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Both copy construction and copy assignment would mutate the source argument, A, by effectively calling this->reset(A.release()). However, this invalidated assumptions made by standard library algorithms such as std::sort(), which may need to make a copy of an object for later comparisons [Hinnant 05]. Consider an implementation the following implementation of std::sort() that implements the quick sort algorithm:.

// ...
value_type pivot_element = *mid_point;
// ...

At this point, the sorting algorithm assumes that pivot_element and *mid_point have equivalent value representations and will compare equal. However, for std::auto_ptr, this is not the case because *mid_point has been mutated, and mutated and results in unexpected behavior.

In C++11, the std::unique_ptr smart pointer class was introduced as a replacement for std::auto_ptr to better specify the ownership semantics of pointer objects. Rather than mutate the source argument in a copy operation, std::unique_ptr explicitly deletes the copy constructor and copy assignment operator, and instead uses a move constructor and move assignment operator. Subsequently, std::auto_ptr was deprecated in C++11.

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Copy operations that mutate the source operand or global state can lead to unexpected program behavior. In the case of using Using such a type in a Standard Template Library container or algorithm , this can also lead to undefined behavior.

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