pointer

C++指针&和*的使用

pointer is a variable that stores the address of another variable. start from scratch, we already know what is variable. A variable store in memory have not only information of its value, but also its memory address.

int x = 5; // declare and initializa a variable

then we can use “address-of operator” - & to get the address of x!

int* ptr = &x //get the x's address and pass it to ptr

you may ask

• why * ptr is a int type data
  •  you are saying that ptr is a pointer to an int.
• where should I place the asteroid *? - Is that with end of int, or with beginning of ptr
  • only matter of style
  • However, the first style (int* ptr) is often preferred because it makes it clear that ptr is a pointer to an int. This can be especially helpful when declaring multiple pointers in a single statement:
  • int* ptr1, ptr2; // Only ptr1 is a pointer, ptr2 is an int
  • better declare one pointer per line
• since the name of & is “address-of operator”取地址符, what is the name of * as operator
  • it is called as “dereference operator”解引用符
• BEAWARE that & also have another way to use, which is reference
  • int &reference_value = value means give it a new name, and all operations on reference_value is same as directly on value You can use the pointer to access the value of x:

int value = *ptr; // Dereferencing the pointer to get the value of x

Example

example code

#include <iostream>
 
int main() {
    int x = 10;       // Declare an integer variable x and initialize it to 10
    int* ptr = &x;    // Declare a pointer to an integer and initialize it with the address of x
 
    std::cout << "Value of x: " << x << std::endl;           // Output the value of x
    std::cout << "Address of x: " << &x << std::endl;        // Output the address of x
    std::cout << "Value of ptr: " << ptr << std::endl;       // Output the value of ptr (address of x)
    std::cout << "Value pointed to by ptr: " << *ptr << std::endl; // Output the value pointed to by ptr (value of x)
 
    return 0;
}

result

Value of x: 10
Address of x: 0x7ffee4b3c8ac
Value of ptr: 0x7ffee4b3c8ac //ptr = &x
Value pointed to by ptr: 10 //*ptr = x

p.s. about address on a 64-bit system, a memory address might look like 0x7ffee4b3c8ac. Here’s why:

• 0x: This prefix indicates that the number is in hexadecimal format.
• 7ffee4b3c8ac: This is the actual address in hexadecimal. Each digit represents 4 bits, so a 64-bit address will have up to 16 hexadecimal digits.

summary

• x is a variable that stores a value.
• &x is the address of the variable x.
• ptr is a pointer that stores the address of x.
• *ptr is the value stored at the address ptr points to.