When Is It Safe to Use `std::memcpy` with Non-Trivially Copyable Objects?

Behavior of std::memcpy with Non-TriviallyCopyable Objects

The behavior of std::memcpy is declared undefined for objects that are not TriviallyCopyable. This is because memcpy does not perform any type checking. It simply copies the bytes of the source object into the destination object. For objects of non-TriviallyCopyable types, this can lead to unexpected behavior.

For example, let's consider the following struct:

struct Entity {
    int health;
    int damage;
};

Both int and Entity are not TriviallyCopyable. If we use std::memcpy to copy objects of this type, we may get unexpected results. For example, the following code may cause the program to crash:

void swapEntities(Entity* e1, Entity* e2) {
    memcpy(e1, e2, sizeof(Entity));
}

This code swaps the values of e1 and e2 by copying the bytes of e2 into e1. However, this doesn't properly handle the problem that Entity is not TriviallyCopyable. The constructor and destructor of Entity are not called, and the values of e1 and e2 may be left in an inconsistent state.

To avoid undefined behavior, it's important to use the appropriate functions for copying objects of non-TriviallyCopyable. std::copy and std::swap are two functions that are guaranteed to be safe to use with objects of any type.

In the updated description of memcpy on cppreference, the following exception is noted: "unless the program does not depend on the effects of the destructor of the target object (which is not run by memcpy) and the lifetime of the target object (which is ended, but not started by memcpy) is started by some other means, such as placement-new."

This exception allows memcpy to be used with non-TriviallyCopyable objects under certain limited circumstances. However, it's important to note that this is an exception, and it's still generally safer to avoid using memcpy with non-TriviallyCopyable objects.

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