Placement New: Why Can\'t We Use `delete` to Deallocate Memory?

Proper Memory Management for Placement New

When utilizing placement new, it becomes the programmer's responsibility to manually invoke the destructor and free the allocated memory. This differs from the default behavior of the delete operator, which typically handles both tasks.

Why Not Delete?

In the code provided:

char* pMemory = new char[sizeof(MyClass)];
MyClass* pMyClass = new(pMemory) MyClass();

Memory is allocated manually using new[] for a character array of size corresponding to the MyClass object. Placement new is then used to construct the object within this memory.

Using delete here would be incorrect because the operator new[] was used to allocate the memory manually. delete is intended for memory allocated using operator new, which is not the case here.

Destructor Responsibility

Since placement new was used, it is the programmer's responsibility to call the destructor to release the object. However, this does not automatically free the allocated memory. To prevent a memory leak, the memory should be freed explicitly after the destructor is called.

Placement New with Internal Buffers

Placement new can also be used with internal buffers that were not allocated with operator new. In such cases, operator delete should not be called as it may result in unexpected behavior or memory corruption.

For example:

struct buffer_struct {
    std::aligned_storage_t<sizeof(MyClass), alignof(MyClass)> buffer;
};

MyClass* pMyClass = new (&a.buffer) MyClass(); //created inside buffer_struct a

In this case, the buffer_struct provides the storage for the MyClass object, but the object construction and destruction are handled independently using placement new and the destructor, respectively.

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