Shouldn't the compiler deduce noexcept for you?
All noexcept does is catch any exception and immediately std::terminate. Confusingly this means that noexcept should really be called deathexcept, since any exception thrown within kills the program.
I don't think there is a rust equivalent of noexcept.
Basically an unrecoverable exception?
> In the situation where no matching [exception] handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate is invoked. In the situation where the search for a handler encounters the outermost block of a function with a non-throwing exception specification, it is implementation-defined whether the stack is unwound, unwound partially, or not unwound at all before the function std::terminate is invoked.
So, the whole thing is basically implementation-defined (including `-fno-exceptions`, since that is something that implementing compilers provide).
No, it doesn't. Try this: https://godbolt.org/z/cvG17cYEs
While that's a possible implementation; the standard is a bit more relaxed: `noexcept` may also call `std::terminate` immediately when an exception is thrown, without calling destructors in the usual way a catch block would do.
https://godbolt.org/z/YTe84M5vq test1 has a ~S() destructor call if maybe_throw() throws; test2 never calls ~S().
MSVC does not appear to support this optimization, so using `noexcept` with MSVC involves overhead similar to the catch-block.
AFAIK itanium ABI exception handling requires two phase unwinding: first the stack is traversed looking for a valid landing pad: if it succeeds then the stack is traversed again calling all destructors. If it fails it calls std terminate. This is actually slower as it need to traverse twice, but the big advantage is that if the program would abort, the state of the program is preserved in the core file. This is easily generalized with noexcept functions: no unwind info is generated for those, so unwind always fail.
MSVC used to do one pass unwind, but I thought they changed it when they implemented table based unwind for x64.
I don't think this is a decent interpretation of what no except does. It misses the whole point of this feature, and confuses a failsafe with the point of using it.
The whole point of noexcept is to tell the compiler that the function does not throw exceptions. This allows the compiler to apply optimizations, such as not needing to track down the necessary info to unwind the call stack when an exception is thrown. Some containers are also designed to only invoke move constructors if they are noexcept and otherwise will copy values around.
As the compiler omits the info required to recover from exceptions, if one is indeed thrown and bubbles up to the noexcept function then it's not possible to do the necessary janitorial work. Therefore, std::terminate is called instead.
Generally what a language feature is intended to do is less relevant than what it actually does, over time. Just like the "inline" keyword was intended as a compiler hint, but what it actually does is change the visibility of symbols across compilation units, and alter the rules for static variables.
Of course, noexcept isn't as useless as inline, yet. There are real uses of it as a hint in template metaprogramming, as you said.
That's the failsafe I mentioned.
> Generally what a language feature is intended to do is less relevant than what it actually does, over time.
Not true, and again this take misses the whole point. The point of noexcept is to declare that a function does not throw exceptions. If a function is declared as noexcept, the whole point is that it does not throw exceptions. How the runtime handles a violation of this contract is something that only expresses in a bug. The same applies for exceptions thrown in destructors. It makes no sense to describe destructors as something that calls std::terminate .
Also, of course destructors are not "something that calls std::terminate", because they do many other things.
For function definitions, noexcept is perfectly equivalent to wrapping the body of your function in try {} catch (...) {std::terminate();}. In fact, noexcept doesn't even add any extra optimization options for the compiler: the compiler can already use the presence of such a block to decide that your function can't throw exceptions and optimize accordingly. Whether it can also ellide this block is a different matter, and noexcept doesn't help with that. It's just a nifty piece of syntax instead of this block.
Conversely, the noexcept declaration on the function doesn't allow the compiler to deduce anything about exceptions not being thrown inside the function, because the standard doesn't forbid throwing exceptions.
The only purpose noexcept serves other than a try/catch block is when used with declarations - there is indeed a difference in how the compiler can optimize a call to a function declared `extern void foo() noexcept;` and a function declared as `extern void foo();`
If a function marked noexcept calls a function that throws an exception, then the program is terminated with an uncaught exception. A called function can throw through a non-noexcept function to a higher-level exception handler no problem.
So in order to avoid changing the semantics of the function, the compiler would have to be able to determine that that transitive closure of called functions dynamically don't throw, and that problem is undecidable, even assuming the requirement that "the compiler can see the source of all those functions" is somehow met, which it won't be.
In LTCG (MSVC)/O3 (GCC/Clang) there are prepasses over the entire callgraph to collect this order
This is exactly the problem. To have made this a useful feature, it should have been more restrictive: a noexcept function should not have been allowed to call any function or operator or lambda that is not marked noexcept. Some extra syntax to allow function templates to be made "conditionally noexcept" would have been necessary, but overall the feature would have had a real use and real power to help make code safer, and more performant.
Java has the first part down, for the class of checked exceptions: a function that doesn't throw can't call functions that do (except in try/catch blocks, but that's largely irrelevant). The annoyances come because of the missing second part - the ability to make a generic function that throws for some type parameters, but doesn't for others.
That's not actually true, it's possible to do "sneaky throws" (https://www.baeldung.com/java-sneaky-throws) of a checked exception from a method which isn't declared to throw that checked exception. The classic example is Class.newInstance(), which propagates any exception from the called constructor. Other ways are calling code from JVM languages other than Java, which do not have the same "checked exception" concept (like Kotlin: see https://kotlinlang.org/docs/exceptions.html and https://kotlinlang.org/docs/java-to-kotlin-interop.html#chec...), generics trickery to confuse the compiler, and manually creating JVM bytecode.
The generics hole is indeed interesting, but it's ultimately a known limitation of how generics were implemented in Java, the presence of type inference, and the design of the exception hierarchy, than an intentional feature. When inferring the type of T to apply in that example, there is no good unique solution: inferring T = Throwable would have been safer, but it makes many simple cases behave unexpectedly, especially with lambdas. Inferring T = RuntimeException is unexpected and unsafe, but in practice it makes many common cases be way more usable, so a call was made to do it, despite the hole.
C++'s templates wouldn't have a similar problem, as they actually instantiate the definition at compile time and can re-check it. Also, there is no equivalent issue to the ambiguous inference, because C++ doesn't do type inference of this kind at all, and anyway there is no problem of the exception hierarchy. Even if there were, C++ could also take the opposite choice than Java, and explicitly infer the safer option when both `noexcept` and `potentially-throws` were possible.
And of course Lombok is a tool for modifying the compilation of Java, so writing Lombok code is not exactly writing pure Java.
Overall this was just another case of the C++ community's preference for speed over safety and robustness. Nothing new, but still a shame to see that the attitude hasn't changed at all.
C++ standard library function yes, C standard library functions, incloding those imported into C++ are more complicated since on most platforms those are not provided by the C++ implementation but some other lower level library. Third-party libraries are the bigger concern though.
> Extern C could also imply noexcept.
In theory this might be correct but in practice this will cause problems with C functions that take callbacks which the user might want to throw from. These are already problematic especially since the C code won't unwind anything but you'd still need to concern yourself with potentially breaking user code.
> Also, explicit try/catch blocks could be required when calling a potentially-throwing function from a noexcept function - slightly annoying, but not a huge problem in practice.
Yes, but avoiding that is the whole point. The try-cach for potentially when you call potentially throwing functions is already implicitly provided by current noexcept and if your noexcept function only calls only noexcept functions then the compiler can already elide all that.
> Overall this was just another case of the C++ community's preference for speed over safety and robustness.
Not at all. If that was the case then noexcept would only be a compiler hint and exceptions bubbling up to noexcept functions would be undefined behavior. Arguably that would be better than what we have now.
Since noexcept is a compilation-level hint, it only matters that the declaration contains them in the <cstdXX> headers, not who provides the implementations. And since C functions can't deal with exceptions thrown from callbacks, it would make sense to annotate any function pointer that you provide with noexcept as well.
> Yes, but avoiding that is the whole point. The try-cach for potentially when you call potentially throwing functions is already implicitly provided by current noexcept and if your noexcept function only calls only noexcept functions then the compiler can already elide all that.
The current implementation calls std::terminate() if an exception is caught. That needn't be the case in practice: you could well handle the exception in a meaningful way and continue execution. Also, having the programmer explicitly do this is valuable in itself, even if they also just call std::terminate(). For example, with the way noexcept is implemented today, you have no notification if a function you're calling goes from noexcept to potentially throwing until your program actually crashes. If noexcept was an actual compiler-checked keyword, your code would stop compiling if one of the functions you rely on started throwing exceptions, and you could decide what to do (maybe catch it, maybe find an alternative, etc).
> Not at all. If that was the case then noexcept would only be a compiler hint and exceptions bubbling up to noexcept functions would be undefined behavior. Arguably that would be better than what we have now.
Crashing the program is of course better than UB. But it's still the worse possible thing to happen other than that. It basically makes noexcept functions be the most scary functions to ever call, instead of being the safest. And if you want to write a piece of code that can never throw an exception, the compiler still won't help you in any way to do that, it's up to you to do it correctly or crash.
Take a hypothetical piece of code
some_library_header.h:
void library_function();
void project_function1();
void project_function2();
void project_function2() {
/* definition */
// no exception
}
void test_function1() {
library_function();
}
void test_function2() {
project_function1();
}
void test_function3() {
project_function2();
}
The best it can do is to say whether a given function is qualified with `noexcept` (see noexcept() operator https://en.cppreference.com/w/cpp/language/noexcept)
Jokes aside - I'm always happy to be corrected, so please go on if I made a mistake somewhere.
Alternatively, Java shows that it is very much possible to do this - the compiler can enforce that a function can't throw exceptions (limited to checked exceptions in Java, but that is besides the point). The way to do it is easy: just check that it doesn't call throw directly, and that all of the functions it calls are also marked noexcept. No need to explore things any deeper than that.
Of course, the designers of C++ didn't want to impose this restriction for various reasons.
Noexcept on the surface is useful, except for the terminate guarantee, which requires a ton of work to avoid metadata size growth and hurts inlining. If violations of noexcept were UB and it was a pure optimization hint the world would be much better
I wonder, why compilers don't expose that information? Some operator returning tri-state "this code provably doesn't throw | could throw | can't see, break the compilation" could help writing generic code immensely. Instead we have to resolve to multi-storey noexcept() operator inside a noexcept qualifier which is very detrimental for the code readability...