The C++ rvalue lifetime disaster

Rvalue references have been with us since C++11. They have originally been introduced to make moving objects more efficient: the object an rvalue reference references is assumed to go out of scope soon and thus may have its resources scavenged without harm. The C++ standard library, for example std::cref or std::ranges, makes use of yet another aspect of rvalue references: since they go out of scope soon, it is assumed unsafe to hold on to them beyond the scope of the current function, while lvalue references are considered safe. We, too, found this assumption to be very useful for smart memory management, in particular in generic code.

Unfortunately, the C++ language itself violates this assumption. Rvalues bind to const&. This means that innocent-looking functions silently convert rvalues to lvalue references, hiding any lifetime limitation of the rvalues. Temporary lifetime extension is meant to make binding a temporary to a reference safe by extending the lifetime of the temporary. But this only works as long as the temporary is a prvalue, and already breaks with rvalue references, let alone spuriously generated lvalue ones. These problems are not merely theoretical. We have had hard-to-find memory corruption in our code because of these problems. In this talk, I will describe the problems in detail, present our library-only approach to mitigate the problems, and finally, make an impossible-to-ever-get-into-the-standard proposal of how to put things right.