35. Placement New

Question.4

A developer sizes the buffer for a base class but constructs a derived class:

class Base { int x; };
class Derived : public Base { int y; float z; };

alignas(Derived) uint8_t buf[sizeof(Base)];  // Only 4 bytes!
Derived* d = new (buf) Derived();  // Needs 12 bytes!

What happens?

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The derived object is truncated to fit the base size
The constructor writes past the buffer end, corrupting adjacent memory -- placement new has no size check
Compilation error -- placement new checks sizeof at compile time
The extra members are allocated on the heap automatically