Emplacing into a polymorphic container

What this C++ polymorphism question tests

This is a medium-difficulty C++ language-knowledge question that probes your understanding of how object slicing and polymorphic containers interact in practice. It rewards careful reasoning about type hierarchies, constructor calls, and memory layout rather than memorisation of standard-library internals.

To reason through problems like this, you need to trace what happens when objects are constructed, copied, or emplaced into containers of a base type. Pay close attention to the distinction between reference semantics and value semantics, and how virtual dispatch behaves (or does not) when the dynamic type is lost. Firms ask these questions to verify that you understand the limits of C++ polymorphism and can predict subtle runtime behaviour.

• Object slicing in containers and value semantics
• Constructor and destructor invocation order
• Virtual function dispatch and dynamic type information
• Emplace operations versus copy/move construction