Pointer

Abstract

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• A Datatype whose value is Memory Address, and itself is located at a memory address too
• In the diagram below shows some Go codes, we create a pointer var p *int32 = new(int32) and perform Pointer Dereference with *p

Memory efficiency

Pass data to functions by pointer is memory-efficient. The data we pass into a function is basically a Memory Address to access that block of data. If we pass data into functions without pointer. We need to create an entire duplicate of data and pass it to the functions.

In Java, data is passed to functions by pointer by default. However in Go, we need to explicity specify the pointer datatype for the function input, then we can pass data by point!

Pointer is the usual way of accessing hardware.

More power!

Function scope rules prevent a function call from directly modifying variables outside its own scope. However, using pointers allows you to pass memory addresses to a function, enabling changes made within the function to affect variables outside its scope.

Pointee

• The actual data that a Pointer points-to inside the Heap Segment

Pointer Dereference

• The process of accessing Pointee of a Pointer

Segmentation fault

int *n;
 
*n = 123;

When *n is uninitialised, *n = 123; will attempt to write the value 123 into a random memory address that *n might be pointing to. This memory address may not be accessible to your program, which will trigger a segmentation fault.

C Pointer

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Printing pointer in C

Use %p as the format string.

Obtaining variable memory address in C

Prefix the variable with the address operator &. The address is in hex format

Declaring a Pointer

int *a_ptr; a_ptr is a pointer to a variable with type int.

Accessing the variable holding the actual data using pointer

Prefix the variable with the indirection operator * which is also a dereferencing operator.

Null Pointer

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• Pointer that doesn’t point to any memory location, basically contains a invalid Memory Address
• In C, it is represented by 0 or nullptr
• In Java, it is represented by null, Custom Datatype can be null
• In Go, it is represented with nil

Void Pointer

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• A Pointer without Datatype aka a pre-defined size
• Used when we aren’t sure what the exact size of a variable like the return type of malloc

Important

We should always try to cast it to a specific type whenever it is possible.

Dangling Pointer

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int *ptr = (int *)malloc(sizeof(int));
*ptr = 1;
 
func(ptr); // func() carelessly does "free(ptr);"
 
*ptr = 2; // ptr is now a dangling pointer

• A pointer that has been freed, but the program logic still assumes it’s valid.

Important

The program may still run without a crash, this will lead to bug that very hard to trace back to.

Rust

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Rust Box

• A smart Pointer that represents Rust Ownership of values allocated on the Heap Segment

Rust References

• Non-owning pointers that points to Rust Box, thus they do not own the data they point to.
• Powers Rust Borrowing