What Happens When You Double Delete Dynamically Allocated Memory?

The Perils of Double Delete

When dealing with dynamic memory allocation, it's crucial to understand the consequences of improper handling, such as double deletion.

Consider the following code snippet:

Obj *op = new Obj;
Obj *op2 = op;
delete op;
delete op2; // What happens here?

Here, a pointer op is created and initialized with a new Obj object. A second pointer op2 is then assigned to point to the same object. Subsequently, both pointers are subjected to deletion.

The Consequences

This code demonstrates an alarming scenario known as double deletion. Deleting a memory location twice leads to undefined behavior, meaning the operating system is free to do whatever it wants.

In practice, the most likely outcome is a runtime crash. The reason for this is that after deleting op, the memory it points to is marked as free and should no longer be used. However, when you delete op2, which points to the same memory location, you attempt to free it again. This confuses the system and can result in a crash.

The Compiler's Role

Compilers typically do not warn you about double deletion as it's considered a logic error. They assume you have implemented proper memory management and do not expect such a violation of the rules.

Severity

The consequences of double deletion are significant. Not only can it cause unexpected crashes, but it can also lead to data corruption, security vulnerabilities, and unpredictable behavior in your application.

Preventing Double Deletion

To avoid this pitfall, it's essential to follow proper memory management practices:

• Always keep track of the ownership of objects and their respective pointers.
• Use smart pointers or other memory management tools to prevent dangling pointers.
• Ensure that objects are only deleted once, preferably by their owner.

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