Identity in C ?

Comparing Pointers for Object Identity

Given two pointers a and b, determining if they reference the same object is a common programming need. The intuitive approach is to use the equality operator (==). This article explores the validity of this approach and provides additional insights from the C standard.

Equality Operator and Pointer Comparison

According to the C 11 standard, pointers of the same type can be compared for equality, with specific rules:

• If both pointers point to the same object or function, or both have a null value, they are equal (a == b).
• If either pointer points to a virtual member function, the result is unspecified.
• If the pointers refer to different objects that are not members of the same object or array elements, the equality comparison is unspecified.

This means that a == b holds true if a and b point to the same object or have a null value.

Relational Operators and Pointers

The relational operators (<, >, <=, >=) have a unique set of rules for pointer comparisons:

• If p and q point to the same object or past the end of the same array, or both are null, the comparisons p <= q and p >= q yield true, while p < q and p > q yield false.
• If p and q point to different objects that are not members of the same object or array elements, or if only one is null, the results of p < q, p > q, p <= q, and p >= q are unspecified.

Implications for Your Code

For your specific case, if both a and b are assigned with something, a == b will indicate if they point to the same object. However, it's important to note that this comparison may yield an unspecified result if something refers to different objects that are not related (e.g., variables in different functions or arrays in different scopes).

Additional Insights from the Standard Library

The C standard library provides additional tools for comparing pointers:

• < and > Specializations: The standard library specializations of < and > for pointer types provide a total order, even when the built-in operators do not. This means that you can compare any void* pointer with std::less<> and friends, ensuring a well-defined ordering.

Conclusion

Using the equality operator == to compare pointers can be a useful technique to determine object identity, but it's crucial to be mindful of the potential caveats and limitations specified in the C standard. In cases where the comparison results in an unspecified condition, alternative approaches, such as tracking object relationships manually or utilizing custom comparison functions, may be necessary.

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