Why is `std::memcpy` Undefined Behavior for Non-Trivially Copyable Objects?

Undefined Behavior in std::memcpy for Non-TriviallyCopyable Objects

The C standard specifies that the behavior of std::memcpy is undefined for objects that are not TriviallyCopyable. This begs the question, why would the behavior be undefined at all?

The undefined behavior arises because, when std::memcpy is used to copy the underlying bytes of a non-trivially copyable source object into a target object of the same type, the target object is technically destroyed. Its storage has been reused without invoking its destructor or re-initializing it with a constructor call.

Consequently, any subsequent use of the target object's member functions or data members is considered undefined. This includes the implicit destructor call for objects with automatic storage duration. The undefined behavior is retrospective, meaning it can impact operations even before the undefined action.

To prevent this undefined behavior, it's crucial to avoid using std::memcpy for non-trivially copyable objects unless the programmer explicitly ensures that it will not lead to any further undefined operations.

It's worth noting that standard libraries can optimize std::copy and std::swap algorithms for trivially copyable types by utilizing memcpy for efficient byte-level copying. Therefore, adhering to generic algorithms and letting the compiler handle optimizations is advisable to avoid potential undefined behavior and ensure expected program semantics.

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