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Subject: Re: [important] C++ Zero-Overhead exception proposal
From: Denis Kotov (redradist_at_[hidden])
Date: 2020-10-19 03:00:51


Also solution could be flexible enough and if user return raw integer
declaration could look like in Herb Sutter suggestion:
```cpp
Subscription subscribe() throws;
```
But if user return error with more complex layout than it should write it
like this:
```cpp
Subscription subscribe() throws SubscriptionError;
```

On Mon, 19 Oct 2020 at 10:56, Denis Kotov <redradist_at_[hidden]> wrote:

> The best way to return error in union
> ```cpp
> union SubscribeResult {
> Subscription res;
> SubscriptionError err;
> };
> ```
>
> In such way compiler can save lots of memory and to distinguish if it is
> an error or return value could be added some boolean value that also
> returns:
> ```cpp
> struct SubscribeResult {
> bool isError;
> union {
> Subscription res;
> SubscriptionError err;
> } result;
> }
> ```
> If there is returned error from function as integer compiler could return
> it as:
> ```cpp
> struct SubscribeResult {
> Subscription res;
> int err;
> }
> ```
>
> Anyway compiler can choose the optimal layout for specific error that
> function is returns
>
>
> On Mon, 19 Oct 2020 at 09:55, Florian Weimer <fweimer_at_[hidden]> wrote:
>
>> * Denis Kotov via Std-Discussion:
>>
>> > This code:
>> > ```cpp
>> > Subscription subscribe() throws SubscriptionError;
>> >
>> > Subscription subscribe() throws {
>> > ...
>> > throw SubscriptionError{"Error", Payload};
>> > ...
>> > }
>> > ```
>> > translates to:
>> > ```cpp
>> > struct SubscribeResult {
>> > Subscription res;
>> > SubscriptionError err;
>> > };
>> >
>> > SubscribeResult subscribe();
>> >
>> > SubscribeResult subscribe() {
>> > ...
>> > return SubscribeResult{ .err = SubscriptionError{"Error", Payload}
>> };
>> > ...
>> > }
>> > ```
>>
>> Not that requires ABI work on many architectures in order to avoid
>> regressions when Subscription can be returned directly in a register,
>> but SubscribeResult cannot. Quite a few ABIs always return structs in
>> memory, or pack multiple fields into a single register (optimizing for
>> the case where the caller stores the result to the final place, rather
>> than unpacking it immediately, as it would with inline exception
>> handling).
>>
>> There is a reason why the present scheme based on unwinding has been
>> called "zero-cost exception handling", too.
>>
>> Thanks,
>> Florian
>> --
>> Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
>> Commercial register: Amtsgericht Muenchen, HRB 153243,
>> Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael
>> O'Neill
>>
>>
>
> --
>
> *Best RegardsDenis Kotov*
>

-- 
*Best RegardsDenis Kotov*


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