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

std::memcpy Behavior with Non-TriviallyCopyable Objects: Why Undefined?

The C standard library's std::memcpy function is well-known for copying a block of memory from one location to another. However, its behavior can become undefined when dealing with non-TriviallyCopyable objects. This arises a question: why is this case deemed undefined in the standard?

Consideration of the definition of trivially copyable sheds light on this issue. A trivially copyable type possesses special properties: it consists solely of primitive types (int, char, etc.) or pointers, and it doesn't involve any user-defined constructors, destructors, or assignments. These properties make it possible for memcpy to operate on these objects reliably.

However, non-TriviallyCopyable objects possess more complex structures. They may involve user-defined operations, references to external resources, or internal state that must be handled properly during copying. Simply replicating the underlying bytes without regard to these elements can lead to undefined behavior downstream.

For instance, if a non-TriviallyCopyable object's destructor is not invoked after memcpy overwrites its data, the program may encounter issues when attempting to access or modify object state. Additionally, if the object's lifetime is not properly established through placement new or similar techniques, the copied object may be left in an unusable or corrupted state.

The standard's specification of undefined behavior in this context serves several purposes. First, it protects programmers from potential errors by prohibiting operations where the outcome is uncertain. Second, it provides implementation flexibility by allowing optimizers to assume that objects are properly created and destroyed, leading to improved performance.

Therefore, it is essential to adhere to the standard's guidelines and avoid using std::memcpy on non-TriviallyCopyable objects. Instead, consider employing alternative approaches or user-defined copying mechanisms that can safely handle the intricacies of such objects.

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