C++: أساسيات المؤشرات

Type* pointerName;

（Program output）

#include <iostream>

int main() {
 int* pInt; // pointer to int
 double* pDouble; // pointer to double
 char* pChar; // pointer to char
 
 return 0;
}

#include <iostream>

int main() {
 int x = 5;
 
 std::cout << "x 's value: " << x << std::endl;
 std::cout << "x 's address: " << &x << std::endl; // Output similar to 0x7ffd4a3b
 
 return 0;
}

x 's value: 5
x 's address: 0x7ffd4a3b

#include <iostream>

int main() {
 int x = 5;
 int* p = &x; // p points to x（p stores x's address）
 
 std::cout << "x 's address: " << &x << std::endl;
 std::cout << "p 's value: " << p << std::endl; // same as &x
 
 return 0;
}

#include <iostream>

int main() {
 int x = 5;
 int* p = &x; // p points to x
 
 std::cout << "x = " << x << std::endl; // 5
 std::cout << "*p = " << *p << std::endl; // 5（Dereference）
 
 return 0;
}

#include <iostream>

int main() {
 int x = 5;
 int* p = &x;
 
 std::cout << "Before: x = " << x << std::endl; // 5
 
 *p = 100; // Modify x's value via pointer
 
 std::cout << "After: x = " << x << std::endl; // 100
 
 return 0;
}

#include <iostream>

int main() {
 int* pInt;
 double* pDouble;
 char* pChar;
 
 std::cout << "int* size: " << sizeof(pInt) << " bytes" << std::endl;
 std::cout << "double* size: " << sizeof(pDouble) << " bytes" << std::endl;
 std::cout << "char* size: " << sizeof(pChar) << " bytes" << std::endl;
 
 return 0;
}

int* size: 8 bytes
double* size: 8 bytes
char* size: 8 bytes

#include <iostream>

int main() {
 int* p = nullptr; // Null pointer (recommended since C++11)
 
 if (p == nullptr) {
 std::cout << "p is null pointer" << std::endl;
 }
 
 return 0;
}

#include <iostream>

int main() {
 int* p = nullptr;
 std::cout << *p << std::endl; // ❌ Undefined behavior! May crash
 return 0;
}

Segmentation fault (core dumped) // Linux

Process finished with exit code -1073741819 // Windows

#include <iostream>

void modifyValue(int x) {
 x = 100; // ❌ modifying a copy, does not affect original variable
}

int main() {
 int a = 5;
 modifyValue(a);
 std::cout << "a = " << a << std::endl; // 5（Unchanged）
 return 0;
}

#include <iostream>

void modifyValue(int* p) {
 *p = 100; // ✅ modify original variable via pointer
}

int main() {
 int a = 5;
 modifyValue(&a); // Pass address
 std::cout << "a = " << a << std::endl; // 100（Changed!）
 return 0;
}

#include <iostream>

void swap(int* a, int* b) {
 int temp = *a;
 *a = *b;
 *b = temp;
}

int main() {
 int x = 3, y = 5;
 std::cout << "Before swap: x = " << x << ", y = " << y << std::endl;
 
 swap(&x, &y); // Pass address
 
 std::cout << "After swap: x = " << x << ", y = " << y << std::endl;
 
 return 0;
}

Before swap: x = 3, y = 5
After swap: x = 5, y = 3

#include <iostream>

int main() {
 int* p; // ❌ Uninitialized (wild pointer)
 std::cout << *p << std::endl; // ❌ Undefined behavior！
 return 0;
}

#include <iostream>

int main() {
 int x = 5;
 double* p = &x; // ❌ Error: int* Cannot point to int
 return 0;
}

error: cannot convert 'int*' to 'double*' in initialization

#include <iostream>

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int* p = arr; // array name is the first element address

    for (int i = 0; i < 5; i++) {
        std::cout << "arr[" << i << "] = " << *(p + i) << std::endl;
    }

    return 0;
}

arr[0] = 10
arr[1] = 20
arr[2] = 30
arr[3] = 40
arr[4] = 50