[cfe-dev] [libcxx] Which headers must include other headers?

[Craig, Ben via cfe-dev]

On 3/9/2016 4:47 PM, Eric Fiselier wrote:
>
>
> On Wed, Mar 9, 2016 at 3:20 PM, Craig, Ben via cfe-dev 
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>     From the C++14 spec...
>
>         17.6.5.2 Headers [res.on.headers]
>         1 A C++ header may include other C++ headers. A C++ header
>         shall provide the declarations and definitions
>         that appear in its synopsis. A C++ header shown in its
>         synopsis as including other C++ headers shall provide
>         the declarations and definitions that appear in the synopses
>         of those other headers.
>
>
>     I'm not 100% sure I know what this means.  If a header (say...
>     <system_error>) mentions a class (like std::string), is it
>     required to provide the full definition for std::string, or is the
>     forward declared template good enough?
>
>
> According to the standard <system_header> does not need <string> even 
> though the synopsis mentions it. A forward declaration is good enough.
> Although <system_header> mentions string, the synopsis for that header 
> doesn't declare or define std::string so [res.on.headers] doesn't apply.
>
>     Right now, libcxx uses the forward declaration.  What about
>     entities that have specializations scattered all over the place,
>     like std::hash?  Is the unspecialized forward declaration good
>     enough, or does the full bulk of all the std::hash specializations
>     have to come along too?
>
>
>
> I don't think all of the specialization need to come along. I'm not 
> sure what the QoI implications are though. Could you provide some 
> motivation as to why you asked this question?
My motivation is that I'm running a conformance test suite on my 
platform (Hexagon + libcxx + libcxxabi + modified Dinkumware C 
library).  For the portion covering <system_error>, I get a lot of 
failures because the test suite tries to use / instantiate std::string, 
and it believes that it should be able to get the template definition 
transitively through <system_error>.

I like the compile time benefits of a forward string declaration, I just 
want to be sure it's conformant.  If not, I'm fine reporting the test as 
faulty.

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>     Employee of Qualcomm Innovation Center, Inc.
>     Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>
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-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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