The missing C++ smart pointer
I think there&#x27;s a bit of confusion here around &quot;value semantics&quot;.<p>No C++ smart pointer has &quot;value semantics&quot;, relative to its target T. You can see this because == performs address comparison, not deep comparison, and `const` methods on the smart pointer can be used to mutate the target (e.g. in C++, operator* on unique_ptr is always const, and yields a T&amp;).<p>This is in contrast to Rust, where Box performs deep equality, and has deep const&#x2F;mut. In Rust, Box is basically just a wrapper around a value to have it on the heap (enabling things like dynamic polymorphism, like in C++). In C++, the pointer is its own entity, with its own separate equality, and so on.<p>Const-ness of operations, operator==, and assignment&#x2F;copying behavior all have to be consistent with each other. For example, if `box` was simply `unique_ptr` with a copy constructor (somehow, and as the table in the blog post basically implies), then you would have that after `auto a = b;`, `a != b`, which obviously doesn&#x27;t work. This means that the hypothetical `std::box` would have to have its comparison and const-ness adjusted as well. In C++ terms, this isn&#x27;t really a pointer at all. The closest thing to what the author is suggesting is actually `polymorphic_value`, I believe, which IIRC has been proposed formally (note that it does not have pointer in the name).<p>Also as an aside, smart pointers are not suitable a) for building data structures in general, and b) building recursive data structures in particular. The former is because meaningfully using smart pointers (i.e. letting them handle destruction) inside an allocator aware data structure (as many C++ data structures tend to be, and even data structures in Rust) would require duplicating the allocator over and over. The latter is because compilers do not perform TCO in many real world examples (and certainly not in debug mode); if you write a linked list using `std::unique_ptr` the destructor will blow your stack.