When it comes to programming in C and C++, pointers are an essential concept to understand. They allow us to manipulate memory addresses and access data in a more efficient way. However, there is a specific type of pointer that can be quite confusing for beginners – the const pointer-to-pointer.
In this article, we will dive into the world of const pointer-to-pointers and understand their purpose, syntax, and how they differ from regular pointers.
To begin with, let's first define what a pointer is. A pointer is a variable that holds the memory address of another variable. It is denoted by the * symbol, and it allows us to access and manipulate the value stored at that memory address.
Now, what does the "const" keyword mean in front of a pointer? In simple terms, it means that the pointer is constant and cannot be reassigned to point to a different memory address. This is useful when we want to ensure that a certain variable or data cannot be modified.
So, a const pointer-to-pointer is a pointer that is constant and points to another pointer, which is also constant. This means that the memory address stored in the first pointer cannot be changed, and the value stored in the second pointer cannot be modified.
Let's take a look at the syntax for declaring a const pointer-to-pointer in C and C++:
```c
const int **ptr;
const char ***ptr;
```
As we can see, the const keyword is placed before the * symbol, indicating that the pointer is constant. It is important to note that the const keyword applies to the data type on its immediate left, and the * symbol is associated with the variable name.
Now, let's understand the difference between a const pointer-to-pointer and a regular pointer. With a regular pointer, we can change the value of the variable it points to, but we cannot change the memory address it holds. On the other hand, with a const pointer-to-pointer, we cannot change the value of the variable it points to, and we cannot change the memory address it holds.
To make this concept clearer, let's look at an example in C:
```c
int num = 10;
int *ptr = #
int **const const_ptr = &ptr;
//trying to change the value of num
*ptr = 20; //valid
**const_ptr = 30; //invalid
//trying to change the memory address of ptr
ptr = &some_other_num; //valid
*const_ptr = &some_other_num; //invalid
```
In the above example, we have a regular pointer (ptr) and a const pointer-to-pointer (const_ptr). We can change the value of the variable num using the regular pointer, but we cannot do the same with the const pointer-to-pointer. Similarly, we can change the memory address stored in ptr, but we cannot do so with const_ptr.
In C++, the concept of const pointer-to-pointers is the same, but the syntax is slightly different. Let's take a look at an example:
