What is the difference between a pointer and a variable in C++?

Both variables and pointers are fundamental concepts in programming, especially in languages like C++. Variables provide a means to store and manipulate data, while pointers serve as powerful tools for managing memory and data access. In this Answer, we’ll delve into the details of variables and pointers in C++, exploring how they work and how they are used effectively in our programs.

Variables

A variable is a container storing a value at some memory location. Depending on the data type, the value can be a number (floating point and integer), characters, string, or a boolean. 

// Declaration and initialization of variables with different datatypes
int age = 23;                // Variable 'age' storing an integer value 
string name = "John";       //  Variable 'name' storing a string
float percentage = 98.5;   //   Variable 'percentage' storing a floating-point value
char grade = 'A';         //    Variable 'grade' storing a character 
bool pass = true;        //     Variable 'pass' storing a boolean value
Variables declaration and initialization in c++

When we declare a variable, it’s allocated a memory location with a unique address and a stored value. The memory locations are typically organized in a stack memory. The names of these variables allow programmers to access the data stored at their respective memory locations.

In the illustration, the hexadecimal notation 0x102 serves as a representation of a memory address. During the initialization of variables, the assigned values are placed in these memory locations. Other values (e.g., 0, NULL, 348234878, and -98786342 ) represent the garbage values in that location.

Variables stored in memory
Variables stored in memory

The pros and cons of using variables in a program are as follows.

Pros

  • Variables are straightforward to understand and use, making code more readable and less error-prone.

  • Fixed memory sizes in variables enable predictable memory usage and prevent memory leaks.

  • Programs using variables enhance the code’s readability, making it easier to debug and maintain.

Cons

  • Passing variables by value can have a performance overhead, especially for large data structures, as they are copied when passed to functions.

  • Variables are typically statically allocated, which makes it challenging to allocate memory dynamically as needed.

  • Using variables can be less efficient in terms of memory and performance for complex data structures like linked lists or trees.

Pointers

When a variable stores the address of another variable’s memory location, it is called a pointer. As the name suggests, the pointer always points to a specific memory location. It points to where the data is stored in the memory.

A pointer pointing to a variable "name"
A pointer pointing to a variable "name"

Pointers use the & and * symbols to facilitate the manipulation and management of memory addresses within a program’s memory space. The & symbol is employed to access the memory address of the allocated locations, while the * symbol is used to dereference a pointer allowing access to the stored value. A pointer can have a special value known as nullptr, indicating that it doesn’t currently point to any valid memory location. Such a pointer is referred to as a null pointer.

int* p = nullptr;  // Initializing a pointer to null

In cases where a pointer continues to reference a memory location after deallocation, it’s called a dangling pointer. A dangling pointer points to an invalid memory location, which results in unpredictable behavior and memory-related problems. To mitigate such issues, the smart pointer composition class is employed. Smart pointers play a crucial role in preventing various pointer-related bugs and ensuring that resources are released correctly when they are no longer needed. In C++, pointers provide a level of indirectionIt allows data to be accessed indirectly via memory addresses., enabling dynamic memory management and data manipulationPointers provide direct access to memory addresses, facilitating tasks like updating values or traversing data structures..

The pros and cons of using pointers in a program are as follows.

Pros

  • Pointers allow for dynamic memory allocation, which can be more efficient and flexible than fixed-size variables.

  • Pointers are fundamental in building complex data structures like linked lists, trees, and graphs, enabling efficient traversal and manipulation.

  • Pointers enable the passing of data by reference, which optimizes performance in functions that work with large data structures.

Cons

  • Pointers introduce complexity and can make code harder to understand and debug.

  • Failing to deallocate memory properly can result in memory leaks, leading to inefficient memory usage.

  • Improper use of pointers can introduce security vulnerabilities like buffer overflows and pointer manipulation attacksUnauthorized modification or redirection of pointers to exploit vulnerabilities in software, often leading to security breaches..

Code example

In the following code, the pointer variable points to the age variable, and its value can be accessed using the pointer and dereferencing symbol.

#include <iostream>
using namespace std;
int main() {
  int age = 23;
  
  cout << "Address of age: " << &age <<'\n';
  int * pointer = &age;
  cout << "Address of age: " << pointer <<'\n'; 
  cout << "Value of age: " << *pointer; 
  return 0;
}

  • In line 8, we’re printing the address of the age variable using the & symbol.

  • In line 10, we’re storing the address of the age variable in the pointer.

  • In line 11, we’re printing the address of the age variable using a pointer.

  • Finally, in line 12, we’re printing the value of the age using the dereferencing symbol with a pointer.

Key Differences

Variables

Pointers

Variables store actual data values directly.

Pointers store memory addresses pointing to the data locations.

Variables consume memory based on the size of the data they hold. For example, variable char consumes 1 byte of memory and int variable consumes 4 bytes of memory.

Pointers consume fixed memory, irrespective of the data size they point to. The hardware architecture determines a fixed size for pointers, i.e., 32 bits on 32-bit systems and 64 bits on 64-bit systems. For example, both char and int variables share the same pointer size as the size depends on the underlying hardware.

Data in variables can be accessed directly using the variable name.


Data pointed to by a pointer can be accessed indirectly by dereferencing the pointer using the * operator.

Variables are ideal for basic data storage.


Pointers offer advanced capabilities like dynamic memory management and data manipulation.

Variables don’t have a null or undefined state.


Pointers can have a null state, indicated by nullptr.

In summary, variables and pointers complement each other in programming, with variables storing and manipulating data directly, while pointers allow advanced data management. Understanding when and how to use variables and pointers is essential for effective C++ development, allowing us to fully utilize the power of the language to solve a wide range of programming challenges.Stack ledwd

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