CurriculumDangling Pointers
Dangling Pointers
A dangling pointer is a pointer that points to memory that has already been deleted or freed. Because the pointer variable itself isn't automatically destroyed or cleared when you call 'delete', it continues to hold the memory address. Using it will cause undefined behavior, data corruption, or a crash.
01The Dangling Pointer
Let's see what a dangling pointer looks like. We allocate memory, use it, and then delete it.
But the pointer still holds the address! If we try to read from it, we get garbage data. If we try to write to it, we might corrupt another program's data and crash.
But the pointer still holds the address! If we try to read from it, we get garbage data. If we try to write to it, we might corrupt another program's data and crash.
main.cpp
Terminal
Waiting for execution...
Visualizer
Run code to see stack and heap.
02The Fix: nullptr
The best practice in manual memory management is to immediately set your pointer to
This turns the dangerous dangling pointer into a safe null pointer. You can safely delete a
nullptr after deleting it.This turns the dangerous dangling pointer into a safe null pointer. You can safely delete a
nullptr multiple times without crashing.main.cpp
Terminal
Waiting for execution...
Visualizer
Run code to see stack and heap.
03The Double Free Crash
If you don't set it to
This is known as a Double Free error.
nullptr, and you accidentally delete the same pointer twice, the OS will detect memory corruption and instantly kill your program.This is known as a Double Free error.
main.cpp
Terminal
Waiting for execution...
04Scope Danger: Returning Locals
Dangling pointers don't just happen with the heap. If you return the address of a local stack variable, that variable is destroyed the second the function ends!
The caller receives a pointer to dead memory.
The caller receives a pointer to dead memory.
main.cpp
Terminal
Waiting for execution...
05The Modern Fix (Preview)
In modern C++, you almost never use raw
We will cover this deeply in Phase 3!
new and delete anymore. We use Smart Pointers which automatically delete themselves when they are no longer needed.We will cover this deeply in Phase 3!
main.cpp
Terminal
Waiting for execution...
AI Tutor
Dangling pointers are responsible for a massive percentage of security vulnerabilities and game crashes worldwide. In many languages, this is impossible. In C++, you must guard against it manually.