When is Manually Calling a Destructor a Good Design Choice?

Can Manual Destructor Calls Indicate Bad Design?

It is widely believed that manually invoking a destructor indicates poor design. However, there are exceptions to this rule.

One scenario where manual destructor calls become necessary is when only the object needs to be destroyed without releasing its allocated memory. This commonly occurs in scenarios where memory allocation and deallocation are handled separately from object construction and destruction. In such cases, objects are constructed using placement new on an existing memory block, and destructors are manually called to obliterate them.

Consider the following example:

char buffer[sizeof(MyClass)];

{
  MyClass* p = new(buffer)MyClass;
  p->dosomething();
  p->~MyClass();
}

{
  MyClass* p = new(buffer)MyClass;
  p->dosomething();
  p->~MyClass();
}

Another notable instance where manual destructor calls arise is the default std::allocator used by std::vector. Elements are constructed within the vector during push_back, but memory is allocated in chunks beforehand. Consequently, vector::erase can annihilate elements without necessarily freeing the underlying memory, particularly if push_back operations are anticipated shortly.

Although manual destructor calls deviate from strict OOP principles, they may be appropriate in situations where memory management is independent of object lifecycle. Additionally, implementing this approach locally within classes designed for such purposes can qualify as sound design.

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