Pointer, a weird beast, suitable for beginners and beyond

For beginners in C/C programming, pointers are one of the most difficult concepts to understand. Many students are troubled by this, and many developers try to avoid using pointers. However, understanding pointers is crucial, so let's start with the basics.

What is a pointer?

A pointer is a data type, similar to int , float , etc. The syntax for declaring a pointer is to add * after the type name. For example, a pointer to an integer is declared as int * , and a pointer to a custom structure mytype is declared as mytype * .

The pointer points to the memory address of the variable. int * Storing the address of integer variables, mytype * Storing the address of mytype variables.

Example:

 <code class="language-c  ">#include<iostream> int main(){ int value = 42; int* p = &value; // p指向value的内存地址std::cout </iostream></code>

Code Example 1

Main pointer operator

In addition to storing the address of a variable, pointers have two main operations: dereference and pointer arithmetic.

Decision

Use the * operator to dereference the pointer to access the value in the memory unit pointed to by the pointer.

 <code class="language-c  ">#include<iostream> int main(){ int var = 42; int *p = &var; // p指向var的内存地址*p = 21; // 修改var的值为21 }</iostream></code>

Code Example 2

An uninitialized pointer is very dangerous because it can point to anywhere in memory, causing the program to crash or undefined behavior.

Pointer arithmetic

Pointer arithmetic allows modifying the value of a pointer (i.e., memory address). Pointers can be added and subtracted, or added and subtracted with integers. It should be noted that the step size of the pointer addition and subtraction operation is the byte size of the data type pointed to. For example, if p is a pointer to an integer, p will make p point to the address of the next integer.

Pointer arithmetic needs to be used with caution, as it allows direct operation of memory.

 <code class="language-c  ">#include<iostream> int main(){ int v = 21; int w = 42; int *p = &w; // p指向w p ; // p指向v *p = 1; // v的值加1 std::cout </iostream></code>

Code Example 3

As shown in the example, we can modify the value of a variable that is not directly assigned through pointer arithmetic.

Empty pointer

Similar to the special position of the number 0 in numbers, nullpointer ( nullptr ) also plays an important role in pointers. A null pointer means that the pointer does not point to any valid memory address. Before using a pointer, be sure to check whether it is empty to avoid program crashes or errors.

 <code class="language-c  ">#include<iostream> int* foo(int value){ static int answ = 42; if(answ == value){ return &answ; } else { return nullptr; } } int main(){ int *p = foo(21); if (p != nullptr) { // 使用p } }</iostream></code>

Code Example 4

Key points

To sum up, we learned:

• A pointer is a variable that stores memory addresses.
• Always initialize the pointer, uninitialized pointer may cause the program to crash.
• Always check if it is empty before dereference to the pointer.
• Use pointer arithmetic with caution.

Hope this guide will help you better understand pointers in C/C. Continue to learn and explore more!

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