Problem Solving with Algorithms and Data Structures using C++ The PreTeXt Interactive Edition

Subsubsection 1.9.4.1 Pointer Syntax

variableType *identifier; // syntax to declare a C++ pointer
int *ptrx; // example of a C++ pointer to an integer

SOMETYPE *variablename; // preferable
SOMETYPE * variablename;
SOMETYPE* variablename;

Subsubsection 1.9.4.2 The address-of operator, &

variableType varN;  // a variable to hold the value
variableType *ptrN = &varN;  // a variable pointing to the address of varN

//variable declaration for a single integer value
int varN = 100;
int *ptrN;
ptrN = &varN;



Image illustrating the memory representation of two variables in C++, showing how a pointer and its associated value are stored in memory. The table is divided into three rows: Variable Names, Stored Values, and Memory Address. The variable "varN" is shown with its value of 100, stored at memory address 0x60. The pointer variable "ptrN" stores the memory address of "varN" (0x60), and its stored value is displayed as 0x80.

Subsubsection 1.9.4.3 Accessing Values from Pointers

//prints a variable by pointer and value #include <iostream> using namespace std; int main( ) { int varN = 100; int *ptrN = &varN; // ptrN points to varN address cout << "varN value: " << varN << endl; cout << "varN location: " << ptrN << endl; cout << "dereference ptrN: " << *ptrN << "endl"; return 0; }

//showcases what happens when reading from unknown memory locations #include <iostream> using namespace std; int main() { int varN = 100; int *ptrN = varN; // Note: MISSING AMPERSAND (&)! // Because we used varN instead of &varN, // ptrN now holds the integer value 100 as if it were a memory address. // You might get an error or you might not! cout << "varN value: " << varN << endl; cout << "varN location: " << ptrN << endl; cout << "ptrN points to varN: " << endl; cout << "dereference ptrN: " << *ptrN << "\n\n"; return 0; }



Image illustrating the concept of a dangling pointer reference in C++ due to incorrect pointer assignment. The diagram shows a pointer variable "ptrN" pointing to memory address 100. However, the value at this memory address is undefined or unknown (represented by a "?" in the memory box). At the same time, the variable "varN" correctly stores the value 100 in its allocated memory. This mismatch occurs because the pointer "ptrN" was assigned the value of "varN" directly, without using the address-of operator (&). As a result, "ptrN" does not point to the actual memory location of "varN" but instead refers to an unintended location, which may lead to undefined behavior.

After changing *ptrN, varN now has: 50

1. You don’t know what is stored in location 100 in memory, and
2. that location is outside of your segment (area in memory reserved for your program), so the operating system will jump in with a message about a “segmentation fault”. Although such an error message looks bad, a “seg fault” is in fact, a helpful error because unlike the elusive logical errors, the reason is fairly localized.

Subsubsection 1.9.4.4 The null pointer