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Evaluating a pointer—including dereferencing the pointer, using it as an operand of an arithmetic operation, type casting it, and using it as the right-hand side of an assignment—into memory that has been deallocated by a memory management function is undefined behavior. Pointers to memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

It is at the memory manager's discretion when to reallocate or recycle the freed memory. When memory is freed, all pointers into it become invalid, and its contents might either be returned to the operating system, making the freed space inaccessible, or remain intact and accessible. As a result, the data at the freed location can appear to be valid but change unexpectedly. Consequently, memory must not be written to or read from once it is freed.

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In this noncompliant code example, an attempt is made to allocate zero bytes of memory through a call to operator new(). If this request succeeds, operator new() is required to return a non-null pointer value. However, according to the C++ Standard, [basic.stc.dynamic.allocation], paragraph 2 [ISO/IEC 14882-2014], attempting to dereference memory through such a pointer results in undefined behavior.

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Bibliography

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