When Comparing Pointers in C , How Reliable Are the Equality and Relational Operators?

Comparison of Pointers for Object Equivalence

When comparing pointers that refer to the same variable, as in the example:

int *a = something;
int *b = something;

The question arises as to whether the equality operator "a == b" provides the expected result.

Equality Operator (==, !=)

According to the C standard, pointers of the same type can be compared for equality as follows:

• They compare equal if they are both null, point to the same function, or represent the same address.
• Null pointers compare equal to each other, while a non-null pointer always compares unequally to a null pointer.

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

Relational operators for pointers are defined only for pointers to objects or functions of the same type. The result depends on whether the pointers point to the same object or function, or whether one or both are null.

For example, in the case of an array, pointers to different elements compare greater or less than each other based on their position in the array. However, comparing pointers to non-array objects or functions that are not members of the same object may yield unspecified results.

Exceptions for Virtual Functions

In cases where either pointer points to a virtual member function, the comparison result is unspecified by the standard.

Comparison in Practice

It should be noted that the comparison of pointers using the equality operator (==, !=) is generally well-defined and reliable for determining if pointers point to the same object. However, relational operators (<, >, <=, >=) should be used with caution, as their behavior can be different depending on the specific context.

Bonus: Comparison with Templates

The standard library also provides templates for pointer comparison, such as std::less<> and friends. These templates guarantee a total order for any pointer type, even when the built-in operators do not provide well-defined behavior.

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