The safest & fastest way I'm aware of to use a unique_ptr factory. It involves adding an error flag to the class; the factory checks the error flag and resets the unique_ptr if it's true.
C++ constructors cannot express fallibility. Rust factory functions cannot placement-initialize (write into a pointer) an object too big to fit on the stack (aside from the perma-unstable `box` syntax, which I don't know if it even works or not). A unique_ptr factory cannot stack-allocate an object.
I don't know if it's possible to create a safe interface to fallible initialization, which is agnostic to whether the object will be constructed via return onto the stack, or onto heap memory. I'm interested to hear about proposals though (I've discussed this in the Rust community discord, but that isn't as public or concrete as a long-form article).
No, they can. They can throw exceptions. My suggestion moves unsafe-construction onto the heap. Not ideal for a whole host of reasons, including a need to null-check the result -- but you can then move the object to the stack, and drop the unique_ptr. But if you're tying one hand behind your back, contortions will be necessary to accomplish the mundane.
I don't know if it's possible to create a safe interface to fallible initialization, which is agnostic to whether the object will be constructed via return onto the stack, or onto heap memory. I'm interested to hear about proposals though (I've discussed this in the Rust community discord, but that isn't as public or concrete as a long-form article).