How to Properly Deallocate Memory Allocated with \'placement new\' in C ?

Proper Cleanup for Manually Allocated Memory: "placement new" and "delete"

In C , memory allocation and deallocation can be managed manually using the "placement new" and "delete" operators. This allows for greater flexibility in memory management, but also introduces potential pitfalls. Let's delve into the correct approach to deallocating memory allocated using "placement new."

Consider the code snippets below:

<code class="cpp">const char* charString = "Hello, World";
void *mem = ::operator new(sizeof(Buffer) + strlen(charString) + 1);
Buffer* buf = new(mem) Buffer(strlen(charString));
delete (char*)buf;</code>

<code class="cpp">const char* charString = "Hello, World";
void *mem = ::operator new(sizeof(Buffer) + strlen(charString) + 1);
Buffer* buf = new(mem) Buffer(strlen(charString));
delete buf;</code>

In both examples, memory is allocated using the low-level operator new function and cast to a void*. Then, a Buffer object is constructed in the allocated memory using the "placement new" syntax.

However, the crucial difference lies in the deallocation. The first snippet incorrectly attempts to delete the buf pointer as a char*, assuming the memory block was allocated using operator new. However, this will lead to memory corruption.

The second snippet demonstrates the correct approach:

<code class="cpp">buf->~Buffer();
::operator delete(mem);</code>

When using "placement new," you must also manually call the destructor. Then, you should call the operator delete function, providing the same pointer as used with operator new. This ensures that both the object is destroyed properly and the allocated memory is freed back to the system.

Remember, only delete what you allocate. Directly calling operator new requires manually calling operator delete and the destructor for complete memory cleanup.

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