What is the difference between (*p)++ and *p++?

The critical distinction between `(*p)++` and `*p++` in C and C++ lies in operator precedence and binding, which fundamentally alters whether the increment operation modifies the pointed-to value or the pointer itself. The expression `(*p)++` uses parentheses to explicitly dictate order: the dereference operator `*` is applied first to obtain the value at the memory location `p` points to, and then the postfix `++` increments that *value*. The pointer `p` remains unchanged. In contrast, `*p++` relies on the language's precedence rules, where postfix `++` has higher precedence than the dereference operator `*`. Consequently, `p++` is evaluated first, yielding the original value of the pointer for use in the expression, but as a side effect, `p` is incremented to point to the next element in memory (scaled by the size of the type). The dereference operator `*` is then applied to the *original* pointer value, accessing the data it pointed to before the increment. Thus, `*p++` reads the value pointed to by `p` and then advances the pointer.

This syntactic nuance leads to profoundly different runtime behaviors and typical use cases. `(*p)++` is an operation on data, commonly used to iterate through and modify the contents of an array or buffer in place. For instance, in a loop processing an integer array, it could be used to increment each element's value sequentially. The pointer `p` would need to be advanced separately within the loop. Conversely, `*p++` is a quintessential idiom for pointer traversal, efficiently combining access and movement. It is the standard pattern for sequentially processing elements in an array or a string without modifying them, such as copying or searching, where the pointer automatically advances to the next element after each access. The accessed value itself is not altered by the `++` in this expression.

Understanding the mechanism requires analyzing the semantics of the postfix increment. The operator yields the operand's value before incrementing, and its side effect—updating the operand—can be sequenced either before or after the rest of the expression's evaluation, a detail governed by sequence points. In `*p++`, the result of `p++` (the old pointer value) is immediately available for dereferencing. The actual update to `p` itself can occur anytime before the next sequence point, allowing compilers to generate highly optimized code. Misinterpreting these expressions can lead to subtle bugs. For example, confusing `*p++` with `*(p++)` is harmless as they are identical, but expecting `*p++` to increment the dereferenced value is a common error; that intent requires `(*p)++`. Similarly, `*++p` differs by using the prefix increment, which would advance the pointer *first* and then dereference the new address.

In practice, this distinction is not merely academic but foundational for writing correct and idiomatic low-level code. The `*p++` construct is pervasive in C standard library function implementations and performance-critical loops, leveraging its atomicity and efficiency. The `(*p)++` form explicitly signals a focus on data mutation. Mastery of these constructs involves recognizing that pointer arithmetic and dereferencing are separable operations, and the choice of expression explicitly communicates the programmer's primary intent: to move through a data structure or to modify the data within it.