AMDG On 4/7/19 2:38 AM, Rainer Deyke via Boost wrote:
On 06.04.19 21:24, Emil Dotchevski via Boost wrote:
I get what you're saying, that if an assignment fails, logically you wish to treat the resulting state something like an uninitialized state. The problem is that your program can no longer assume that all objects it works with are valid -- that is, RAII is out the window -- except if the special state is (by definition) a valid state. But this contradicts your wish, because a valid state is nothing like an uninitialized state. It has to be on your mind all the time, because now objects in this state are valid (by definition) and you must define behavior for functions that are handed such objects.
You keep using the term "valid" as if its a clear-cut binary distinction. However, I can think of at least four different degrees of validity:
1: Garbage. A variable (of a class type) was not properly constructed and contains complete garbage. It is undefined behavior to perform any operation on the object, including assignment and destruction.
2: Uninitialized. A variable (of a built-in type) is uninitialized. It is undefined behavior to read the value of this variable, but the variable is "valid" in the sense that you can assign a new value to it and that you can destruct it.
3: Indeterminate. A variable (of a class type) has an indeterminate, semantically meaningless state (after throwing an exception from a member function with the basic exception guarantee, or after being pulled from an object pool). It is technically allowed by not semantically meaningful to read the value of this variable, but the variable is "valid" in the sense that you can assign a new value to it and that you can destruct it.
4: Correct. A variable (of any type) is valid and contains a semantically meaningful and correct value.
You seem to categorize degrees 3 and 4 as "valid" and degrees 1 and 2 as "invalid". I'm saying that the distinction between degrees 1 and 2 is huge,
I disagree. The rules for (2) regarding construction and destruction are a bit relaxed, but most uses are still undefined behavior.
as is the distinction between degrees 3 and 4, but the distinction between 2 and 3 is relatively small. At the physical level, there is no distinction between degrees 2 and 3, since reading from uninitialized memory doesn't cause any actual problems on actual hardware.
The hardware behavior is irrelevant. It only applies if you assume a simple translation from the source to machine code. This is not a valid assumption due to compiler optimizations.
At the high conceptual level, there is also no distinction between degrees 2 and 3, because the same set of operations is semantically meaningful for both. The distinction still exists, technically, but I consider any code that takes advantage of this distinction suspect, and I'd love for static code analysis tools to give the same diagnostics for degree 3 as for degree 2. If they cannot, then that's a problem with the tools, not with the idea.
The distinction is in fact quite simple: Does the expression: x.foo() have undefined behavior according to the language. C++ itself makes no distinction between (3) and (4). In Christ, Steven Watanabe