Comment by mxz3000
1 year ago
If the compiler can prove there isn't action-at-a-distance between those lines (this might be non-trivial), then can't the destructor be called before running bar ? Does the C++ spec necessarily say that destructors are called at the end of the block, compared to, let's say, as soon as the variable is no longer used?
> If the compiler can prove there isn't action-at-a-distance between those lines (this might be non-trivial), then can't the destructor be called before running bar?
Yes; there is an "as-if" rule in the C++ standard that says that the implementation must emulate only the "observable behavior" of the abstract machine defined by the standard. The observable behavior is: access through volatile lvalues, data written to files, and I/O to interactive devices.
Does the C++ spec necessarily say that destructors are called at the end of the block, compared to, let's say, as soon as the variable is no longer used?
Yes, as long as the destructor has any side effect:
> If a variable with automatic storage duration has initialization or a destructor with side effects, an implementation shall not destroy it before the end of its block nor eliminate it as an optimization, even if it appears to be unused, except that a class object or its copy/move may be eliminated as specified in 11.10 [1].
Other wise things like std::lock_guard[2] would not work; that's an example of a type that is never explicitly used after definition, but used purely for the side-effects of its constructor/destructor.
1: https://isocpp.org/files/papers/N4860.pdf
2: https://en.cppreference.com/w/cpp/thread/lock_guard