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

The Undefined Behavior of std::memcpy for Non-TriviallyCopyable Objects

The header string/byte/memcpy in C specifies that the behavior of std::memcpy is undefined if the objects being copied are not trivially copyable. This means that if you attempt to use memcpy on a non-trivially copyable type, the program will enter an undefined state, and anything can happen, including crashes, data corruption, or unexpected results.

So, why does the standard deem it necessary to specify this?

In the case of trivially copyable types, the copy operation is a simple bitwise copy from the source to the destination. The objects have the same memory layout, and there are no special requirements for the copy operation to be performed.

However, non-trivially copyable types may have constructors, destructors, or other special behavior that needs to be executed when the object is copied. For example, a class may have a constructor that initializes pointers, or a destructor that releases resources.

If you were to use memcpy on a non-trivially copyable type, you would bypass these special behaviors, which could lead to undefined behavior. For example, if you copied an object without calling its constructor, the object might not be properly initialized, and accessing its members could lead to a crash.

The standard specifies that the behavior of memcpy is undefined for non-trivially copyable types to protect you from these potential problems. By forcing you to use the correct copy constructor or assignment operator, the standard ensures that the objects are copied in a way that preserves their intended behavior.

While it is possible to skirt around the undefined behavior in some cases, it is not recommended. It is always best to follow the standard and use the correct copy mechanisms for your non-trivially copyable types.

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