[cfe-users] Fwd: Warnings for implicit constructors and wrong usage of auto

[Andrea Arteaga via cfe-users]

Dear all,
Recently, my team suffered from a bug due to a double bad usage of C++.

We have a function returning a reference to an object:

Object& GetObject();

Sometimes we use this function like this:

auto obj = GetObject();

This triggers a copy of the object, which we really don't mean. The two
problems are:
1. Object does not delete the copy constructor, nor does it declare one. We
have a policy of never using implicitly-declared constructors, we either
use `=delete` or `=default`. Nevertheless we missed this class.
2. A reference is demoted to a rvalue due to the usage of `auto` instead of
`auto&`.

We would like clang to issue warnings in these two cases. The latter case
does not really seem easy to warn about, but the former looks
straightforward.

Does clang offer such warnings?

Thanks
Andrea Arteaga
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Re: [cfe-users] Warnings for implicit constructors and wrong usage of auto

[George Karpenkov via cfe-users]

+cfe-dev

Hi Andrea,

I think you might get more luck asking on the cfe-dev mailing list.

George

> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users 
>  wrote:
> 
> Dear all,
> Recently, my team suffered from a bug due to a double bad usage of C++.
> 
> We have a function returning a reference to an object:
> 
> Object& GetObject();
> 
> Sometimes we use this function like this:
> 
> auto obj = GetObject();
> 
> This triggers a copy of the object, which we really don't mean. The two 
> problems are:
> 1. Object does not delete the copy constructor, nor does it declare one. We 
> have a policy of never using implicitly-declared constructors, we either use 
> `=delete` or `=default`. Nevertheless we missed this class.
> 2. A reference is demoted to a rvalue due to the usage of `auto` instead of 
> `auto&`.
> 
> We would like clang to issue warnings in these two cases. The latter case 
> does not really seem easy to warn about, but the former looks straightforward.
> 
> Does clang offer such warnings?
> 
> Thanks
> Andrea Arteaga 
> 
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Re: [cfe-users] [cfe-dev] Warnings for implicit constructors and wrong usage of auto

[Richard Smith via cfe-users]

On 15 May 2018 at 16:01, John McCall via cfe-dev 
wrote:

> On May 15, 2018, at 6:05 PM, George Karpenkov via cfe-dev <
> cfe-...@lists.llvm.org> wrote:
>
> +cfe-dev
>
> Hi Andrea,
>
> I think you might get more luck asking on the cfe-dev mailing list.
>
>
> George
>
> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users <
> cfe-users@lists.llvm.org> wrote:
>
> Dear all,
> Recently, my team suffered from a bug due to a double bad usage of C++.
>
> We have a function returning a reference to an object:
>
> Object& GetObject();
>
> Sometimes we use this function like this:
>
> auto obj = GetObject();
>
> This triggers a copy of the object, which we really don't mean. The two
> problems are:
> 1. Object does not delete the copy constructor, nor does it declare one.
> We have a policy of never using implicitly-declared constructors, we either
> use `=delete` or `=default`. Nevertheless we missed this class.
>
>
> Implicitly-defined copy constructors are ubiquitous in idiomatic C++.
> Maybe that's not true in your project, but still, this seems too
> special-case for the compiler.  Maybe a linter that has a more
> sophisticated model of what code is yours vs. the standard library.
>
> 2. A reference is demoted to a rvalue due to the usage of `auto` instead
> of `auto&`.
>
>
> This is a more reasonable thing to try to warn about.  I have two concerns:
>   - I don't know a reasonable way to suppress the warning if you really do
> want to load from the l-value.
>   - I have a non-specific worry that it'll disrupt some important idiom
> that I'm just not thinking of.
>

How about:

  vector v = get_vector();
  auto x = v.front();

or, worse:

  auto y = get_vector().front();

But those are concerns that we could explore during iterative design and
> implementation.
>
> John.
>
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>
>
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Re: [cfe-users] [cfe-dev] Warnings for implicit constructors and wrong usage of auto

[John McCall via cfe-users]

> On May 15, 2018, at 6:05 PM, George Karpenkov via cfe-dev 
>  wrote:
> 
> +cfe-dev
> 
> Hi Andrea,
> 
> I think you might get more luck asking on the cfe-dev mailing list.
> 
> George
> 
>> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users 
>> mailto:cfe-users@lists.llvm.org>> wrote:
>> 
>> Dear all,
>> Recently, my team suffered from a bug due to a double bad usage of C++.
>> 
>> We have a function returning a reference to an object:
>> 
>> Object& GetObject();
>> 
>> Sometimes we use this function like this:
>> 
>> auto obj = GetObject();
>> 
>> This triggers a copy of the object, which we really don't mean. The two 
>> problems are:
>> 1. Object does not delete the copy constructor, nor does it declare one. We 
>> have a policy of never using implicitly-declared constructors, we either use 
>> `=delete` or `=default`. Nevertheless we missed this class.

Implicitly-defined copy constructors are ubiquitous in idiomatic C++.  Maybe 
that's not true in your project, but still, this seems too special-case for the 
compiler.  Maybe a linter that has a more sophisticated model of what code is 
yours vs. the standard library.

>> 2. A reference is demoted to a rvalue due to the usage of `auto` instead of 
>> `auto&`.

This is a more reasonable thing to try to warn about.  I have two concerns:
  - I don't know a reasonable way to suppress the warning if you really do want 
to load from the l-value.
  - I have a non-specific worry that it'll disrupt some important idiom that 
I'm just not thinking of.
But those are concerns that we could explore during iterative design and 
implementation.

John.
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Re: [cfe-users] [cfe-dev] Warnings for implicit constructors and wrong usage of auto

[John McCall via cfe-users]

> On May 15, 2018, at 7:23 PM, Richard Smith  wrote:
> On 15 May 2018 at 16:01, John McCall via cfe-dev  > wrote:
>> On May 15, 2018, at 6:05 PM, George Karpenkov via cfe-dev 
>> mailto:cfe-...@lists.llvm.org>> wrote:
>> 
>> +cfe-dev
>> 
>> Hi Andrea,
>> 
>> I think you might get more luck asking on the cfe-dev mailing list.
>> 
>> George
>> 
>>> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users 
>>> mailto:cfe-users@lists.llvm.org>> wrote:
>>> 
>>> Dear all,
>>> Recently, my team suffered from a bug due to a double bad usage of C++.
>>> 
>>> We have a function returning a reference to an object:
>>> 
>>> Object& GetObject();
>>> 
>>> Sometimes we use this function like this:
>>> 
>>> auto obj = GetObject();
>>> 
>>> This triggers a copy of the object, which we really don't mean. The two 
>>> problems are:
>>> 1. Object does not delete the copy constructor, nor does it declare one. We 
>>> have a policy of never using implicitly-declared constructors, we either 
>>> use `=delete` or `=default`. Nevertheless we missed this class.
> 
> Implicitly-defined copy constructors are ubiquitous in idiomatic C++.  Maybe 
> that's not true in your project, but still, this seems too special-case for 
> the compiler.  Maybe a linter that has a more sophisticated model of what 
> code is yours vs. the standard library.
> 
>>> 2. A reference is demoted to a rvalue due to the usage of `auto` instead of 
>>> `auto&`.
> 
> This is a more reasonable thing to try to warn about.  I have two concerns:
>   - I don't know a reasonable way to suppress the warning if you really do 
> want to load from the l-value.
>   - I have a non-specific worry that it'll disrupt some important idiom that 
> I'm just not thinking of.
> 
> How about:
> 
>   vector v = get_vector();
>   auto x = v.front();
> 
> or, worse:
> 
>   auto y = get_vector().front();

Yep, good example.

John.
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Re: [cfe-users] [cfe-dev] Warnings for implicit constructors and wrong usage of auto

[Andrea Arteaga via cfe-users]

Thanks for your reply, John.

I fully understand your points, I actually had the same concern that a
reference-to-rvalue warning would be issued for perfectly reasonable and
intended code. It's still one of those warnings I would like to see when
writing code in an IDE, not when compiling the full project (we would setup
builds with different warning options for these two cases).

For the default constructor warning, I did not mean to be warned when a
default copy constructor exists, just when one is used.

In any case, it does not seem like clang currently implements warnings like
those, right?

Thank you.
Andrea

On Wed, May 16, 2018 at 1:55 AM, John McCall  wrote:

> On May 15, 2018, at 7:23 PM, Richard Smith  wrote:
> On 15 May 2018 at 16:01, John McCall via cfe-dev 
> wrote:
>
>> On May 15, 2018, at 6:05 PM, George Karpenkov via cfe-dev <
>> cfe-...@lists.llvm.org> wrote:
>>
>> +cfe-dev
>>
>> Hi Andrea,
>>
>> I think you might get more luck asking on the cfe-dev mailing list.
>>
>>
>> George
>>
>> On May 15, 2018, at 1:15 PM, Andrea Arteaga via cfe-users <
>> cfe-users@lists.llvm.org> wrote:
>>
>> Dear all,
>> Recently, my team suffered from a bug due to a double bad usage of C++.
>>
>> We have a function returning a reference to an object:
>>
>> Object& GetObject();
>>
>> Sometimes we use this function like this:
>>
>> auto obj = GetObject();
>>
>> This triggers a copy of the object, which we really don't mean. The two
>> problems are:
>> 1. Object does not delete the copy constructor, nor does it declare one.
>> We have a policy of never using implicitly-declared constructors, we either
>> use `=delete` or `=default`. Nevertheless we missed this class.
>>
>>
>> Implicitly-defined copy constructors are ubiquitous in idiomatic C++.
>> Maybe that's not true in your project, but still, this seems too
>> special-case for the compiler.  Maybe a linter that has a more
>> sophisticated model of what code is yours vs. the standard library.
>>
>> 2. A reference is demoted to a rvalue due to the usage of `auto` instead
>> of `auto&`.
>>
>>
>> This is a more reasonable thing to try to warn about.  I have two
>> concerns:
>>   - I don't know a reasonable way to suppress the warning if you really
>> do want to load from the l-value.
>>   - I have a non-specific worry that it'll disrupt some important idiom
>> that I'm just not thinking of.
>>
>
> How about:
>
>   vector v = get_vector();
>   auto x = v.front();
>
> or, worse:
>
>   auto y = get_vector().front();
>
>
> Yep, good example.
>
> John.
>
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