Understanding Pointer Expressions in C and C++

Pointer expressions that combine the dereference operator (*) with increment (++) or decrement (--) operators are among the most confusing topics for beginners in C and C++ programming. Expressions such as *ptr++, *++ptr, ++*ptr, and (*ptr)++ may look similar, but they produce completely different results because of operator precedence and associativity.

Understanding how these expressions are evaluated is essential for writing correct pointer-based programs. These concepts are also frequently asked in competitive examinations, university exams, and technical interviews because they test your understanding of pointers and operator evaluation.

Operators Used in Pointer Expressions

Before understanding complex pointer expressions, let's briefly review the operators involved.

Operator Precedence and Associativity

The result of a pointer expression depends on the precedence and associativity of the operators involved. Therefore, before solving any pointer expression, it is important to know which operator is evaluated first.

Comparison of Common Pointer Expressions

The following table summarizes the most commonly used pointer expressions and their meanings. Understanding these four expressions will eliminate most of the confusion related to pointers and increment operators.

Sample Expressions Used Throughout This Article

To understand the behavior of different pointer expressions, we will use the following declarations in all upcoming examples.

int *ptr;
int result;

/* Pointer Expressions */

result = *ptr++;
result = *++ptr;
result = ++*ptr;
result = (*ptr)++;

In the next sections, we will analyze each expression individually. For every expression, we will understand:

• How the compiler evaluates the expression.
• Which operator is executed first.
• Whether the pointer changes or the stored value changes.
• A complete C++ program demonstrating the behavior.
• The corresponding program output with detailed explanation.

Expression 1: *ptr++

The expression *ptr++ is one of the most frequently asked pointer expressions in C and C++. At first glance, many students assume that the value pointed to by ptr is incremented. However, this interpretation is incorrect.

Since the postfix increment operator (++) has higher precedence than the dereference operator (*), the compiler evaluates the expression as:

*(ptr++)

This means the compiler first uses the current pointer to access the value and then moves the pointer to the next memory location. The value stored at the original memory location remains unchanged.

result = *ptr;
ptr = ptr + 1;

Programming Example

#include <iostream>
using namespace std;
int main()
{
    int arr[] = {11,22,33,44,55};
    int *ptr;
    int result;
    ptr = arr;
    cout << "Pointer Initially : " << ptr << endl;
    result = *ptr++;
    cout << "After Executing Expression" << endl;
    cout << "Result = " << result << endl;
    cout << "Pointer = " << ptr << endl;
    return 0;
}

Output

Pointer Initially : 0x....
After Executing Expression
Result = 11
Pointer = Next Address

Expression 2: *++ptr

The expression *++ptr looks very similar to *ptr++, but its behavior is completely different.

Here, the compiler evaluates the expression as:

*(++ptr)

The prefix increment operator increases the pointer before it is used. Therefore, the pointer first moves to the next memory location, and only then is the value at the new location accessed.

ptr = ptr + 1;
result = *ptr;

Programming Example

#include <iostream>
using namespace std;
int main()
{
    int arr[] = {11,22,33,44,55};
    int *ptr;
    int result;
    ptr = arr;
    cout << "Pointer Initially : " << ptr << endl;
    result = *++ptr;
    cout << "After Executing Expression" << endl;
    cout << "Result = " << result << endl;
    cout << "Pointer = " << ptr << endl;
    return 0;
}

Output

Pointer Initially : 0x....
After Executing Expression
Result = 22
Pointer = Next Address

Comparison: *ptr++ vs *++ptr

Expression 3: ++*ptr

Unlike the previous two expressions, ++*ptr does not move the pointer. Instead, it increments the value stored at the memory location pointed to by ptr.

The compiler interprets this expression as:

++(*ptr)

Here, the dereference operator accesses the value first, and then the prefix increment operator increases that value before it is used. The pointer continues to point to the same memory location throughout the operation.

*ptr = *ptr + 1;
result = *ptr;

Programming Example

#include <iostream>
using namespace std;
int main()
{
    int value = 10;
    int *ptr = &value;
    int result;
    result = ++*ptr;
    cout <<"Result = " << result << endl;
    cout <<"Value = " << value << endl;
    cout <<"Pointer = " << ptr << endl;
    return 0;
}

Output

Result = 11
Value = 11
Pointer = Same Address

Expression 4: (*ptr)++

This expression also modifies the value stored at the memory location, but unlike the previous example, it uses the postfix increment operator.

The current value is used first, and then the value stored in memory is increased by one. The pointer remains unchanged throughout the operation.

(*ptr)++

result = *ptr;
*ptr = *ptr + 1;

Programming Example

#include <iostream>
using namespace std;
int main()
{
    int value = 10;
    int *ptr = &value;
    int result;
    result = (*ptr)++;
    cout <<"Result = " << result << endl;
    cout <<"Value = " << value << endl;
    cout <<"Pointer = " << ptr << endl;
    return 0;
}

Output

Result = 10
Value = 11
Pointer = Same Address

Comparison of All Four Pointer Expressions

Previous Year Examination Question

result = *ptr++;

Step-by-Step Compiler Evaluation

Whenever you encounter a pointer expression in an examination, do not try to guess the answer. Instead, follow a systematic approach.

Let us now examine a widely discussed question from the BCI (Basic Computer Instructor) Exam 2022. Although this question was later removed because it had two equivalent correct options, making it ambiguous, it is still an excellent example for understanding operator precedence and associativity in C++. In this section, we will analyze the question carefully and explain the reasoning behind each option.

BCI 2022 Previous Year Question

#include <iostream>
using namespace std;
int main()
{
    int arr[] = {10, 20, 30, 40, 50};
    int *ptrdata;
    ptrdata = arr;
    *ptrdata++;
    // Original expression given in the question
    cout << "Using Original Expression *ptrdata++" << endl;
    cout << "Value : " << *ptrdata << endl;
    cout << "Pointer Points : " << ptrdata << endl;
    // Option (A)
    cout << "\nUsing Option (A) Expression *(ptrdata++)" << endl;
    ptrdata = arr; // Reset Pointer
    *(ptrdata++);
    cout << "Value : " << *ptrdata << endl;
    cout << "Pointer Points : " << ptrdata << endl;
    // Option (B)
    cout << "\nUsing Option (B) Expression (*ptrdata)++" << endl;
    ptrdata = arr; // Reset Pointer
    (*ptrdata)++;
    cout << "Value : " << *ptrdata << endl;
    cout << "Pointer Points : " << ptrdata << endl;
    // Option (C)
    cout << "\nUsing Option (C) Expression *(ptrdata)++" << endl;
    ptrdata = arr; // Reset Pointer
    *(ptrdata)++;
    cout << "Value : " << *ptrdata << endl;
    cout << "Pointer Points : " << ptrdata << endl;
    // Option (D)
    cout << "\nUsing Option (D) Expression, Depends on Compiler" << endl;
    cout << "Not applicable. All behaviors are standard in C++." << endl;
    return 0;
}